Anastrozole, sold under the brand name Arimidex among others, is a medication used in addition to other treatments for breast cancer.Specifically it is used for hormone receptor-positive breast cancer. It has also been used to prevent breast cancer in those at high risk. It is taken by mouth.

Common side effects of anastrozole include hot flashes, altered mood, joint pain, and nausea. Severe side effects include an increased risk of heart disease and osteoporosis. Use during pregnancy may harm the baby. Anastrozole is in the aromatase-inhibiting family of medications. It works by blocking the production of estrogens in the body, and hence has antiestrogenic effects.