Tamoxifen is a hormonal therapy used in the treatment of hormone receptor-positive breast cancer, particularly in premenopausal and postmenopausal women.

It is classified as a selective estrogen receptor modulator (SERM).

Tamoxifen works by binding to estrogen receptors in breast tissue, blocking the effects of estrogen, a hormone that may promote the growth of certain breast cancer cells. By doing so, it may help slow or prevent tumor growth.

Because estrogen also plays roles in normal body functions, tamoxifen may have effects on other tissues, including the uterus and bones, which is why long-term monitoring may be recommended.