Etidronic acid (INN) or 1-hydroxyethane 1,1-diphosphonic acid (HEDP) is a bisphosphonate used in detergents, water treatment, cosmetics, and pharmaceutical treatment. Etidronic acid is a chelating agent and may be added to bind or, to some extent, counter the effects of substances, such as calcium, iron, or other metal ions, which may be discharged as a component of gray wastewater and could conceivably contaminate groundwater supplies. As a phosphonate it has corrosion inhibiting properties on unalloyed steel.
Clinically, etidronate disodium 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.