Anastrozole (Arimidex) is a drug indicated in the treatment of breast cancer in post-menopausal women. It is used in both adjuvant therapy (i.e. following surgery) and in metastatic breast cancer. It has the effect of decreasing the amount of estrogen that the body makes.

Anastrozole belongs in the class of drugs known as aromatase inhibitors. It inhibits the enzyme aromatase, which is responsible for converting androgens (produced by women in the adrenal glands) to estrogen.

The ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial represents a long-term follow-up study of 9366 women with localized breast cancer who received either anastrozole or/and tamoxifen. After more than 5 years the group that received anastrozole had significantly better clinical results than the tamoxifen group. The trial suggested that anastrozole is the preferred medical therapy for postmenopausal women with localized breast cancer that is estrogen receptor positive.