Chemical Formula: C17H19N5 Anastrozole is used with other treatments, such as surgery or radiation, to treat early breast cancer in women who have experienced menopause (change of life; end of monthly menstrual periods). This medication is also used in women, who have experienced menopause, as a first treatment of breast cancer that has spread within the breast or to other areas of the body. This medication is also used to treat breast cancer in women whose breast cancer has worsened after taking tamoxifen (Nolvadex). Anastrozole is in a class of medications called nonsteroidal aromatase inhibitors. It works by decreasing the amount of estrogen the body makes. This can slow or stop the growth of many types of breast cancer cells that need estrogen to grow.. Side effects Like any drug, anastrozole can cause side effects. Everyone reacts differently to drugs and some people have more side effects than others. Common side effects of anastrozole include: