Illustration of osteoporotic bone tissue and an osteoclast cell. Osteoporosis (brittle bone disease) causes a reduction in overall bone mass and an increase in the bones porosity, making it more brittle and likely to fracture. Osteoclasts are normally present in bones to absorb and remove unwanted bone tissue during normal bone regeneration. In osteoporosis increased activity of the cells contributes to the weakening of the bone. Osteoporosis commonly affects the elderly and post-menopausal women, who experience a decrease in levels of the hormone oestrogen. Oestrogen replacements and drugs that slow the rate of bone loss are used to treat the disease.