Fosamax (Alendronate) is in the group of medicines called bisphosphonates. Fosamax alters the cycle of bone formation and breakdown in the body. Alendronate slows bone loss while increasing bone mass, which may prevent bone fractures. Fosamax (Alendronate) is used to treat or prevent postmenopausal osteoporosis and steroid-induced osteoporosis. Fosamax (Alendronate) is also used to treat Paget's disease of bone.