Many breast tumors are "estrogen sensitive," meaning the hormone estrogen helps them to grow. Aromatase inhibitors (AIs) can help block the growth of these tumors by lowering the amount of estrogen in the body.
Using a substance called aromatase, the ovaries and other tissues of the body produce estrogen. AIs do not block estrogen production by the ovaries, but they can block other tissues from making this hormone. That's why AIs are used mostly in women who have reached menopause, when the ovaries are no longer producing estrogen.
Tamoxifen (Nolvadex®), also helps to prevent the growth of estrogen-sensitive breast tumors, but it works differently from AIs. Whereas AIs reduce the amount of estrogen in the body, tamoxifen blocks a tumor's ability to use estrogen.
Currently, three AIs are approved by the U.S. Food and Drug Administration: anastrazole (Arimidex®), exemestane (Aromasin®), and letrozole (Femara®).