Didronel is used primarily to treat Paget's disease of bone, although it is sometimes used for bone therapy following hip replacement or surgery. The drug inhibits the formation, growth and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces. The drug is not metabolized and only half of the drug is excreted through urine, the other half being distributed throughout bone.Side effects include bone pain or tenderness, and more rarely, hives and skin rash.