Osteoporosis is a disease of bones that leads to an increased risk of fracture (bone fracture). In osteoporosis the bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of proteins in bone is altered. Osteoporosis is defined by the World Health Organization (WHO) in women as a bone mineral density 2.5 standard deviations below peak bone mass (20-year-old healthy female average) as measured by DXA (Dual energy X-ray absorptiometry); the term "established osteoporosis" includes the presence of a fragility fracture.

Osteoporosis is most common in women after menopause, when it is called postmenopausal osteoporosis, but may also develop in men, and may occur in anyone in the presence of particular hormonal disorders and other chronic (Chronic (medicine)) diseases or as a result of medications, specifically glucocorticoids, when the disease is called steroid- or glucocorticoid-induced osteoporosis (SIOP or GIOP). Given its influence in the risk of fragility fracture, osteoporosis may significantly affect life expectancy and quality of life.

Disease of the parathyroid glands (hyperparathyroidism) is also a major cause of osteoporosis. Hyperparathyroidism should be high on the list of causes in any patient with severe osteoporosis, osteoporosis occurring at a young age, or osteoporosis in a male.

Over consumption of dietary protein is another often neglected cause of osteoporosis. The excess protein causes calcium to be taken from the bones and excreted in the urine.

Osteoporosis can be prevented with lifestyle changes and sometimes medication; in people with osteoporosis, treatment may involve both. Lifestyle change includes exercise and preventing falls (Fall prevention) as well as reducing protein intake. Medication includes calcium (calcium in biology), vitamin D, bisphosphonates and several others. Fall-prevention advice includes exercise to tone deambulatory muscles, proprioception-improvement exercises; equilibrium therapies may be included. Exercise with its anabolic effect, may at the same time stop or reverse osteoporosis. Osteoporosis is a component of the frailty syndrome.