Tamoxifen
A selective estrogen receptor modulator: it blocks estrogen in breast tissue. Men use it off-label to treat or prevent gynecomastia and sometimes after a testosterone or steroid cycle.
FDA-approved (Nolvadex; Soltamox liquid) for breast cancer treatment and risk reduction; off-label in men for gynecomastia.
Used for
- Breast cancer treatment and risk reduction (its approved use)
- Gynecomastia (breast tissue growth in men; off-label)
- Post-cycle therapy (community use)
How it works
Blocks estrogen receptors in breast tissue, so it can shrink or stop gynecomastia without lowering estradiol itself. In the brain it also blocks estrogen feedback, which can raise LH and testosterone (the same idea as clomiphene).
Half-life: about 5–7 days (its active metabolite N-desmethyltamoxifen about 14 days); steady state takes about 4 weeks
What the research shows
- A 2012 systematic review of 4 trials in prostate cancer patients on antiandrogens: tamoxifen cut gynecomastia (risk ratio 0.10) and breast pain (0.06) at 6 months against no treatment, and prevented them better than anastrozole (risk ratios 0.22 and 0.25).
- A randomized trial of 176 men on bicalutamide: 10 mg a day started with it prevented most breast events (35% had them at 12 months), while 20 mg a day started after breast events appeared left them persisting in 27.7%.
- In 81 men with idiopathic gynecomastia treated with 10 mg daily at a breast clinic, 90.1% had complete resolution (cohort study, no control group).
- A 2022 review of treatments for pubertal gynecomastia found tamoxifen the most-used drug and effective with few side effects, but with little high-quality research.
- A review of tamoxifen in men found few side effects in men treated for infertility or idiopathic gynecomastia and under 5% stopping for toxicity; long-term safety in men has not been well studied.
- Women on tamoxifen who also took a strong CYP2D6-blocking antidepressant (paroxetine, fluoxetine) did not have more deaths than those on other SSRIs (hazard ratio 0.96, 95% CI 0.88–1.06), though the label says the effect is not settled.
Dosing
Dosing isn’t listed on the public site. In the app, each published option is shown with its source (a drug label, a clinical guideline, a trial or community reports) and can be turned into a protocol you review.
Work out the units to draw for your vial →Side effects
- Hot flashes, nausea, mood changes, fluid retention
- In men treated for gynecomastia or infertility: few side effects reported, under 5% stopping for toxicity (review)
- Eye problems: cataracts, retinopathy, color vision changes (label)
- Vaginal bleeding or discharge in women (label)
Cautions
- Boxed warning: stroke, pulmonary embolism and uterine cancers occurred in women taking it for risk reduction. Blood clots (legs, lungs, eyes) are the main risk to know.
- Report any vision change or leg swelling right away.
- Tell your doctor about CYP2D6-blocking drugs (paroxetine, fluoxetine) and warfarin (a large rise in its effect). The label says not to combine with anastrozole or letrozole.
- Can cause fetal harm: not in pregnancy (label).
- Liver abnormalities and liver cancer are reported (label); a doctor may check liver tests.
- It does not lower estradiol: on TRT, estradiol may rise, so check bloodwork.
- Prohibited in tested sports (WADA).
Storage
Unmixed powder: freezer or fridge, away from light.
Once mixed with bacteriostatic water: fridge; the common rule is to use it within 28 days.
There is no label or published stability data for this one, so treat these as habits, not limits.
Interactions
Warfarin: its effect can rise a lot. Monitor clotting closely; the label says not to use warfarin with tamoxifen taken for DCIS or to lower breast cancer risk.
Do not combine with anastrozole; combining with letrozole is not recommended (tamoxifen lowered letrozole levels by 38%). Avoid strong CYP3A4 inducers such as rifampin.
Strong CYP2D6 blockers (paroxetine): the label says the effect on tamoxifen’s benefit is not well established: some studies found less of its active forms and a weaker effect, others did not.
A large cohort of women on tamoxifen found no extra deaths with paroxetine or fluoxetine compared with other SSRIs; this does not settle the question.
Not a full list. Tell your doctor and pharmacist everything you take.
Reference only, not medical advice. Written for this app from the cited research. Doses marked community-reported are not from trials.