Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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.