A drug used to treat advanced breast cancer or early-stage breast cancer that is estrogen receptor positive (ER+). It is used in postmenopausal women whose cancer has already been treated with tamoxifen. It is also being studied in the treatment of other types of cancer. Exemestane lowers the amount of estrogen made by the body. This may stop the growth of cancer cells that need estrogen to grow. Exemestane is a type of aromatase inhibitor. Also called Aromasin.