Anastrozole, sold as Arimidex, is an oral drug that blocks aromatase, the enzyme that turns androgens such as testosterone and androstenedione into estrogens. The FDA approved it in December 1995. Its approved uses are all in breast cancer: it treats hormone receptor-positive breast cancer in postmenopausal women.
Outside oncology, anastrozole is also used off-label in men, including by people taking testosterone or other anabolic-androgenic steroids (AAS), to lower estradiol. It has no FDA-approved use in men. The controlled trials in men are small, and they show a trade-off: testosterone goes up, but estradiol, which men need for bone and fat regulation, goes down.
This page summarizes what the FDA label, clinical trials and anti-doping rules report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Drug class | Nonsteroidal (triazole) aromatase inhibitor |
| Brand / approval | Arimidex, NDA 020541; FDA approved December 27, 1995; generics available |
| Route | Oral tablet |
| Strength and labeled dose | 1 mg tablet; 1 mg once daily |
| Half-life (label) | About 50 h; near steady state after about 7 days of daily dosing |
| Estradiol suppression (label) | About 70% within 24 h and about 80% after 14 days at 1 mg/day, in postmenopausal women |
| Approved use | Hormone receptor-positive breast cancer in postmenopausal women |
| US status | Prescription only; not a controlled substance |
| WADA status | Prohibited at all times (class S4.1, aromatase inhibitors) |
What it is
Anastrozole’s formula is C₁₇H₁₉N₅, with a molecular weight of 293.4 g/mol (FDA label; PubChem). It is not a steroid; the label describes it as a selective, nonsteroidal aromatase inhibitor.
Aromatase converts androgens into estrogens:
- androstenedione becomes estrone;
- testosterone becomes estradiol.
In postmenopausal women, estrogens come mainly from this conversion of adrenal androgens in tissues outside the ovaries. Blocking aromatase therefore lowers estrogen levels sharply. The label notes that anastrozole has no detectable effect on cortisol or aldosterone production, and no direct estrogenic, androgenic or progestogenic activity in animals.
Related aromatase inhibitors include letrozole, also nonsteroidal, and exemestane. Anastrozole works differently from tamoxifen, a selective estrogen receptor modulator (SERM) that acts at the estrogen receptor rather than lowering estrogen production.
How strongly it blocks aromatase
Isotope studies in postmenopausal women with breast cancer measured total-body aromatization (the share of androgen converted to estrogen) before and during treatment:
| Study | Design | Aromatization inhibited | Fall in plasma estradiol |
|---|---|---|---|
| Geisler et al., Br J Cancer 1996 | 12 women, 1 mg vs 10 mg/day, 28-day crossover | 96.7% (1 mg); 98.1% (10 mg) | At least 83.5%, same at either dose |
| Geisler et al., J Clin Oncol 2002 | 12 women, anastrozole 1 mg vs letrozole 2.5 mg, 6 weeks each | 97.3% (anastrozole); over 99.1% (letrozole) | 84.9% (anastrozole); 87.8% (letrozole) |
Two points follow. First, 10 mg did not block aromatase meaningfully more than 1 mg. Second, letrozole at its labeled dose suppressed estrogen slightly more than anastrozole at its labeled dose.
Pharmacokinetics
From the FDA label:
| Parameter | Value |
|---|---|
| Time to peak (fasted) | Within about 2 h |
| Effect of food | Slows absorption (peak delayed from 2 to 5 h, peak level 16% lower) but not the total amount absorbed |
| Dose linearity | Linear from 1 to 20 mg |
| Steady state | About 7 days; levels 3–4× those after a single dose |
| Plasma protein binding | 40% |
| Metabolism | Liver (≈85% of elimination): N-dealkylation, hydroxylation, glucuronidation; main metabolite (triazole) is inactive |
| Renal elimination | ≈10% of total clearance |
| Mean elimination half-life | 50 h (46.8 h in a pediatric population analysis) |
According to the label, estradiol stays suppressed for up to 6 days after the last 1 mg dose.
No dose change is needed for kidney impairment, for mild to moderate liver impairment, or in older adults. The drug has not been studied in severe liver impairment.
Approved medical use
The label approves anastrozole 1 mg once daily, with or without food, for postmenopausal women with:
- hormone receptor-positive early breast cancer, as adjuvant treatment after surgery;
- hormone receptor-positive or receptor-unknown locally advanced or metastatic breast cancer, as first-line treatment;
- advanced breast cancer that has progressed on tamoxifen.
The label also notes that anastrozole would not be expected to lower estradiol in premenopausal women, because their estrogen comes mainly from the ovaries. Its effect in that group has not been studied.
There is no labeled use in men or children. The label states that efficacy was not demonstrated in two pediatric trials (pubertal gynecomastia in boys and McCune-Albright syndrome in girls).
What the research shows
Early breast cancer: the ATAC trial
ATAC (Arimidex, Tamoxifen, Alone or in Combination) was a double-blind trial in 9,366 postmenopausal women with operable breast cancer. They took anastrozole 1 mg, tamoxifen 20 mg, or both, daily for 5 years. The combination arm was stopped early because it did no better than tamoxifen alone.
Results for anastrozole vs tamoxifen alone (hazard ratio below 1 favors anastrozole), from the FDA label:
| Outcome | 68-month follow-up | 10-year follow-up |
|---|---|---|
| Disease-free survival, all patients | HR 0.87 (95% CI 0.78–0.97) | HR 0.91 (95% CI 0.83–0.99) |
| Disease-free survival, hormone receptor-positive | HR 0.83 (95% CI 0.73–0.94) | HR 0.86 (95% CI 0.78–0.95) |
| Overall survival, all patients | HR 0.97 (95% CI 0.85–1.12) | HR 0.97 (95% CI 0.88–1.08) |
In the 10-year analysis, recurrence in receptor-positive patients stayed lower on anastrozole after treatment ended (HR 0.81, 95% CI 0.67–0.98), and the absolute gap in time to recurrence in receptor-positive patients grew from 2.7% at 5 years to 4.3% at 10 years (Cuzick et al., Lancet Oncol 2010). Overall survival did not differ significantly between the two drugs.
Breast cancer prevention: IBIS-II
IBIS-II randomized 3,864 postmenopausal women aged 40–70 at increased risk of breast cancer to anastrozole 1 mg/day or placebo for 5 years (Cuzick et al., Lancet 2014; Cuzick et al., Lancet 2020):
| Outcome | Anastrozole | Placebo | Effect |
|---|---|---|---|
| Breast cancer, median 5.0 years | 40 (2%) | 85 (4%) | HR 0.47 (95% CI 0.32–0.68) |
| Breast cancer, median 131 months | 85 cases | 165 cases | HR 0.51 (95% CI 0.39–0.66) |
| Deaths, median 131 months | 69 | 70 | HR 0.96 (95% CI 0.69–1.34) |
The long-term follow-up found no excess of fractures or cardiovascular disease. Prevention is not one of the indications on the US label.
Men: testosterone and estradiol
Because estradiol feeds back on the pituitary, blocking aromatase in men raises LH (luteinizing hormone) and the testes make more testosterone. Three randomized trials in men show what that looks like:
| Trial | Men studied | Regimen | Testosterone | Estradiol |
|---|---|---|---|---|
| Leder et al., JCEM 2004 | 37 men aged 62–74, testosterone under 350 ng/dL; 12 weeks | 1 mg daily | 343 → 572 ng/dL | 26 → 17 pg/mL |
| Same trial | Same | 1 mg twice weekly | 397 → 520 ng/dL | 27 → 17 pg/mL |
| Burnett-Bowie et al., JCEM 2009 | 69 men aged 60+, low or borderline testosterone; 1 year vs placebo | 1 mg daily | 319 → 524 ng/dL at 3 months; 474 at 1 year | 15 → 12 pg/mL |
| Mauras et al., JCEM 2000 | 8 males aged 15–22; 10 weeks, no control group | 1 mg daily | +58% | −48% |
In the Leder trial, quality-of-life scores, erectile function scores and hematocrit did not change over 12 weeks. PSA rose slightly in the twice-weekly group (1.7 to 2.2 ng/mL).
What the hormone rise did not do is notable. In the Mauras study, 10 weeks of estrogen suppression produced no significant change in fat mass, fat-free mass, protein synthesis or muscle strength, and IGF-1 fell 18%. In the year-long Burnett-Bowie trial, spine bone density fell on anastrozole while it rose slightly on placebo (p = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in older men.
What estradiol does in men
The clearest evidence comes from a trial that used anastrozole as a research tool. Investigators shut down hormone production in 400 healthy men aged 20–50 with goserelin, then gave graded doses of testosterone gel for 16 weeks, with or without anastrozole to block conversion to estradiol (Finkelstein et al., NEJM 2013). They found:
- low testosterone accounted for losses of lean mass, muscle size and strength;
- low estrogen primarily accounted for increases in body fat;
- both contributed to the decline in sexual function.
In other words, estradiol is not a waste product in men. Driving it too low has its own costs.
Gynecomastia
Anastrozole is often assumed to treat or prevent gynecomastia (male breast tissue growth). Two randomized trials did not support this:
| Trial | Population | Result |
|---|---|---|
| Plourde et al., JCEM 2004 | 80 boys aged 11–18 with pubertal gynecomastia; 1 mg/day vs placebo for 6 months | 50% or more reduction in breast volume in 38.5% on anastrozole vs 31.4% on placebo (p = 0.47) |
| Boccardo et al., J Clin Oncol 2005 | 114 men with prostate cancer on bicalutamide 150 mg; 48 weeks | Gynecomastia in 73% (placebo), 51% (anastrozole 1 mg), 10% (tamoxifen 20 mg) |
In the prostate cancer trial, the authors concluded that anastrozole did not significantly reduce gynecomastia, while tamoxifen did. Adverse events were reported in 69% of the anastrozole group vs 37% on bicalutamide alone.
About “anastrozole is needed on testosterone.” This idea is common in online advice, but no randomized trial shows that adding anastrozole to testosterone improves outcomes in men. The trials above show that it lowers estradiol, which in men is linked to bone loss and fat gain, and that it did not prevent gynecomastia in the two settings tested.
Male infertility
In an uncontrolled series of 140 subfertile men with a low testosterone-to-estradiol ratio, anastrozole 1 mg/day raised that ratio from 7.2 to 18.1. In 25 men with low sperm counts, sperm concentration rose from 5.5 to 15.6 million/mL (Raman and Schlegel, J Urol 2002). There was no placebo group. A 2024 network meta-analysis of 14 randomized trials concluded that the evidence is insufficient to support routine use of aromatase inhibitors, clomiphene or tamoxifen to improve semen parameters (Al Wattar et al., BMC Urol 2024).
Height in boys
In a randomized trial of 52 adolescent boys with growth hormone deficiency, anastrozole added to growth hormone slowed bone-age advancement. Predicted adult height rose by 6.7 cm at 36 months vs 1 cm on placebo (Mauras et al., JCEM 2008). Predicted height is not final height, and this is not a labeled use.
Side effects and risks
Adverse effects in the ATAC trial
The label lists pre-specified adverse events in 3,092 women on anastrozole and 3,094 on tamoxifen, over a median of about 5 years of treatment (FDA label):
| Event | Anastrozole | Tamoxifen | Odds ratio (95% CI) |
|---|---|---|---|
| Hot flashes | 36% | 41% | 0.80 (0.73–0.89) |
| Joint symptoms (arthritis, arthralgia, arthrosis) | 36% | 29% | 1.32 (1.19–1.47) |
| All fractures | 10% | 7% | 1.57 (1.30–1.88) |
| Spine, hip or wrist fractures | 4% | 3% | 1.48 (1.13–1.95) |
| High cholesterol (reported) | 9% | 3.5% | — |
| Ischemic cardiovascular disease | 4% | 3% | 1.23 (0.95–1.60) |
| Venous thromboembolism (blood clots) | 3% | 5% | 0.61 (0.47–0.80) |
| Vaginal bleeding | 5% | 10% | 0.50 (0.41–0.61) |
| Endometrial cancer (women with a uterus) | 0.2% | 0.6% | 0.31 (0.10–0.94) |
Carpal tunnel syndrome was also more common on anastrozole (2.5% vs 0.7%). The overall pattern reflects low estrogen: more joint pain and fractures, but fewer clots and uterine problems than tamoxifen.
Bone loss
In the ATAC bone substudy, women on anastrozole lost bone density at the spine and hip over 2 years, while women on tamoxifen gained. The label advises considering bone density monitoring. At 10 years, fractures were more frequent during active treatment (451 vs 351; OR 1.33, 95% CI 1.15–1.55) but similar after treatment ended (Cuzick et al., Lancet Oncol 2010). In older men, a year of anastrozole also reduced spine bone density (Burnett-Bowie et al., JCEM 2009).
Heart and cholesterol
In the overall ATAC population, ischemic cardiovascular events did not differ significantly between drugs. In women who already had ischemic heart disease (7.5% of the trial), they occurred in 17% on anastrozole vs 10% on tamoxifen. The label tells prescribers to weigh this risk, and to consider cholesterol monitoring.
Rare serious reactions
The label lists reactions occurring in fewer than 1 in 10,000 patients:
- skin reactions such as lesions, ulcers or blisters;
- allergic swelling of the face, lips, tongue or throat (angioedema), and anaphylaxis;
- liver test changes, including hepatitis.
Pregnancy
Based on animal studies and its mechanism, anastrozole can cause fetal harm. In rats and rabbits it increased pregnancy loss. The label advises effective contraception during treatment and for at least 3 weeks after the last dose.
Interactions
- Tamoxifen: do not combine. Tamoxifen lowered anastrozole levels by 27%, and the combination gave no benefit over tamoxifen alone in ATAC.
- Estrogen-containing products: may reduce anastrozole’s effect and should not be used with it.
- Warfarin and CYP450 drugs: in a study of 16 male volunteers, anastrozole did not change warfarin levels or clotting tests. The label considers CYP450-based interactions unlikely at 1 mg.
Who should not use it
The only labeled contraindication is a prior hypersensitivity reaction to anastrozole or any tablet ingredient.
Overdose
Single doses up to 60 mg in healthy men and 10 mg daily in women with advanced breast cancer were tolerated in trials. There is no specific antidote.
Legal status and use outside medicine
In the US, anastrozole is a prescription drug and is not a controlled substance (DailyMed).
The World Anti-Doping Agency lists aromatase inhibitors, with anastrozole named, under S4.1 of its 2026 Prohibited List. They are banned in and out of competition, and S4.1 substances are classed as “specified substances” (NADA Germany, official translation of the 2026 WADA list).
Key takeaways
- A breast cancer drug. Anastrozole is approved only for hormone receptor-positive breast cancer in postmenopausal women, at 1 mg once daily.
- Potent and long-acting. At 1 mg/day it blocks about 97% of total-body aromatization, and its 50-hour half-life means estradiol stays suppressed for days after a dose.
- Raises testosterone in men, but lowers estradiol too. Trials in older men showed testosterone rising by about 120–230 ng/dL. A small study found no gain in muscle or strength, and a year-long trial found bone loss.
- Estradiol matters in men. Suppressing it was linked to more body fat and worse sexual function, and to bone density loss over a year.
- Not proven for gynecomastia. It failed in randomized trials in pubertal boys and in men on bicalutamide, where tamoxifen worked.
- Main risks are joints, bone and lipids. In ATAC, joint symptoms, fractures and high cholesterol were more common than with tamoxifen.
Sources
- ANI Pharmaceuticals. Arimidex (anastrozole) tablets, prescribing information. DailyMed; Drugs@FDA, NDA 020541.
- National Center for Biotechnology Information. Anastrozole, PubChem CID 2187. PubChem.
- Geisler J, King N, Dowsett M, et al. Influence of anastrozole (Arimidex), a selective, non-steroidal aromatase inhibitor, on in vivo aromatisation and plasma oestrogen levels in postmenopausal women with breast cancer. Br J Cancer. 1996;74(8):1286–1291. PubMed.
- Geisler J, Haynes B, Anker G, Dowsett M, Lønning PE. Influence of letrozole and anastrozole on total body aromatization and plasma estrogen levels in postmenopausal breast cancer patients evaluated in a randomized, cross-over study. J Clin Oncol. 2002;20(3):751–757. PubMed.
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- Mauras N, O’Brien KO, Klein KO, Hayes V. Estrogen suppression in males: metabolic effects. J Clin Endocrinol Metab. 2000;85(7):2370–2377. PubMed.
- Finkelstein JS, Lee H, Burnett-Bowie SA, et al. Gonadal steroids and body composition, strength, and sexual function in men. N Engl J Med. 2013;369(11):1011–1022. PubMed.
- Plourde PV, Reiter EO, Jou HC, et al. Safety and efficacy of anastrozole for the treatment of pubertal gynecomastia: a randomized, double-blind, placebo-controlled trial. J Clin Endocrinol Metab. 2004;89(9):4428–4433. PubMed.
- Boccardo F, Rubagotti A, Battaglia M, et al. Evaluation of tamoxifen and anastrozole in the prevention of gynecomastia and breast pain induced by bicalutamide monotherapy of prostate cancer. J Clin Oncol. 2005;23(4):808–815. PubMed.
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- Al Wattar BH, Rimmer MP, Teh JJ, et al. Pharmacological non-hormonal treatment options for male infertility: a systematic review and network meta-analysis. BMC Urol. 2024;24(1):158. PubMed.
- Mauras N, Gonzalez de Pijem L, Hsiang HY, et al. Anastrozole increases predicted adult height of short adolescent males treated with growth hormone: a randomized, placebo-controlled, multicenter trial for one to three years. J Clin Endocrinol Metab. 2008;93(3):823–831. PubMed.
- Nationale Anti Doping Agentur Deutschland. WADA Prohibited List 2026, informational German translation. NADA.