Tamoxifen is an antagonist of the estrogen receptor in breast tissue. It has been the standard endocrine (anti-estrogen) therapy for hormone-positive early breast cancer, although aromatase inhibitors have been proposed for postmenopausal women. In men, tamoxifen is sometimes used to treat gynecomastia that arises for example as a side effect of antiandrogen prostate cancer treatment.
Tamoxifen competitively binds to estrogen receptors on tumors and other tissue targets, producing a nuclear complex that decreases DNA synthesis and inhibits estrogen effects. It is a nonsteroidal agent with potent antiestrogenic properties which compete with estrogen for binding sites in breast and other tissues.