Arimidex or Anastrozole is a drug used to be regularly prescribed for the treatment of breast cancer in women. It has been approved for use in the US in 1995. Arimidex belongs to the class of third-generation aromatase inhibitors. It acts as an aromatase inhibitor to block the production of estrogen. Since many forms of breast cancer are stimulated by estrogen, it is believed that Arimidex reduces the amount of estrogen and stops the progression of the disease (unlike Nolvadex, which works without blocking the production of estrogen).