Osteoporosis or "brittle bones" is a metabolic bone disease in which the amount of bone tissue is reduced sufficiently to increase the likelihood of fracture. Fractures of the vertebrae, femur (hip), and wrist are the most common osteoporotic fractures, but other bones such as the ribs, upper arm, and pelvis may also fracture.

Osteoporosis affects 25 percent of women over the age of 60. It is much less common and not as severe in men. It is more common after menopause, when estrogen levels decline.

Although low bone mass is the major factor in osteoporotic fractures, there may also be qualitative and architectural changes in bone with aging that lead to increased fragility. Osteoporosis can be primary or secondary. Primary osteoporosis occurs independently of other causes. The secondary osteoporoses result from identifiable causes, such as exogenous cortisone administration, Cushing's disease, hyperparathyroidism, hyperthyroidism, hypogonadism, multiple myeloma, prolonged immobilization, alcoholism, anorexia nervosa, and various gastrointestinal disorders.