Tamoxifen

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
Some breast cancer cells require estrogen to grow. Estrogen binds to and activates the estrogen receptor in these cells. Tamoxifen is metabolized into compounds that also bind to the estrogen receptor but do not activate it. Furthermore tamoxifen prevents estrogen from binding to its receptor. Hence breast cancer cell growth is blocked.