Description: The American College of Physicians (ACP) developed this guideline to present the available evidence on various pharmacologic treatments to prevent fractures in men and women with low bone density or osteoporosis. Methods: Published literature on this topic was identified by using MEDLINE (1966 to December 2006), the ACP Journal Club database, the Cochrane Central Register of Controlled Trials (no date limits), the Cochrane Database of Systematic Reviews (no date limits), Web sites of the United Kingdom National Institute of Health and Clinical Excellence (no date limits), and the United Kingdom Health Technology Assessment Program (January 1998 to December 2006). Searches were limited to English-language publications and human studies. Keywords for search included terms for osteoporosis, osteopenia, low bone density, and the drugs listed in the key questions. This guideline grades the evidence and recommendations according to the ACP's clinical practice guidelines grading system.