Short answer: Both are selective estrogen receptor modulators. Tamoxifen is approved for breast cancer, including metastatic breast cancer in men, and has a half-life of about 5 to 7 days.1 Clomiphene is approved only for ovulation problems in women.2 Both are used off label in men for low testosterone or infertility, with limited evidence.3
| Tamoxifen | Clomiphene (Clomid) | |
|---|---|---|
| Class | Nonsteroidal antiestrogen (SERM)1 | Nonsteroidal ovulatory stimulant (SERM), a mix of two isomers2 |
| Approved uses | Metastatic breast cancer in women and men; adjuvant treatment; DCIS; risk reduction in high-risk women1 | Ovulatory dysfunction in women who want to become pregnant2 |
| Label dosing schedule | Breast cancer: 20 to 40 mg daily; above 20 mg split into two doses1 | 50 mg daily for 5 days per cycle to start2 |
| Half-life | About 5 to 7 days; main metabolite about 14 days1 | No single value; zuclomiphene detectable for over a month2 |
| Time to peak (Tmax) | About 5 hours after a 20 mg dose1 | About 3 hours (enclomiphene) and 7 hours (zuclomiphene) after 50 mg4 |
| Steady state | About 4 weeks; metabolite about 8 weeks1 | Not stated |
| Boxed warning | Uterine cancers, stroke and pulmonary embolism in the risk-reduction setting1 | None |
| Other notable warnings | Cataracts; blood clots; liver effects1 | Visual symptoms; ovarian hyperstimulation; testicular tumors and gynecomastia reported in men2 |
| Use in men | Labeled for metastatic male breast cancer; other uses (infertility, gynecomastia) off label5 | Off label; label says no adequate studies show it works for male infertility2 |
Both block estrogen at some tissues and act like it at others. Tamoxifen's label is built on breast cancer trials, and it includes men: published reports of 132 evaluable men with breast cancer showed a 50% response rate.1 Clomiphene's label is about triggering ovulation; it acts at the hypothalamus and pituitary to raise the hormones that drive the ovaries.2
In men, blocking estrogen feedback at the brain raises LH and FSH, which is why both get used off label for low testosterone or sperm production.3
It is thin for both. A 2025 meta-analysis of 10 randomized trials in men with unexplained subfertility found pregnancy rates of 10.4% on clomiphene vs 7.1% on placebo, a difference that was not significant; there was not enough data to pool tamoxifen trials.3 A review of tamoxifen side effects in men found fewer reported problems in men treated for infertility or gynecomastia than in men with prostate or breast cancer, and noted that long-term safety data are lacking.5 No randomized trial compares the two directly for male hypogonadism. Any use in men is a decision for a prescriber.