Selective estrogen receptor modulator (SERM)
ATC code: L02BA01 (Tamoxifen)
Brand names
Nolvadex, Soltamox, Tamoxen
A selective estrogen receptor modulator: antagonist in breast tissue, partial agonist in endometrium, bone, and liver. Competitively binds estrogen receptors in breast tumor cells, blocking proliferative signaling. CYP2D6-metabolized to active endoxifen with 30–100-fold higher estrogen receptor affinity. In CYP2D6 poor metabolizers (~7% of Caucasians), efficacy is reduced.
First line
Drug of choice for adjuvant endocrine therapy of hormone-receptor positive breast cancer in premenopausal women per ESMO 2024 and NCCN 2024. Dose 20 mg once daily for 5–10 years (ATLAS trial showed 10-year benefit over 5 in high-risk patients). Also used for chemoprevention in women at high breast cancer risk (BRCA carriers, LCIS, DCIS).
Drug of choice in premenopausal women and an alternative in postmenopausal patients.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Tamoxifen is used by amateur bodybuilders and AAS users in post-cycle therapy to block estrogen receptors and restore endogenous testosterone production. ASCO 2022 and the AEMPS Ficha Técnica confine tamoxifen to hormone-receptor-positive breast cancer and risk reduction in high-risk women. Approved indications in male athletes do not exist. Use carries venous thromboembolism, endometrial cancer ( boxed warning) and thromboembolic stroke risk.
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Boxed warning
FDA boxed warning: increased risk of endometrial cancer, stroke, and venous thromboembolism. In women with risk factors, decisions are individualized.
FDA category D. Teratogen (spontaneous abortion has been described). Non-hormonal contraception during therapy and for 2 months after. When planning pregnancy, stop tamoxifen at least 2 months before conception.
Contraindicated during lactation – suppresses milk production.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Tamoxifen is evaluated for the following indications with varying evidence strength: ER-positive breast cancer (evidence tier A), Athletic endurance (evidence tier F). See the full indication matrix with dosing and citations above on this page.
Common side effects of Tamoxifen (≥ 1 in 100): Hot flashes and night sweats, Vaginal bleeding and discharge, Weight gain, Fatigue, Nausea, Reduced libido. See the Safety section for uncommon and serious reactions.
FDA category D. FDA category D. Teratogen (spontaneous abortion has been described). Non-hormonal contraception during therapy and for 2 months after. When planning pregnancy, stop tamoxifen at least 2 months before conception.
Contraindicated during lactation – suppresses milk production.
Tamoxifen is contraindicated in: Tamoxifen hypersensitivity; Active thromboembolic disease; Pregnancy and lactation; Severe hepatic impairment; Concomitant strong CYP2D6 inhibitors (paroxetine, fluoxetine, bupropion). Full list in the Safety section.
FDA boxed warning: increased risk of endometrial cancer, stroke, and venous thromboembolism. In women with risk factors, decisions are individualized.