Etidronate is a salt of etidronic acid (brand name Didronel, also known as EHDP) a diphosphonate, which is indicated for the treatment of symptomatic Paget’s disease of bone and in the prevention and treatment of heterotopic ossification following total hip replacement or due to spinal cord injury. Didronel is not approved for the treatment of osteoporosis. This drugs acts primarily on bone. It can inhibit the formation, growth, and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces. Inhibition of crystal resorption occurs at lower doses than are required to inhibit crystal growth. Both effects increase as the dose increases. Preclinical studies indicate etidronate disodium does not cross the blood-brain barrier. Didronel is not metabolized. The amount of drug absorbed after an oral dose is approximately 3 percent. Bisphosphonates, when attached to bone tissue, are absorbed by osteoclasts, the bone cells that breaks down bone tissue. Although the mechanism of action of non-nitrogenous bisphosphonates has not been fully elucidated, available data suggest that they bind strongly to hydroxyapatite crystals in the bone matrix, preferentially at the sites of increased bone turnover and inhibit the formation and dissolution of the crystals. Other actions may include direct inhibition of mature osteoclast function, promotion of osteoclast apoptosis, and interference with osteoblast-mediated osteoclast activation. Etidronic acid may promote osteoclast apoptosis by competing with adenosine triphosphate (ATP) in the cellular energy metabolism. The osteoclast initiates apoptosis and dies, leading to an overall decrease in the breakdown of bone.