Vitamin D3 tablets are mainly used for preventing and treating rickets, a disease that is caused by not having enough vitamin D. It is also used for treating weak bones (osteoporosis), bone pain (osteomalacia) and 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. It is also used for boosting the immune system, preventing autoimmune diseases, and preventing cancer.