Aromatase inhibitors (AI) are a class of drugs used in the treatment of breast cancer and ovarian cancer in postmenopausal women. Some cancers require estrogen to grow. Aromatase is an enzyme that synthesizes estrogen. Aromatase inhibitors block the synthesis of estrogen. This lowers the estrogen level, and slows the growth of cancers. In pre-menopausal women, estrogen is made in the ovaries. In post-menopausal women, estrogen is made in the fatty tissues elsewhere in the body. Aromatase inhibitors can't stop the ovaries from making estrogen, so they only block estrogen production effectively in post-menopausal women.