Arimidex (anastrozole) is an anti-estrogenic drug developed for the treatment of advanced breast cancer in women. Specifically, this agent is the first in a newer class of third-generation selective oral aromatase inhibitors.646 It acts by blocking the enzyme aromatase, subsequently blocking the production of estrogen in the body. Since many forms of breast cancer cells are stimulated by estrogen, reducing levels of this hormone in the body may retard the progression of the disease. This is also the fundamental use of tamoxifen citrate (Nolvadex®), except Nolvadex blocks the action of estrogen at the receptor, not its actual endogenous production. The effects of anastrozole can be very substantial, with a daily dose of 1 mg (commonly 1 tablet) capable of producing estrogen suppression greater than 80% in treated patients. With the powerful effect this drug has on hormone levels, it is usually only prescribed to post-menopausal women. For the steroid-using male athlete, anastrozole is applied to minimize the side effects associated with elevated estrogen levels secondary to anabolic/androgenic steroid use. In comparison with traditional methods such as Nolvadex and Proviron, anastrozole is significantly more effective at controlling estrogen.