In January 1996, the U.S. Food and Drug Administration (FDA) approved anastrozole (Arimidex) as a treatment for breast cancer commercialized by AstraZeneca.

The effectiveness in the adjuvant setting was based on analysis of data from the ATAC trial at a median duration of treatment of 31 months, with anastrozole showing a statistically significant increase in recurrence-free survival compared with tamoxifen. Anastrozole blocks the production of estrogen.