Short answer: they solve different problems, and for gynecomastia tamoxifen has far better evidence. Anastrozole (Arimidex) blocks the aromatase enzyme, so the body makes less estradiol. Tamoxifen (Nolvadex) leaves estradiol in place but blocks it at the estrogen receptor in breast tissue and the brain. In a randomized trial in men taking the anti-androgen bicalutamide, gynecomastia occurred in 10% on tamoxifen, 51% on anastrozole and 73% on placebo. Lowering estrogen also has costs in men: in older men, a year of anastrozole lowered spine bone density.
Anastrozole vs tamoxifen at a glance
| Anastrozole | Tamoxifen | |
|---|---|---|
| What it is | Non-steroidal aromatase inhibitor (AI) | Selective estrogen receptor modulator (SERM) |
| Approval | Approved (FDA 1995, Arimidex) for breast cancer in postmenopausal women | Approved (FDA 1977, Nolvadex) for breast cancer in women and men |
| Human studies (counted in our records) | 396 | 2342 |
| Half-life | ~50 h | ~5–7 days (tamoxifen); ~14 days (N-desmethyltamoxifen, main active metabolite) |
| Testosterone suppression score | 0/3 | 0/3 |
| Liver risk score | 1/3 | 1/3 |
| Evidence score | 2/3 | 2/3 |
| WADA | Prohibited at all times (in and out of competition) (S4.1 Aromatase inhibitors) | Prohibited at all times (in and out of competition) (S4.2 Anti-estrogenic substances) |
| How it handles estrogen | Lowers estradiol production (aromatase inhibitor) | Blocks estrogen receptors in breast and brain (SERM) |
| Gynecomastia prevention (men on bicalutamide) | 51% vs 73% placebo, not significant | 10% vs 73% placebo |
| SwissChems product | Anastrozole (0.5mg/capsule), 60 Capsules | Tamoxifen (20mg/capsule), 60 Capsules |
Scores run from 0 (none) to 3 (high); see our methodology. Data from the SARMs database.
Source: Sarmguide compound database; Anastrozole value: ARIMIDEX (anastrozole) tablets – prescribing information (DailyMed, ANI) (2026).
How they work in men
In men, some testosterone is converted to estradiol by aromatase. Estradiol does useful things (bone, fat regulation, libido) and also feeds back to the brain to limit LH. Anastrozole lowers estradiol, so LH and testosterone rise: in men aged 62–74, testosterone rose from 343 to 572 ng/dL in 12 weeks while estradiol fell from about 26 to 17 pg/mL. Tamoxifen blocks the receptor instead: LH and testosterone rise, estradiol stays, and breast tissue is protected.
Gynecomastia: the key difference
The best data come from men with prostate cancer on bicalutamide, which causes gynecomastia in most men. In a 48-week randomized trial of 114 men, gynecomastia occurred in 10% on tamoxifen 20 mg/day, 51% on anastrozole 1 mg/day and 73% on placebo; breast pain in 6%, 27% and 39%. A 2025 meta-analysis of nine randomized trials found tamoxifen cut the risk of gynecomastia and breast pain by about 82%, while anastrozole showed no significant benefit. In boys with pubertal gynecomastia, an 80-boy trial found anastrozole no better than placebo. These were not SARM users, but they are the best evidence for the question.
Effects on testosterone, body composition and bone
| Outcome | Anastrozole | Tamoxifen |
|---|---|---|
| Testosterone | Rose in older men (e.g. 319 to 524 ng/dL at 3 months) | Rose about 40% in healthy men; SHBG rose too |
| Body composition and strength | No change after 1 year in older men | Not studied for this |
| Bone | Spine bone density fell over 1 year; testosterone gel improved it, anastrozole did not | Not a concern in male data reviewed |
| Metabolic | Insulin sensitivity fell in healthy men (6 weeks) | IGF-1 fell 25% in healthy men |
| Fertility | Improved sperm counts in a subset of subfertile men with low T/E2 ratio | Raised gonadotropins in subfertile men |
A large study of 400 healthy men whose own hormones were replaced in controlled amounts showed what low estrogen does: estrogen deficiency mainly increased body fat, and both low testosterone and low estrogen lowered sexual function. That is the main argument against driving estradiol too low.
Side effects
| Anastrozole | Tamoxifen |
|---|---|
| Bone loss (men and women); more fractures than tamoxifen in women’s breast cancer trials | Blood clots and stroke (label warning) |
| Joint pain, hot flashes, raised cholesterol (women) | Loss of libido and impotence in some men |
| Lower insulin sensitivity (healthy men) | Hot flushes; liver enzyme changes; eye changes |
| Acne and headache (boys) | Fewer than 5% of men stopped for side effects in a review |
After SARMs
SARMs do not convert to estrogen, so estrogen-driven gyno is less likely than with testosterone; it is reported mainly with mixed products or ones containing undeclared hormones. That makes routine estrogen blocking on SARMs hard to justify, and low estrogen has its own costs. Neither drug has been tested in SARM users. See SARMs PCT, SARMs bloodwork and estrogen blockers for men.
Legal status and sport
Both are prescription medicines for breast cancer (Arimidex approved 1995, Nolvadex 1977); use in men for estrogen control is off-label. WADA bans aromatase inhibitors under S4.1 and SERMs under S4.2, at all times.
Verdict
If the goal is preventing or treating gynecomastia, tamoxifen has the evidence and anastrozole does not. If the goal is lowering estradiol itself, anastrozole does that, but in men it brought no body-composition or strength benefit and cost spine bone density over a year. Related: Nolvadex vs Clomid · raloxifene for gyno · MK-677 gyno
Frequently asked questions
Is anastrozole or tamoxifen better for gyno?
TamoxifenEvidence: Trial
In men on bicalutamide, gynecomastia occurred in 10% on tamoxifen vs 51% on anastrozole and 73% on placebo. A 2025 meta-analysis found no significant benefit for anastrozole.
What is the difference between Arimidex and Nolvadex?
Lower estrogen vs block itEvidence: Human data
Arimidex (anastrozole) blocks the aromatase enzyme, so the body makes less estradiol. Nolvadex (tamoxifen) leaves estradiol in place but blocks it at the receptor in breast tissue and the brain.
Does anastrozole raise testosterone?
YesEvidence: Trial
In older men, testosterone rose from 343 to 572 ng/dL in 12 weeks. Body composition and strength did not improve over a year.
Is it bad to lower estrogen too much in men?
YesEvidence: Trial
In trials, very low estrogen raised body fat, lowered sexual function and reduced insulin sensitivity. A year of anastrozole lowered spine bone density in older men.
Can you take anastrozole and tamoxifen together?
Not studied togetherEvidence: Limited
There is no trial of the combination in men outside cancer settings. Lowering estrogen while also blocking its receptor adds the costs of low estrogen without proven benefit.
Do you need an estrogen blocker on SARMs?
Hard to justifyEvidence: Limited
SARMs do not convert to estrogen, so routine estrogen blocking is hard to justify, and low estrogen has its own costs. Gyno with SARMs is reported mainly with mixed products or ones containing undeclared hormones.
Are anastrozole and tamoxifen banned in sport?
Yes, banned by WADAEvidence: Law
WADA bans aromatase inhibitors (S4.1) and SERMs (S4.2) at all times. Both are also prescription drugs.
Sources
- Evaluation of tamoxifen and anastrozole in the prevention of gynecomastia and breast pain induced by bicalutamide monotherapy of prostate cancer
- Prevention of bicalutamide-induced breast events in patients with prostate cancer: a meta-analysis of randomized controlled trials
- Pharmacokinetics and pharmacodynamics of anastrozole in pubertal boys with recent-onset gynecomastia
- Effects of aromatase inhibition in elderly men with low or borderline-low serum testosterone levels
- Effects of aromatase inhibition in hypogonadal older men: a randomized, double-blind, placebo-controlled trial
- Effects of aromatase inhibition on bone mineral density and bone turnover in older men with low testosterone levels
- Effects of aromatase inhibition vs. testosterone in older men with low testosterone: randomized-controlled trial
- Aromatase Inhibition Reduces Insulin Sensitivity in Healthy Men
- Gonadal steroids and body composition, strength, and sexual function in men
- Outcomes of anastrozole in oligozoospermic hypoandrogenic subfertile men
- Neuroendocrine regulation of growth hormone and androgen axes by selective estrogen receptor modulators in healthy men
- Tamoxifen in men: a review of adverse events
- ARIMIDEX (anastrozole) tablets – prescribing information (DailyMed, ANI)
- Tamoxifen citrate tablets – prescribing information (DailyMed, Mylan)
- Reversible Gynecomastia and Hypogonadism Due to Usage of Commercial Performance-Enhancing Supplement Use
- World Anti-Doping Code International Standard: Prohibited List 2026
