antiestrogen
n. a substance that reduces or blocks the physiological effects of estrogens, the female sex hormones, on tissues normally responsive to these hormones. Examples include tamoxifen (U.S. trade name: Nolvadex), toremifene (U.S. trade name: Fareston), fulvestrant (U.S. trade name: Faslodex), and selective estrogen receptor modulators (SERMs), such as raloxifene (U.S. trade name: Evista), which have both inhibitory and facilitative effects upon different pathways mediated by estrogen receptors. Antiestrogens are used in the treatment or prevention of breast cancer and some estrogenically mediated effects of menopause, as well as in the treatment of some types of female infertility. Also called estrogen antagonist.