Anastrozole is a non-steroidal aromatase inhibitor used in medication to treat breast cancer among women after menopause. It decreases the number of estrogen the body is making and helps to reverse the growth of these cancers. It is still the most popular estrogen control drug. Doctors give this to women whose cancer has progressed even after taking tamoxifen. Common side effects include hot flashes, headache, trouble sleeping, stomach upset, vomiting, and others, if these condition get worse, promptly tell your doctor.