Bisphosphonate: A class of drugs used to strengthen bone. Bisphosphonates are used to treat osteoporosis and the bone pain from diseases such as metastatic breast cancer, multiple myeloma, and Paget's disease. Bone is in a constant state of remodeling, whereby new bone is laid down by cells called osteoblasts, while old bone is removed by cells called osteoclasts. Bisphosphonates inhibit bone removal (resorption) by the osteoclasts. The bisphosphonates include Fosamax (alendronate), Actonel (risedronate), Boniva (ibandronate), and Reclast (zoledronate).