A class of medical drugs used in the treatment of osteoporosis and other conditions that involve fragile bones. The bisphosphonates attack osteclasts (i.e. the bone cells that break down bone tissue). The general formula is O₃P–C(R¹R³)–PO₃. The side chain R¹ is a simple group (–H, –OH, –Cl). R² is usually a longer chain (e.g. –S–C₆H₄–Cl or –(CH₂)₅–NH₂.