Aromatase inhibitors are used to treat and prevent hormone-positive breast cancer. They work by blocking the enzyme aromatase, which your body uses to make estrogen. Lowering estrogen levels can help slow the growth of certain cancers that need estrogen to grow. Aromatase inhibitors are usually taken as oral pills. Examples include anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin).