Estrogen antagonists are used to treat breast cancer. They work by blocking the effects of estrogen, a hormone that can promote the growth of certain types of breast cancer. Estrogen antagonists come in different forms, including oral pills, oral liquids, and injections. Examples include tamoxifen (Soltamox), fulvestrant (Faslodex), and elacestrant (Orserdu). You can discuss with your healthcare team which option might be right for you.