Alendronic acid is a bisphosphonate drug used for osteoporosis, osteogenesis imperfecta, and several other bone diseases. It is marketed alone as well as in combination with vitamin D. Alendronate inhibits osteoclast-mediated bone-resorption. Like all bisphosphonates, it is chemically related to inorganic pyrophosphate, the endogenous regulator of bone turnover. But while pyrophosphate inhibits both osteoclastic bone resorption and the mineralization of the bone newly formed by osteoblasts, alendronate specifically inhibits bone resorption without any effect on mineralization at pharmacologically achievable doses. Its inhibition of bone-resorption is dose-dependent and approximately 1,000 times stronger than the equimolar effect of the first bisphosphonate drug, etidronate. Under therapy, normal bone tissue develops, and alendronate is deposited in the bone-matrix in a pharmacologically inactive form. For optimal action, enough calcium and vitamin D are needed in the body in order to promote normal bone development. Hypocalcemia should, therefore, be corrected before starting therapy. Treatment of post-menopausal women and people with osteogenesis imperfecta over the age of 22 with alendronic acid has demonstrated normalization of the rate of bone turnover, significant increase in BMD (bone mineral density) of the spine, hip, wrist and total body, and significant reductions in the risk of vertebral (spine) fractures, wrist fractures, hip fractures, and all non-vertebral fractures. In the Fracture Intervention Trial, the women with the highest risk of fracture (by virtue of pre-existing vertebral fractures) were treated with Fosamax 5 mg/day for two years followed by 10 mg/day for the third year. This resulted in approximately 50% reductions in fractures of the spine, hip, and wrist compared with the control group taking placebos. Both groups also took calcium and vitamin D.