Osteoporosis is a condition in which bones become less dense and bone fractures occur more easily. Two approaches help prevent osteoporosis-related fractures. One is to promote measures that can prevent osteoporosis itself, such as adequate calcium and vitamin D intake, physical activity, and avoidance of tobacco and alcohol. Some drugs, such as estrogen and bisphosphonates, can also prevent osteoporosis, but experts do not recommend these drugs for general use because of side effects and expense. The second approach is to test for osteoporosis in order to identify people with low bone density. These people are at highest risk for fracture and will benefit most from drugs that prevent fracture. Testing for osteoporosis involves using a special test called dual-energy x-ray absorptiometry (DEXA). Although some groups recommend osteoporosis screening, no study has proven that osteoporosis screening reduces fractures.