Tamoxifen

Key facts

• Tamoxifen (sold as Nolvadex) is a selective estrogen receptor modulator: it blocks estrogen's effect in breast tissue while behaving more like estrogen in others, such as the uterus and bone.
• It's usually taken as one tablet a day, with or without food, often for 5 to 10 years depending on your cancer stage and risk.
• Tamoxifen increases the risk of endometrial (uterine) cancer and of blood clots in the legs or lungs; report any abnormal vaginal bleeding, leg swelling, or breathlessness straightaway.
• Seek urgent care for sudden chest pain, one-sided leg swelling, sudden shortness of breath, or sudden vision changes.

What tamoxifen treats

Tamoxifen treats breast cancer that depends on estrogen to grow (estrogen-receptor-positive breast cancer), in both early and advanced stages, in men as well as women. It also reduces the risk of breast cancer developing in people at high risk, and lowers the chance that a previous breast cancer returns. It is not a treatment for uterine conditions; it in fact raises the risk of uterine cancer, so new pelvic symptoms need review, not reassurance.

How tamoxifen works

Estrogen binds to receptors on cells and drives some breast cancers to grow. Tamoxifen attaches to those same receptors, blocking estrogen's growth signal in breast tissue. In the lining of the uterus, however, tamoxifen behaves more like a weak estrogen itself, which is why it can thicken the endometrium and, over years of use, raise the risk of endometrial cancer.

Before you take it

• Tell your doctor about any history of blood clots, stroke, or a family history of clotting disorders; tamoxifen raises clot risk.
• Report abnormal vaginal bleeding or spotting immediately; it needs prompt investigation, not just monitoring.
• Do not take tamoxifen during pregnancy; it can harm a developing fetus, and reliable contraception is required during treatment and for a period after stopping.
• Tell your doctor about liver disease and other medicines, including some antidepressants (strong CYP2D6 inhibitors such as paroxetine and fluoxetine), which can reduce tamoxifen's effectiveness.

Side effects

Common effects include hot flushes, night sweats, vaginal dryness or discharge, irregular periods, and mild nausea.

Stop and seek urgent medical care for any of these:

• Leg pain, swelling, or redness, or sudden shortness of breath (possible blood clot).
• Abnormal vaginal bleeding or unusual pelvic pain (possible endometrial change).
• Sudden vision loss or eye pain.
• Yellowing of the skin or eyes, or severe abdominal pain (possible liver problem).

Safety essentials

• Tamoxifen's two defining risks are endometrial cancer and blood clots (deep vein thrombosis and pulmonary embolism). Any abnormal vaginal bleeding is a red flag that needs prompt medical review, not something to wait out.
• Have regular gynecological checks while on treatment, and report bleeding between periods or after menopause without delay.
• Do not start tamoxifen if you are or might be pregnant, and use reliable contraception throughout treatment.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.