Tamoxifen is an estrogen antagonist.
It has been used for several years as a therapy for the prevention of breast cancer in women. It blocks receptors on cells that estrogen normally would be attracted to. Estrogen is said to have both beneficial and harmful effects, depending on the area of the body that is targeted. Women are often advised to take estrogen supplementation after menopause due to its reported ability to protect the cardiovascular system. At the same time, they are warned that estrogen might predispose them to certain types of breast cancers.