tamoxifen /təmok″səfin/ , a nonsteroidal antiestrogen used in the palliative treatment of advanced breast cancer in premenopausal and postmenopausal women whose tumors are estrogen dependent. Tamoxifen has also been used to reduce the incidence of breast cancer in women with a high risk for developing it and for treating gynecomastia, precocious puberty, and other instances of estrogen excess.
