It has been estimated that 13% of women in the United States will develop breast cancer during their lifetimes. In many women, growth of breast cancer can be reduced by blocking the formation of estrogen with drugs known as aromatase inhibitors. Although these drugs improve the survival of women who have estrogen-responsive breast cancer, blocking estrogen production can cause substantial decreases in bone density, increasing the risk of osteoporosis and bone fractures. Tamoxifen and goserelin have also been used as antiestrogen agents during breast cancer chemotherapy and they can have similar adverse effects on bone density.