Anastrozole

updated

Anastrozole is an FDA-approved medicine for hormone receptor-positive breast cancer in postmenopausal women, with the 9,366-woman ATAC trial behind that use. Sellers promote it to control estrogen on steroid cycles and to prevent gynecomastia, but it did not beat placebo in 80 boys with pubertal gynecomastia and did not significantly reduce gynecomastia in men starting bicalutamide. Two PubMed searches on 9 October 2026 found no study of anastrozole for estrogen control or side effects in anabolic steroid users.

Aromatase inhibitor

  • FDA approved ARIMIDEX under NDA 020541 on 27 December 1995 for hormone receptor-positive (or unknown) breast cancer in postmenopausal women.
  • ATAC (9,366 postmenopausal women, 5 years) found disease-free survival favored anastrozole over tamoxifen (HR 0.87, 95% CI 0.78 to 0.97 at a median 68 months), and overall survival did not differ at a median 120 months.
  • In 80 boys aged 11 to 18 with pubertal gynecomastia, 38.5% on anastrozole versus 31.4% on placebo reached a 50% or greater fall in breast volume (P = 0.47).
  • In 114 men starting bicalutamide, gynecomastia developed in 51% on anastrozole, 10% on tamoxifen and 73% on placebo over 48 weeks.
  • The label's estradiol falls of about 70% and 80% were measured in postmenopausal women. In older men estradiol fell from 26 to 17 pg/mL.
  • In 69 older men, 1 year of anastrozole lowered spine bone mineral density compared with placebo (P = 0.0014).

What is anastrozole approved to do?

FDA approved ARIMIDEX (anastrozole) on 27 December 1995. It is labeled only for hormone receptor-positive or unknown breast cancer in postmenopausal women, so use in men for estrogen control is not a labeled use.

The ARIMIDEX label covers hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. The label version used here took effect on 2 March 2026.

FDA approved ARIMIDEX under NDA 020541 on 27 December 1995. The current application holder is ANI Pharmaceuticals. Approvals outside the United States are not covered here.

Anastrozole is C17H19N5 (293.4 g/mol, CAS 120511-73-1, PubChem CID 2187). The label's active ingredient is anastrozole itself, not a salt, so label doses are anastrozole.

ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source
Drugs@FDA: ARIMIDEX approved 27 December 1995

Regulatory record

U.S. Food and Drug Administration. Drugs@FDA application record for NDA 020541 (ARIMIDEX). Date read from the openFDA Drugs@FDA record.

FDA approved ARIMIDEX under NDA 020541 on 27 December 1995. The current application holder is ANI Pharmaceuticals.

Participants / model
Not a clinical population
Treatment
Drug application
Follow-up
Approved 1995
Study design
FDA application record
Funding
not stated in the source

This record gives the approval date and holder. Indications come from the product label.

Read the original source
PubChem: anastrozole identity

Chemical identity record

National Library of Medicine. PubChem compound record CID 2187 (anastrozole).

Anastrozole: PubChem CID 2187, C17H19N5, 293.4 g/mol, CAS 120511-73-1. The label's active ingredient is anastrozole itself, not a salt, so label doses are anastrozole.

Participants / model
Chemical identity, not a clinical population
Treatment
Defined molecule
Follow-up
Not applicable
Study design
NLM chemical record
Funding
not stated in the source

Identity record only; it says nothing about efficacy or safety.

Read the original source

What did the large breast cancer trial find?

ATAC randomized 9,366 postmenopausal women with operable breast cancer. Disease-free survival favored anastrozole over tamoxifen at 68 and 120 months, and overall survival did not differ.

ATAC randomized 9,366 postmenopausal women with operable breast cancer to anastrozole 1 mg, tamoxifen 20 mg, or both for 5 years. At a median 68 months of follow-up, disease-free survival favored anastrozole over tamoxifen (HR 0.87, 95% CI 0.78 to 0.97; hormone receptor-positive subgroup HR 0.83, 95% CI 0.73 to 0.94). At a median 120 months the disease-free survival HRs were 0.91 (95% CI 0.83 to 0.99) and 0.86 (95% CI 0.78 to 0.95), and overall survival did not differ (HR 0.97, 95% CI 0.88 to 1.08).

The combination arm showed no benefit over tamoxifen and was closed, and the label says tamoxifen should not be given with anastrozole. The trial was in women with breast cancer, so none of these results describe men or steroid-cycle use.

ATAC: 9,366 postmenopausal women, anastrozole versus tamoxifen, 5 years

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) prescribing information, label version effective 2 March 2026, section 14.1 (ATAC phase 3 trial). Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

ATAC randomized 9,366 postmenopausal women with operable breast cancer to anastrozole, tamoxifen or both for 5 years. At a median 68 months, disease-free survival favored anastrozole over tamoxifen (HR 0.87, 95% CI 0.78 to 0.97; hormone receptor-positive subgroup HR 0.83, 0.73 to 0.94). At a median 120 months the disease-free survival HRs were 0.91 (0.83 to 0.99) and 0.86 (0.78 to 0.95), and overall survival did not differ (HR 0.97, 0.88 to 1.08). The combination arm showed no benefit over tamoxifen and was closed, and the label says tamoxifen should not be given with anastrozole.

Participants / model
9,366 postmenopausal women with operable breast cancer
Treatment
Oral anastrozole 1 mg daily versus tamoxifen 20 mg daily, or both, for 5 years
Follow-up
5 years of treatment; median follow-up 68 and 120 months
Study design
Phase 3 randomized trial, results read from label section 14.1
Funding
not stated in the source

Women only. The result does not apply to men or to steroid-cycle use.

Read the original source

Does anastrozole treat or prevent gynecomastia?

Not in the randomized studies found. Anastrozole did not beat placebo in 80 boys with pubertal gynecomastia, and it did not significantly reduce gynecomastia in 114 men starting bicalutamide, where tamoxifen did. A guideline recommends against aromatase inhibitors for gynecomastia in general.

In a randomized double-blind trial in 80 boys aged 11 to 18 whose pubertal gynecomastia had not shrunk over 3 months, anastrozole 1 mg daily for 6 months did not beat placebo: 38.5% versus 31.4% reached a 50% or greater fall in ultrasound breast volume (odds ratio 1.513, 95% CI 0.496 to 4.844, P = 0.47), even though the median testosterone-to-estradiol ratio rose 166% versus 39%. The ARIMIDEX label (section 8.4) states that efficacy was not demonstrated for pubertal gynecomastia and describes a placebo-controlled trial in 80 boys aged 11 to 18 in which estradiol fell 15.4% versus 4.5% and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

In 114 men with prostate cancer starting bicalutamide 150 mg daily, gynecomastia developed over 48 weeks in 73% with added placebo, 10% with tamoxifen 20 mg and 51% with anastrozole 1 mg (breast pain in 39%, 6% and 27%). The authors concluded that anastrozole did not significantly reduce gynecomastia or breast pain while tamoxifen did. This is antiandrogen-induced gynecomastia, not gynecomastia from anabolic steroids.

A meta-analysis of 9 randomized trials in men on bicalutamide found that tamoxifen lowered the risk of gynecomastia (RR 0.18, 95% CI 0.08 to 0.38) and breast pain (RR 0.18, 95% CI 0.07 to 0.43), radiotherapy also lowered both, and anastrozole showed no significant benefit.

The European Academy of Andrology guideline recommends against using SERMs, aromatase inhibitors or non-aromatizable androgens to treat gynecomastia in general (strong recommendation, low-quality evidence). It recommends watchful waiting after a causative substance is stopped, reserves surgery for long-lasting gynecomastia, and states that more than 90% of pubertal gynecomastia resolves on its own within 24 months.

80 boys, 6 months: no benefit over placebo

Randomized controlled trial

Plourde PV et al. J Clin Endocrinol Metab. 2004;89(9):4428-33. PMID 15356042.

Anastrozole did not beat placebo: 38.5% versus 31.4% reached a 50% or greater fall in ultrasound breast volume (odds ratio 1.513, 95% CI 0.496 to 4.844, P = 0.47), even though the median testosterone-to-estradiol ratio rose 166% versus 39%.

Participants / model
80 boys aged 11 to 18 with pubertal gynecomastia that had not shrunk over 3 months
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 months
Study design
Randomized double-blind trial
Funding
not stated in the source

Boys with pubertal gynecomastia. The result does not describe adult men or steroid users.

Read the original source
114 men on bicalutamide: gynecomastia 73% placebo, 10% tamoxifen, 51% anastrozole

Randomized controlled trial

Boccardo F et al. J Clin Oncol. 2005;23(4):808-15. PMID 15681525.

Over 48 weeks, gynecomastia developed in 73% with added placebo, 10% with tamoxifen and 51% with anastrozole (breast pain in 39%, 6% and 27%). The authors concluded that anastrozole did not significantly reduce gynecomastia or breast pain while tamoxifen did. Adverse events were reported in 69% on anastrozole versus 37% on placebo.

Participants / model
114 men with prostate cancer starting bicalutamide monotherapy
Treatment
Oral anastrozole 1 mg, tamoxifen 20 mg or placebo daily, with bicalutamide 150 mg daily
Follow-up
48 weeks
Study design
Randomized trial
Funding
not stated in the source

This is antiandrogen-induced gynecomastia, not gynecomastia from anabolic steroids.

Read the original source
Meta-analysis of 9 randomized trials: tamoxifen RR 0.18, anastrozole no significant benefit

Systematic review and meta-analysis

Spagnolo L et al. J Endocrinol Invest. 2025;48(7):1659-1668. PMID 40244528.

Tamoxifen lowered the risk of gynecomastia (RR 0.18, 95% CI 0.08 to 0.38) and breast pain (RR 0.18, 95% CI 0.07 to 0.43). Radiotherapy also lowered both. Anastrozole showed no significant benefit.

Participants / model
Men with prostate cancer on bicalutamide, 9 randomized trials
Treatment
Oral tamoxifen, oral anastrozole or breast radiotherapy versus bicalutamide alone or with placebo or sham
Follow-up
Pooled randomized trials
Study design
Systematic review and meta-analysis of 9 randomized trials
Funding
not stated in the source

Applies to men on bicalutamide.

Read the original source
European Academy of Andrology guideline: no SERMs or aromatase inhibitors for gynecomastia in general

Human study

Kanakis GA et al. Andrology. 2019;7(6):778-793. PMID 31099174.

The guideline recommends against using SERMs, aromatase inhibitors or non-aromatizable androgens to treat gynecomastia in general (strong recommendation, low-quality evidence). It recommends watchful waiting after a causative substance is stopped, reserves surgery for long-lasting gynecomastia, and states that more than 90% of pubertal gynecomastia resolves on its own within 24 months.

Participants / model
Boys and men with gynecomastia (guideline scope)
Treatment
Not drug-specific
Follow-up
Not applicable
Study design
Clinical practice guideline (GRADE)
Funding
not stated in the source

A treatment recommendation for gynecomastia in general. It is not specific to anastrozole.

Read the original source
ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source

What happened to hormones and the body in men given anastrozole?

In older men with low testosterone, 12 weeks of anastrozole raised testosterone and lowered estradiol without changing quality of life or erectile function. A 1-year trial found lower spine bone density than on placebo, and a crossover trial in healthy men found lower insulin sensitivity.

In 37 men aged 62 to 74 with screening testosterone below 350 ng/dL, 12 weeks of anastrozole raised mean total testosterone from 343 to 572 ng/dL on 1 mg daily and from 397 to 520 ng/dL on 1 mg twice weekly. Estradiol fell from 26 to 17 and from 27 to 17 pg/mL and stayed within the normal male range. Quality of life, erectile function (IIEF) and prostate symptom scores did not change, and PSA rose in the twice-weekly group.

In a 1-year placebo-controlled trial in 69 men aged 60 or older with low or borderline testosterone, anastrozole 1 mg daily raised mean testosterone (319 to 524 ng/dL at month 3), lowered estradiol (15 to 12 pg/mL) and lowered posterior-anterior spine bone mineral density compared with placebo (P = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in these men. No fracture outcome in men was found.

In a randomized double-blind crossover trial in 17 healthy men aged 18 to 50, 6 weeks of anastrozole 1 mg daily lowered estradiol (59.9 versus 102.0 pmol/L on placebo), raised total testosterone modestly (25.8 versus 21.4 nmol/L) and lowered the glucose infusion rate during low-dose insulin infusion (12.16 versus 14.15 micromol/kg/min), which the authors interpret as reduced peripheral insulin sensitivity.

In a separate design, 400 healthy men aged 20 to 50 had their own testosterone and estradiol suppressed with goserelin and were randomized to placebo or testosterone gel for 16 weeks; 202 of them also took anastrozole to block conversion to estradiol. Estrogen deficiency mainly accounted for increases in body fat, and androgen and estrogen deficiency both contributed to the decline in sexual function. Anastrozole was a research tool here, not a treatment, and this design shows that estradiol has effects in men. It does not test a steroid cycle.

37 older men, 12 weeks: testosterone up, estradiol down, no change in quality of life

Randomized controlled trial

Leder BZ et al. J Clin Endocrinol Metab. 2004;89(3):1174-80. PMID 15001605.

Mean total testosterone rose from 343 to 572 ng/dL on 1 mg daily and from 397 to 520 ng/dL on 1 mg twice weekly. Estradiol fell from 26 to 17 and from 27 to 17 pg/mL and stayed within the normal male range. Quality of life, erectile function (IIEF) and prostate symptom scores did not change, and PSA rose in the twice-weekly group.

Participants / model
37 men aged 62 to 74 with screening testosterone below 350 ng/dL
Treatment
Oral anastrozole 1 mg daily or 1 mg twice weekly, or placebo
Follow-up
12 weeks
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Short hormone study in older men with low or borderline-low testosterone.

Read the original source
69 men, 1 year: spine bone density lower than on placebo

Randomized controlled trial

Burnett-Bowie SA et al. J Clin Endocrinol Metab. 2009;94(12):4785-92. PMID 19820017.

Mean testosterone rose (319 to 524 ng/dL at month 3) and estradiol fell (15 to 12 pg/mL). Posterior-anterior spine bone mineral density was lower than with placebo (P = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in these men.

Participants / model
69 men aged 60 or older with low or borderline testosterone
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
1 year
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Older men with low or borderline testosterone. No fracture outcome in men was found.

Read the original source
17 healthy men, 6 weeks each: estradiol down, insulin sensitivity down

Randomized controlled trial

Gibb FW et al. J Clin Endocrinol Metab. 2016;101(5):2040-6. PMID 26967690.

Anastrozole lowered estradiol (59.9 versus 102.0 pmol/L on placebo), raised total testosterone modestly (25.8 versus 21.4 nmol/L) and lowered the glucose infusion rate during low-dose insulin infusion (12.16 versus 14.15 micromol/kg/min), which the authors interpret as reduced peripheral insulin sensitivity.

Participants / model
17 healthy men aged 18 to 50
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 weeks per arm
Study design
Randomized double-blind crossover trial with hyperinsulinemic euglycemic clamp
Funding
not stated in the source

Healthy men, short term. It measures a laboratory insulin-sensitivity endpoint, not a clinical outcome.

Read the original source
400 healthy men, 16 weeks: estrogen deficiency raised body fat

Randomized controlled trial

Finkelstein JS et al. N Engl J Med. 2013;369(11):1011-22. PMID 24024838.

400 healthy men had their own testosterone and estradiol suppressed with goserelin and were randomized to placebo or testosterone gel; 202 of them also took anastrozole to block conversion to estradiol. Estrogen deficiency mainly accounted for increases in body fat, and androgen and estrogen deficiency both contributed to the decline in sexual function.

Participants / model
400 healthy men aged 20 to 50
Treatment
Goserelin plus testosterone gel (1.25 to 10 g daily) or placebo, with or without oral anastrozole
Follow-up
16 weeks
Study design
Randomized trial
Funding
not stated in the source

Anastrozole was used as a research tool under goserelin, not as a treatment. The trial shows that estradiol has effects in men. It does not test a steroid cycle.

Read the original source

Is anastrozole the first-line way to control estrogen on a steroid cycle?

No study found tests it. Two PubMed searches on 9 October 2026 found no study of anastrozole for estrogen control, gynecomastia or side effects in anabolic steroid users, and none of the water-retention records studied men.

Sellers describe anastrozole as the first-line aromatase inhibitor for controlling estrogen on steroid cycles that aromatize, and as preventing and treating gynecomastia, water retention and other estrogenic side effects.

A PubMed search on 9 October 2026 for anastrozole with anabolic steroid or bodybuilding terms returned 8 records: a commentary, a mouse cell-culture study, three drug-detection method papers, a gym survey, and the forum and seized-product analyses below. None tested anastrozole for estrogen control, gynecomastia or side effects in anabolic steroid users. A second search for anastrozole or aromatase inhibitors with water or fluid retention returned 4 records, all reviews or trials in women with breast cancer or with estrogen-deprivation arthralgia; none studied men.

An analysis of 1,792 posts on a French bodybuilding forum (2013 to 2019), all by men with a mean age of 28.2 years, found aromatase inhibitors used mainly to counter aromatization of androgens, with a median anastrozole course of 70 days at a mean 0.4 mg a day, obtained mainly online. This records how the drug is used, not whether it works. For the doses users report, the only hormone data found are from the 1 mg twice-weekly arm in older men above, and no outcome data were found.

Poland's official medicines control laboratory tested 63 seized illegal products sold for post-cycle therapy (2020 to 2024). 34.9% did not contain the declared active ingredient (20.6% contained no active ingredient and 14.3% a different one), and 6.4% contained an extra undeclared ingredient. Per-drug results are not given in the abstract, so a gray-market product may not be the drug tested in the studies on this page.

PubMed searches, 9 October 2026: 8 records and 4 records, none testing the supplier claims

PubMed search record

PubMed searches run on 9 October 2026: (1) anastrozole[tiab] AND (anabolic steroid*[tiab] OR anabolic-androgenic[tiab] OR anabolic androgenic[tiab] OR bodybuild*[tiab]); (2) (anastrozole[tiab] OR aromatase inhibitor*[tiab]) AND ("water retention"[tiab] OR "fluid retention"[tiab]). 8 and 4 records returned.

The first search returned 8 records: a commentary, a mouse cell-culture study, three drug-detection method papers, a gym survey, and the forum and seized-product analyses described on this page. None tested anastrozole for estrogen control, gynecomastia or side effects in anabolic steroid users. The second search returned 4 records, all reviews or trials in women with breast cancer or with estrogen-deprivation arthralgia; none studied men.

Participants / model
Records returned, not a population
Treatment
Not applicable
Follow-up
Searches run on 9 October 2026
Study design
Literature search
Funding
not stated in the source

A search result shows what PubMed returned on that date. It is not proof that no unindexed study exists.

Read the original source
1,792 forum posts, 2013 to 2019: how aromatase inhibitors are used

Human study

Rochoy M et al. Therapie. 2022;77(6):683-691. PMID 35660259.

Aromatase inhibitors were used mainly to counter aromatization of androgens. The median anastrozole course was 70 days at a mean 0.4 mg a day, obtained mainly online. Users described 62 adverse reactions attributed to aromatase inhibitors as a group.

Participants / model
Men posting on one French bodybuilding forum, mean age 28.2 years
Treatment
Self-sourced oral anastrozole, self-reported doses
Follow-up
Posts from 2013 to 2019
Study design
Observational analysis of 1,792 forum posts
Funding
not stated in the source

This records how the drug is used, not whether it works. Doses and reactions are self-reported.

Read the original source
63 seized products, 2020 to 2024: 34.9% lacked the declared ingredient

Regulatory record

Blazewicz A et al. Front Chem. 2025;13:1536858. PMID 40177353.

Of 63 seized products sold for post-cycle therapy and labeled as SERMs, aromatase inhibitors (anastrozole, letrozole, exemestane) or other agents such as hCG and cabergoline, 34.9% did not contain the declared active ingredient (20.6% contained no active ingredient and 14.3% a different one), and 6.4% contained an extra undeclared ingredient. Per-drug results are not given in the abstract.

Participants / model
Seized products, not a clinical population (63 of 601 samples)
Treatment
Illegal post-cycle therapy products as seized
Follow-up
Products seized 2020 to 2024
Study design
Official laboratory product analysis
Funding
not stated in the source

A gray-market product may not be the drug tested in the studies on this page.

Read the original source

How does anastrozole work, and what does the body do with it?

Anastrozole is an aromatase inhibitor that lowers estradiol. The label reports about 70% lower estradiol within 24 hours and about 80% after 14 days in postmenopausal women with advanced breast cancer, a mean elimination half-life of 50 hours and about 85% hepatic elimination. The falls measured in men were smaller.

Anastrozole is used to block the conversion of androgens to estradiol (aromatization). In postmenopausal women with advanced breast cancer, the label reports estradiol about 70% lower within 24 hours and about 80% lower after 14 days, with suppression lasting up to 6 days after stopping. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals.

Those percentages come from postmenopausal women and should not be transferred to men. In men the measured falls were smaller: from 26 to 17 pg/mL in older men with low testosterone, and 59.9 versus 102.0 pmol/L in healthy men aged 18 to 50.

In the label, peak plasma levels come within 2 hours when fasted, steady state is reached at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic.

ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source
37 older men, 12 weeks: testosterone up, estradiol down, no change in quality of life

Randomized controlled trial

Leder BZ et al. J Clin Endocrinol Metab. 2004;89(3):1174-80. PMID 15001605.

Mean total testosterone rose from 343 to 572 ng/dL on 1 mg daily and from 397 to 520 ng/dL on 1 mg twice weekly. Estradiol fell from 26 to 17 and from 27 to 17 pg/mL and stayed within the normal male range. Quality of life, erectile function (IIEF) and prostate symptom scores did not change, and PSA rose in the twice-weekly group.

Participants / model
37 men aged 62 to 74 with screening testosterone below 350 ng/dL
Treatment
Oral anastrozole 1 mg daily or 1 mg twice weekly, or placebo
Follow-up
12 weeks
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Short hormone study in older men with low or borderline-low testosterone.

Read the original source
17 healthy men, 6 weeks each: estradiol down, insulin sensitivity down

Randomized controlled trial

Gibb FW et al. J Clin Endocrinol Metab. 2016;101(5):2040-6. PMID 26967690.

Anastrozole lowered estradiol (59.9 versus 102.0 pmol/L on placebo), raised total testosterone modestly (25.8 versus 21.4 nmol/L) and lowered the glucose infusion rate during low-dose insulin infusion (12.16 versus 14.15 micromol/kg/min), which the authors interpret as reduced peripheral insulin sensitivity.

Participants / model
17 healthy men aged 18 to 50
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 weeks per arm
Study design
Randomized double-blind crossover trial with hyperinsulinemic euglycemic clamp
Funding
not stated in the source

Healthy men, short term. It measures a laboratory insulin-sensitivity endpoint, not a clinical outcome.

Read the original source
400 healthy men, 16 weeks: estrogen deficiency raised body fat

Randomized controlled trial

Finkelstein JS et al. N Engl J Med. 2013;369(11):1011-22. PMID 24024838.

400 healthy men had their own testosterone and estradiol suppressed with goserelin and were randomized to placebo or testosterone gel; 202 of them also took anastrozole to block conversion to estradiol. Estrogen deficiency mainly accounted for increases in body fat, and androgen and estrogen deficiency both contributed to the decline in sexual function.

Participants / model
400 healthy men aged 20 to 50
Treatment
Goserelin plus testosterone gel (1.25 to 10 g daily) or placebo, with or without oral anastrozole
Follow-up
16 weeks
Study design
Randomized trial
Funding
not stated in the source

Anastrozole was used as a research tool under goserelin, not as a treatment. The trial shows that estradiol has effects in men. It does not test a steroid cycle.

Read the original source
1,792 forum posts, 2013 to 2019: how aromatase inhibitors are used

Human study

Rochoy M et al. Therapie. 2022;77(6):683-691. PMID 35660259.

Aromatase inhibitors were used mainly to counter aromatization of androgens. The median anastrozole course was 70 days at a mean 0.4 mg a day, obtained mainly online. Users described 62 adverse reactions attributed to aromatase inhibitors as a group.

Participants / model
Men posting on one French bodybuilding forum, mean age 28.2 years
Treatment
Self-sourced oral anastrozole, self-reported doses
Follow-up
Posts from 2013 to 2019
Study design
Observational analysis of 1,792 forum posts
Funding
not stated in the source

This records how the drug is used, not whether it works. Doses and reactions are self-reported.

Read the original source

What risks do the label and the male studies report?

The label warns of more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), lower bone mineral density, higher total cholesterol and embryo-fetal toxicity. In men, trials found lower spine bone density and lower insulin sensitivity.

The ARIMIDEX label warns of more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol and embryo-fetal toxicity. These warnings come from women with breast cancer. In the label's trial in 80 boys with pubertal gynecomastia, treatment-related adverse reactions occurred in 16.3% versus 8.1% on placebo.

In men, adverse events were reported in 69% on anastrozole versus 37% on placebo in the 114-man bicalutamide trial. Spine bone mineral density was lower than with placebo after 1 year in 69 older men with low or borderline testosterone, and insulin sensitivity was lower in 17 healthy men. In the 400-man hormone-suppression study, estrogen deficiency contributed to the decline in sexual function.

Users on the French forum described 62 adverse reactions attributed to aromatase inhibitors as a group, not to anastrozole alone. Product identity is a separate concern: 34.9% of 63 seized post-cycle therapy products did not contain the declared active ingredient.

ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source
114 men on bicalutamide: gynecomastia 73% placebo, 10% tamoxifen, 51% anastrozole

Randomized controlled trial

Boccardo F et al. J Clin Oncol. 2005;23(4):808-15. PMID 15681525.

Over 48 weeks, gynecomastia developed in 73% with added placebo, 10% with tamoxifen and 51% with anastrozole (breast pain in 39%, 6% and 27%). The authors concluded that anastrozole did not significantly reduce gynecomastia or breast pain while tamoxifen did. Adverse events were reported in 69% on anastrozole versus 37% on placebo.

Participants / model
114 men with prostate cancer starting bicalutamide monotherapy
Treatment
Oral anastrozole 1 mg, tamoxifen 20 mg or placebo daily, with bicalutamide 150 mg daily
Follow-up
48 weeks
Study design
Randomized trial
Funding
not stated in the source

This is antiandrogen-induced gynecomastia, not gynecomastia from anabolic steroids.

Read the original source
69 men, 1 year: spine bone density lower than on placebo

Randomized controlled trial

Burnett-Bowie SA et al. J Clin Endocrinol Metab. 2009;94(12):4785-92. PMID 19820017.

Mean testosterone rose (319 to 524 ng/dL at month 3) and estradiol fell (15 to 12 pg/mL). Posterior-anterior spine bone mineral density was lower than with placebo (P = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in these men.

Participants / model
69 men aged 60 or older with low or borderline testosterone
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
1 year
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Older men with low or borderline testosterone. No fracture outcome in men was found.

Read the original source
17 healthy men, 6 weeks each: estradiol down, insulin sensitivity down

Randomized controlled trial

Gibb FW et al. J Clin Endocrinol Metab. 2016;101(5):2040-6. PMID 26967690.

Anastrozole lowered estradiol (59.9 versus 102.0 pmol/L on placebo), raised total testosterone modestly (25.8 versus 21.4 nmol/L) and lowered the glucose infusion rate during low-dose insulin infusion (12.16 versus 14.15 micromol/kg/min), which the authors interpret as reduced peripheral insulin sensitivity.

Participants / model
17 healthy men aged 18 to 50
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 weeks per arm
Study design
Randomized double-blind crossover trial with hyperinsulinemic euglycemic clamp
Funding
not stated in the source

Healthy men, short term. It measures a laboratory insulin-sensitivity endpoint, not a clinical outcome.

Read the original source
400 healthy men, 16 weeks: estrogen deficiency raised body fat

Randomized controlled trial

Finkelstein JS et al. N Engl J Med. 2013;369(11):1011-22. PMID 24024838.

400 healthy men had their own testosterone and estradiol suppressed with goserelin and were randomized to placebo or testosterone gel; 202 of them also took anastrozole to block conversion to estradiol. Estrogen deficiency mainly accounted for increases in body fat, and androgen and estrogen deficiency both contributed to the decline in sexual function.

Participants / model
400 healthy men aged 20 to 50
Treatment
Goserelin plus testosterone gel (1.25 to 10 g daily) or placebo, with or without oral anastrozole
Follow-up
16 weeks
Study design
Randomized trial
Funding
not stated in the source

Anastrozole was used as a research tool under goserelin, not as a treatment. The trial shows that estradiol has effects in men. It does not test a steroid cycle.

Read the original source
1,792 forum posts, 2013 to 2019: how aromatase inhibitors are used

Human study

Rochoy M et al. Therapie. 2022;77(6):683-691. PMID 35660259.

Aromatase inhibitors were used mainly to counter aromatization of androgens. The median anastrozole course was 70 days at a mean 0.4 mg a day, obtained mainly online. Users described 62 adverse reactions attributed to aromatase inhibitors as a group.

Participants / model
Men posting on one French bodybuilding forum, mean age 28.2 years
Treatment
Self-sourced oral anastrozole, self-reported doses
Follow-up
Posts from 2013 to 2019
Study design
Observational analysis of 1,792 forum posts
Funding
not stated in the source

This records how the drug is used, not whether it works. Doses and reactions are self-reported.

Read the original source
63 seized products, 2020 to 2024: 34.9% lacked the declared ingredient

Regulatory record

Blazewicz A et al. Front Chem. 2025;13:1536858. PMID 40177353.

Of 63 seized products sold for post-cycle therapy and labeled as SERMs, aromatase inhibitors (anastrozole, letrozole, exemestane) or other agents such as hCG and cabergoline, 34.9% did not contain the declared active ingredient (20.6% contained no active ingredient and 14.3% a different one), and 6.4% contained an extra undeclared ingredient. Per-drug results are not given in the abstract.

Participants / model
Seized products, not a clinical population (63 of 601 samples)
Treatment
Illegal post-cycle therapy products as seized
Follow-up
Products seized 2020 to 2024
Study design
Official laboratory product analysis
Funding
not stated in the source

A gray-market product may not be the drug tested in the studies on this page.

Read the original source

Why does anastrozole sit in A tier?

The A rests on an FDA approval since 1995 and the 9,366-woman ATAC trial for breast cancer. It does not cover the supplier's use in males, where the gynecomastia trials were negative and estrogen control on steroid cycles had no study behind it.

The A applies to the breast cancer use in postmenopausal women. The supplier's use is a separate question. The randomized gynecomastia trials in boys and in men starting bicalutamide were negative, a European guideline recommends against aromatase inhibitors for gynecomastia in general, and the searches run on 9 October 2026 found no study of estrogen control, gynecomastia or water retention in steroid users.

Trials in men also report lower spine bone density and lower insulin sensitivity, and for the doses steroid users report only hormone data were found.

ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source
ATAC: 9,366 postmenopausal women, anastrozole versus tamoxifen, 5 years

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) prescribing information, label version effective 2 March 2026, section 14.1 (ATAC phase 3 trial). Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

ATAC randomized 9,366 postmenopausal women with operable breast cancer to anastrozole, tamoxifen or both for 5 years. At a median 68 months, disease-free survival favored anastrozole over tamoxifen (HR 0.87, 95% CI 0.78 to 0.97; hormone receptor-positive subgroup HR 0.83, 0.73 to 0.94). At a median 120 months the disease-free survival HRs were 0.91 (0.83 to 0.99) and 0.86 (0.78 to 0.95), and overall survival did not differ (HR 0.97, 0.88 to 1.08). The combination arm showed no benefit over tamoxifen and was closed, and the label says tamoxifen should not be given with anastrozole.

Participants / model
9,366 postmenopausal women with operable breast cancer
Treatment
Oral anastrozole 1 mg daily versus tamoxifen 20 mg daily, or both, for 5 years
Follow-up
5 years of treatment; median follow-up 68 and 120 months
Study design
Phase 3 randomized trial, results read from label section 14.1
Funding
not stated in the source

Women only. The result does not apply to men or to steroid-cycle use.

Read the original source
80 boys, 6 months: no benefit over placebo

Randomized controlled trial

Plourde PV et al. J Clin Endocrinol Metab. 2004;89(9):4428-33. PMID 15356042.

Anastrozole did not beat placebo: 38.5% versus 31.4% reached a 50% or greater fall in ultrasound breast volume (odds ratio 1.513, 95% CI 0.496 to 4.844, P = 0.47), even though the median testosterone-to-estradiol ratio rose 166% versus 39%.

Participants / model
80 boys aged 11 to 18 with pubertal gynecomastia that had not shrunk over 3 months
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 months
Study design
Randomized double-blind trial
Funding
not stated in the source

Boys with pubertal gynecomastia. The result does not describe adult men or steroid users.

Read the original source
114 men on bicalutamide: gynecomastia 73% placebo, 10% tamoxifen, 51% anastrozole

Randomized controlled trial

Boccardo F et al. J Clin Oncol. 2005;23(4):808-15. PMID 15681525.

Over 48 weeks, gynecomastia developed in 73% with added placebo, 10% with tamoxifen and 51% with anastrozole (breast pain in 39%, 6% and 27%). The authors concluded that anastrozole did not significantly reduce gynecomastia or breast pain while tamoxifen did. Adverse events were reported in 69% on anastrozole versus 37% on placebo.

Participants / model
114 men with prostate cancer starting bicalutamide monotherapy
Treatment
Oral anastrozole 1 mg, tamoxifen 20 mg or placebo daily, with bicalutamide 150 mg daily
Follow-up
48 weeks
Study design
Randomized trial
Funding
not stated in the source

This is antiandrogen-induced gynecomastia, not gynecomastia from anabolic steroids.

Read the original source
European Academy of Andrology guideline: no SERMs or aromatase inhibitors for gynecomastia in general

Human study

Kanakis GA et al. Andrology. 2019;7(6):778-793. PMID 31099174.

The guideline recommends against using SERMs, aromatase inhibitors or non-aromatizable androgens to treat gynecomastia in general (strong recommendation, low-quality evidence). It recommends watchful waiting after a causative substance is stopped, reserves surgery for long-lasting gynecomastia, and states that more than 90% of pubertal gynecomastia resolves on its own within 24 months.

Participants / model
Boys and men with gynecomastia (guideline scope)
Treatment
Not drug-specific
Follow-up
Not applicable
Study design
Clinical practice guideline (GRADE)
Funding
not stated in the source

A treatment recommendation for gynecomastia in general. It is not specific to anastrozole.

Read the original source
PubMed searches, 9 October 2026: 8 records and 4 records, none testing the supplier claims

PubMed search record

PubMed searches run on 9 October 2026: (1) anastrozole[tiab] AND (anabolic steroid*[tiab] OR anabolic-androgenic[tiab] OR anabolic androgenic[tiab] OR bodybuild*[tiab]); (2) (anastrozole[tiab] OR aromatase inhibitor*[tiab]) AND ("water retention"[tiab] OR "fluid retention"[tiab]). 8 and 4 records returned.

The first search returned 8 records: a commentary, a mouse cell-culture study, three drug-detection method papers, a gym survey, and the forum and seized-product analyses described on this page. None tested anastrozole for estrogen control, gynecomastia or side effects in anabolic steroid users. The second search returned 4 records, all reviews or trials in women with breast cancer or with estrogen-deprivation arthralgia; none studied men.

Participants / model
Records returned, not a population
Treatment
Not applicable
Follow-up
Searches run on 9 October 2026
Study design
Literature search
Funding
not stated in the source

A search result shows what PubMed returned on that date. It is not proof that no unindexed study exists.

Read the original source

Studies and sources

ARIMIDEX label, version effective 2 March 2026: indication, PK, warnings, boys

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) oral tablets, prescribing information, label version effective 2 March 2026. Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

The label approves ARIMIDEX only for hormone receptor-positive (or unknown) breast cancer in postmenopausal women: adjuvant treatment, first-line treatment of advanced disease, and advanced disease after tamoxifen. In postmenopausal women with advanced breast cancer, estradiol fell by about 70% within 24 hours and about 80% after 14 days, and suppression lasted up to 6 days after stopping. Peak plasma levels come within 2 hours when fasted, steady state at about 7 days, the mean elimination half-life is 50 hours, and about 85% of elimination is hepatic. The label says anastrozole has no direct progestogenic, androgenic or estrogenic activity in animals. Warnings cover more ischemic cardiovascular events than with tamoxifen in women with pre-existing ischemic heart disease (17% versus 10% in ATAC), decreases in bone mineral density, increases in total cholesterol, and embryo-fetal toxicity. Section 8.4 states that efficacy was not demonstrated for pubertal gynecomastia: in a placebo-controlled trial of 80 boys aged 11 to 18 given anastrozole for 6 months, the rate of 50% or greater breast reduction did not differ from placebo, estradiol fell 15.4% versus 4.5%, and treatment-related adverse reactions occurred in 16.3% versus 8.1%.

Participants / model
Postmenopausal women with breast cancer; 80 boys aged 11 to 18 with pubertal gynecomastia (section 8.4)
Treatment
ARIMIDEX oral tablets
Follow-up
Label version effective 2 March 2026
Study design
FDA prescribing information
Funding
not stated in the source

The estradiol figures come from postmenopausal women and should not be transferred to men. The label does not describe use in men for estrogen control.

Read the original source
ATAC: 9,366 postmenopausal women, anastrozole versus tamoxifen, 5 years

FDA prescribing information

DailyMed, U.S. National Library of Medicine. ARIMIDEX (anastrozole) prescribing information, label version effective 2 March 2026, section 14.1 (ATAC phase 3 trial). Set ID acbfaaa9-503c-4691-9828-76a7146ed6de.

ATAC randomized 9,366 postmenopausal women with operable breast cancer to anastrozole, tamoxifen or both for 5 years. At a median 68 months, disease-free survival favored anastrozole over tamoxifen (HR 0.87, 95% CI 0.78 to 0.97; hormone receptor-positive subgroup HR 0.83, 0.73 to 0.94). At a median 120 months the disease-free survival HRs were 0.91 (0.83 to 0.99) and 0.86 (0.78 to 0.95), and overall survival did not differ (HR 0.97, 0.88 to 1.08). The combination arm showed no benefit over tamoxifen and was closed, and the label says tamoxifen should not be given with anastrozole.

Participants / model
9,366 postmenopausal women with operable breast cancer
Treatment
Oral anastrozole 1 mg daily versus tamoxifen 20 mg daily, or both, for 5 years
Follow-up
5 years of treatment; median follow-up 68 and 120 months
Study design
Phase 3 randomized trial, results read from label section 14.1
Funding
not stated in the source

Women only. The result does not apply to men or to steroid-cycle use.

Read the original source
Drugs@FDA: ARIMIDEX approved 27 December 1995

Regulatory record

U.S. Food and Drug Administration. Drugs@FDA application record for NDA 020541 (ARIMIDEX). Date read from the openFDA Drugs@FDA record.

FDA approved ARIMIDEX under NDA 020541 on 27 December 1995. The current application holder is ANI Pharmaceuticals.

Participants / model
Not a clinical population
Treatment
Drug application
Follow-up
Approved 1995
Study design
FDA application record
Funding
not stated in the source

This record gives the approval date and holder. Indications come from the product label.

Read the original source
80 boys, 6 months: no benefit over placebo

Randomized controlled trial

Plourde PV et al. J Clin Endocrinol Metab. 2004;89(9):4428-33. PMID 15356042.

Anastrozole did not beat placebo: 38.5% versus 31.4% reached a 50% or greater fall in ultrasound breast volume (odds ratio 1.513, 95% CI 0.496 to 4.844, P = 0.47), even though the median testosterone-to-estradiol ratio rose 166% versus 39%.

Participants / model
80 boys aged 11 to 18 with pubertal gynecomastia that had not shrunk over 3 months
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 months
Study design
Randomized double-blind trial
Funding
not stated in the source

Boys with pubertal gynecomastia. The result does not describe adult men or steroid users.

Read the original source
114 men on bicalutamide: gynecomastia 73% placebo, 10% tamoxifen, 51% anastrozole

Randomized controlled trial

Boccardo F et al. J Clin Oncol. 2005;23(4):808-15. PMID 15681525.

Over 48 weeks, gynecomastia developed in 73% with added placebo, 10% with tamoxifen and 51% with anastrozole (breast pain in 39%, 6% and 27%). The authors concluded that anastrozole did not significantly reduce gynecomastia or breast pain while tamoxifen did. Adverse events were reported in 69% on anastrozole versus 37% on placebo.

Participants / model
114 men with prostate cancer starting bicalutamide monotherapy
Treatment
Oral anastrozole 1 mg, tamoxifen 20 mg or placebo daily, with bicalutamide 150 mg daily
Follow-up
48 weeks
Study design
Randomized trial
Funding
not stated in the source

This is antiandrogen-induced gynecomastia, not gynecomastia from anabolic steroids.

Read the original source
Meta-analysis of 9 randomized trials: tamoxifen RR 0.18, anastrozole no significant benefit

Systematic review and meta-analysis

Spagnolo L et al. J Endocrinol Invest. 2025;48(7):1659-1668. PMID 40244528.

Tamoxifen lowered the risk of gynecomastia (RR 0.18, 95% CI 0.08 to 0.38) and breast pain (RR 0.18, 95% CI 0.07 to 0.43). Radiotherapy also lowered both. Anastrozole showed no significant benefit.

Participants / model
Men with prostate cancer on bicalutamide, 9 randomized trials
Treatment
Oral tamoxifen, oral anastrozole or breast radiotherapy versus bicalutamide alone or with placebo or sham
Follow-up
Pooled randomized trials
Study design
Systematic review and meta-analysis of 9 randomized trials
Funding
not stated in the source

Applies to men on bicalutamide.

Read the original source
European Academy of Andrology guideline: no SERMs or aromatase inhibitors for gynecomastia in general

Human study

Kanakis GA et al. Andrology. 2019;7(6):778-793. PMID 31099174.

The guideline recommends against using SERMs, aromatase inhibitors or non-aromatizable androgens to treat gynecomastia in general (strong recommendation, low-quality evidence). It recommends watchful waiting after a causative substance is stopped, reserves surgery for long-lasting gynecomastia, and states that more than 90% of pubertal gynecomastia resolves on its own within 24 months.

Participants / model
Boys and men with gynecomastia (guideline scope)
Treatment
Not drug-specific
Follow-up
Not applicable
Study design
Clinical practice guideline (GRADE)
Funding
not stated in the source

A treatment recommendation for gynecomastia in general. It is not specific to anastrozole.

Read the original source
37 older men, 12 weeks: testosterone up, estradiol down, no change in quality of life

Randomized controlled trial

Leder BZ et al. J Clin Endocrinol Metab. 2004;89(3):1174-80. PMID 15001605.

Mean total testosterone rose from 343 to 572 ng/dL on 1 mg daily and from 397 to 520 ng/dL on 1 mg twice weekly. Estradiol fell from 26 to 17 and from 27 to 17 pg/mL and stayed within the normal male range. Quality of life, erectile function (IIEF) and prostate symptom scores did not change, and PSA rose in the twice-weekly group.

Participants / model
37 men aged 62 to 74 with screening testosterone below 350 ng/dL
Treatment
Oral anastrozole 1 mg daily or 1 mg twice weekly, or placebo
Follow-up
12 weeks
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Short hormone study in older men with low or borderline-low testosterone.

Read the original source
69 men, 1 year: spine bone density lower than on placebo

Randomized controlled trial

Burnett-Bowie SA et al. J Clin Endocrinol Metab. 2009;94(12):4785-92. PMID 19820017.

Mean testosterone rose (319 to 524 ng/dL at month 3) and estradiol fell (15 to 12 pg/mL). Posterior-anterior spine bone mineral density was lower than with placebo (P = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in these men.

Participants / model
69 men aged 60 or older with low or borderline testosterone
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
1 year
Study design
Randomized placebo-controlled trial
Funding
not stated in the source

Older men with low or borderline testosterone. No fracture outcome in men was found.

Read the original source
17 healthy men, 6 weeks each: estradiol down, insulin sensitivity down

Randomized controlled trial

Gibb FW et al. J Clin Endocrinol Metab. 2016;101(5):2040-6. PMID 26967690.

Anastrozole lowered estradiol (59.9 versus 102.0 pmol/L on placebo), raised total testosterone modestly (25.8 versus 21.4 nmol/L) and lowered the glucose infusion rate during low-dose insulin infusion (12.16 versus 14.15 micromol/kg/min), which the authors interpret as reduced peripheral insulin sensitivity.

Participants / model
17 healthy men aged 18 to 50
Treatment
Oral anastrozole 1 mg daily or placebo
Follow-up
6 weeks per arm
Study design
Randomized double-blind crossover trial with hyperinsulinemic euglycemic clamp
Funding
not stated in the source

Healthy men, short term. It measures a laboratory insulin-sensitivity endpoint, not a clinical outcome.

Read the original source
400 healthy men, 16 weeks: estrogen deficiency raised body fat

Randomized controlled trial

Finkelstein JS et al. N Engl J Med. 2013;369(11):1011-22. PMID 24024838.

400 healthy men had their own testosterone and estradiol suppressed with goserelin and were randomized to placebo or testosterone gel; 202 of them also took anastrozole to block conversion to estradiol. Estrogen deficiency mainly accounted for increases in body fat, and androgen and estrogen deficiency both contributed to the decline in sexual function.

Participants / model
400 healthy men aged 20 to 50
Treatment
Goserelin plus testosterone gel (1.25 to 10 g daily) or placebo, with or without oral anastrozole
Follow-up
16 weeks
Study design
Randomized trial
Funding
not stated in the source

Anastrozole was used as a research tool under goserelin, not as a treatment. The trial shows that estradiol has effects in men. It does not test a steroid cycle.

Read the original source
1,792 forum posts, 2013 to 2019: how aromatase inhibitors are used

Human study

Rochoy M et al. Therapie. 2022;77(6):683-691. PMID 35660259.

Aromatase inhibitors were used mainly to counter aromatization of androgens. The median anastrozole course was 70 days at a mean 0.4 mg a day, obtained mainly online. Users described 62 adverse reactions attributed to aromatase inhibitors as a group.

Participants / model
Men posting on one French bodybuilding forum, mean age 28.2 years
Treatment
Self-sourced oral anastrozole, self-reported doses
Follow-up
Posts from 2013 to 2019
Study design
Observational analysis of 1,792 forum posts
Funding
not stated in the source

This records how the drug is used, not whether it works. Doses and reactions are self-reported.

Read the original source
63 seized products, 2020 to 2024: 34.9% lacked the declared ingredient

Regulatory record

Blazewicz A et al. Front Chem. 2025;13:1536858. PMID 40177353.

Of 63 seized products sold for post-cycle therapy and labeled as SERMs, aromatase inhibitors (anastrozole, letrozole, exemestane) or other agents such as hCG and cabergoline, 34.9% did not contain the declared active ingredient (20.6% contained no active ingredient and 14.3% a different one), and 6.4% contained an extra undeclared ingredient. Per-drug results are not given in the abstract.

Participants / model
Seized products, not a clinical population (63 of 601 samples)
Treatment
Illegal post-cycle therapy products as seized
Follow-up
Products seized 2020 to 2024
Study design
Official laboratory product analysis
Funding
not stated in the source

A gray-market product may not be the drug tested in the studies on this page.

Read the original source
PubChem: anastrozole identity

Chemical identity record

National Library of Medicine. PubChem compound record CID 2187 (anastrozole).

Anastrozole: PubChem CID 2187, C17H19N5, 293.4 g/mol, CAS 120511-73-1. The label's active ingredient is anastrozole itself, not a salt, so label doses are anastrozole.

Participants / model
Chemical identity, not a clinical population
Treatment
Defined molecule
Follow-up
Not applicable
Study design
NLM chemical record
Funding
not stated in the source

Identity record only; it says nothing about efficacy or safety.

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Why is Anastrozole in A tier?

A fits an approved drug promoted for an off-label use. Anastrozole has been FDA-approved since 1995 and has a 9,366-woman phase 3 trial, but that grade belongs to the breast cancer use. In males the randomized gynecomastia trials were negative, and the steroid-cycle estrogen-control and water-retention claims had no study behind them in the searches run on 9 October 2026.