As people age, their bones become thinner. Some loss of bone density with age is natural, but in many older adults, the extent of bone loss results in a brittle bone condition called osteoporosis. It results from an imbalance between the natural breakdown and rebuilding of bone. Thin, osteoporotic bones are fragile and break more easily than dense bones. Fortunately, many things may help prevent osteoporosis. They include adequate intake of vitamin D and calcium; regular weight-bearing exercise, such as walking; and avoidance of smoking. There also are many drug therapies that may help treat osteoporosis and prevent broken bones. Few studies, however, have compared the benefits of the different drug therapies for osteoporosis.