An aromatase inhibitor is a drug that lowers the amount of hormones in the body. It is prescribed as a type of hormone therapy for postmenopausal women who have hormone receptor-positive breast cancer. It is thought to decrease the risk of subsequent breast cancer by half (i.e. 50%). There are three commonly used aromatase inhibitors: letrozole (Femara), anastrozole (Arimidex), and exemestane (Aromasin).