Arimidex lowers bone mineral density, which may enhance your chance of developing osteoporosis. Your bone mineral density may require to be checked before and while treatment with this medicine. FDA pregnancy category D. Do not apply Arimidex if you are pregnant. It could offend the unborn baby. Apply effective birth control, and inform your doctor if you become pregnant while treatment. It has been said that anastrozole passes into breast milk or if it could offend a nursing baby.