Tamoxifen
Tamoxifen is a prescription medicine that blocks oestrogen in breast tissue. It is approved to treat hormone-sensitive breast cancer in women and men, and to lower breast cancer risk in some women at high risk.
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What it is
Tamoxifen is a medicine taken by mouth, as tablets or an oral solution, from the group called selective oestrogen receptor modulators (SERMs). It is a hormone treatment for breast cancer [1, 2]. It is a prescription-only medicine and is used under the care of a cancer specialist or other doctor.
How it works
Tamoxifen is a non-steroidal anti-oestrogen. It competes with oestrogen for binding to oestrogen receptors in target tissues such as the breast, which is how it slows the growth of oestrogen-sensitive tumours [1]. In other tissues, such as the lining of the womb, it can act more like oestrogen, which explains some of its risks [1]. Tamoxifen has a half-life of about 5 to 7 days, and its main active breakdown product (N-desmethyl tamoxifen) has a half-life of about 14 days. Steady levels in the blood take several weeks to build up [1].
Approved medical uses
- Metastatic breast cancer in women and men [1, 2].
- Adjuvant treatment of early breast cancer after surgery, to reduce the chance of the cancer returning [1].
- Ductal carcinoma in situ (DCIS) after surgery and radiotherapy, to reduce the risk of invasive breast cancer [1].
- Reducing breast cancer risk in women at high risk [1].
What the research shows
- Risk reduction trial results. In the risk-reduction studies summarised in the US label, women taking tamoxifen had higher rates of some serious events than women taking placebo. Per 1,000 women per year, endometrial cancer occurred in 2.20 on tamoxifen versus 0.71 on placebo, uterine sarcoma in 0.17 versus 0.04, stroke in 1.43 versus 1.00, and pulmonary embolism in 0.75 versus 0.25 [1]. Some of these events were fatal [1].
- Breast cancer in men. The label states that tamoxifen is effective in metastatic breast cancer in men as well as women [1, 2].
- Male fertility and low testosterone. Tamoxifen is not approved for treating low testosterone or male infertility. The American Urological Association (AUA) says clinicians may use selective oestrogen receptor modulators, aromatase inhibitors or hCG in men with testosterone deficiency who want to keep their fertility. This is a conditional recommendation based on lower-quality evidence [3].
- Gaps in the evidence. A 2020 review noted that SERMs are commonly used for men with symptoms of low testosterone, but that no studies had assessed reproductive outcomes when SERMs are given together with testosterone therapy [4]. The same review described these fertility approaches as not well established and noted that they carry side effects [4].
Known risks and side effects
- Boxed warning (US): in women with DCIS and women at high risk of breast cancer, tamoxifen increases the risk of uterine cancers, stroke and pulmonary embolism, and some cases have been fatal [1].
- Uterine cancers: endometrial cancer and the rarer uterine sarcoma, which tends to have a poorer outlook. Abnormal vaginal bleeding should be reported promptly [1].
- Blood clots: deep vein thrombosis and pulmonary embolism. The risk is higher when tamoxifen is combined with chemotherapy [1].
- Eye problems: a higher risk of cataracts and cataract surgery [1].
- Liver effects: changes in liver enzymes and, rarely, fatty liver, cholestasis, hepatitis and liver necrosis, including some fatal cases [1].
- High calcium: hypercalcaemia has been reported in people with breast cancer that has spread to the bone, within a few weeks of starting treatment [1].
- Raised triglycerides: rarely, with pancreatitis in some cases [2].
- In men: loss of libido and impotence have led some male patients to stop treatment [2].
- Pregnancy: tamoxifen may harm an unborn baby, and pregnancy should be avoided during treatment and for 2 months after stopping [1].
- Common effects include hot flushes [1].
- Sport: tamoxifen is prohibited at all times under the World Anti-Doping Agency (WADA) Prohibited List [5].
Blood tests commonly monitored
- Full blood count, including platelets: the label recommends periodic checks [1].
- Liver function tests: recommended periodically during treatment [1].
- Calcium: relevant in people with breast cancer that has spread to the bone, because of the risk of hypercalcaemia [1].
- Triglycerides: relevant because of rare reports of raised triglycerides with pancreatitis [2].
- Other checks (not blood tests): yearly gynaecological examinations, eye examinations, and breast examination and mammography before and during treatment [1].
If you and your doctor are monitoring treatment, Enhanced Training Studio keeps a private log on your phone: what you take and when, your blood test results over time, and how you feel. How tracking works.
Sources
- US National Library of Medicine. Tamoxifen citrate tablets: prescribing information. DailyMed. dailymed.nlm.nih.gov
- US Food and Drug Administration. Nolvadex (tamoxifen citrate) tablets label. FDA, 2005. accessdata.fda.gov
- American Urological Association (Mulhall, et al.). Evaluation and Management of Testosterone Deficiency: AUA Guideline. Journal of Urology and auanet.org, 2018 (validity confirmed 2024). auanet.org
- Herati AS, Kohn TP, Kassiri B. New frontiers in fertility preservation: a hypothesis on fertility optimization in men with hypergonadotrophic hypogonadism. Translational Andrology and Urology, 2020. PMC
- World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026. WADA, 2025. Prohibited List 2026 (PDF copy hosted by CONMEBOL)
Related
Last reviewed 7 October 2026. General information, not medical advice. Found a mistake? Email [email protected].