Vitamin D3 is vitamin produced by the body when exposed to ultraviolet light or obtained from dietary sources. Vitamin D3 is a hormone that has an important role in calcium and phosphorus metabolism. Technically, vitamin D3 is not a vitamin because the body can produce it. Also known as cholecalciferol. Calcium citrate is used to prevent or treat low blood calcium levels in people who do not get enough calcium from their diets.It is also used for treating weak bones (osteoporosis), bone pain (osteomalacia), bone loss in people with a condition called hyperparathyroidism, and an inherited disease (osteogenesis imperfecta) in which the bones are especially brittle and easily broken. It is also used for preventing falls and fractures in people at risk for osteoporosis, and preventing low calcium and bone loss (renal osteodystrophy) in people with kidney failure.