A drug used to treat certain types of breast cancer. It is used alone in postmenopausal women who have not been treated with hormone therapy or whose disease got worse after treatment with hormone therapy. It is used with palbociclib or abemaciclib in women whose disease got worse after treatment with hormone therapy. It is also being studied in the treatment of other types of cancer. Fulvestrant binds to proteins called estrogen receptors, which are found in some breast cancer cells. These proteins may cause cancer cells to grow. Fulvestrant blocks these proteins and may keep cancer cells from growing. It is a type of antiestrogen. Also called Faslodex and ICI 182780.