Anastrozole (Arimidex) is the aromatase inhibitor of choice. The drug is appropriately used when using substantial amounts of aromatizing agents, or when one is prone to gynecomastia and using moderate amounts of such agents. As an aromatase inhibitor, Arimidex's mechanism of action -- blocking conversion of aromatizable agents to estrogen -- is in contrast to the mechanism of action of anti-estrogens such as clomiphene (Clomid) or tamoxifen (Nolvadex), which block estrogen receptors in some tissues, and activate estrogen receptors in others.