Clodronic acid is an organochlorine compound derived from methylene chloride with both hydrogens replaced by phosphonic acid groups. It inhibits bone resorption and soft tissue calcification. Used primarily as the disodium salt tetrahydrate, it treats severe hypercalcaemia associated with malignancy, osteolytic lesions, and bone pain from skeletal metastases. It functions as a bone density conservation agent and antineoplastic agent. It is also a one-carbon compound, a 1,1-bis(phosphonic acid), and the conjugate acid of clondronate(2-).