Vitamins are compounds that you must have for growth and health. They are needed in small amounts only and are available in the foods that you eat. Vitamin D is necessary for strong bones and teeth. Lack of vitamin D may lead to a condition called rickets, especially in children, in which bones and teeth are weak. In adults it may cause a condition called osteomalacia, in which calcium is lost from bones so that they become weak. Your doctor may treat these problems by prescribing vitamin D for you. Vitamin D is also sometimes used to treat other diseases in which calcium is not used properly by the body. Ergocalciferol is the form of vitamin D used in vitamin supplements. Some conditions may increase your need for vitamin D. These include: Alcoholism Intestine diseases Kidney disease Liver disease OverActivity of the parathyroid glands with kidney failure Pancreas disease Surgical removal of stomach In addition, individuals and breast-fed infants who lack exposure to sunlight, as well as dark-skinned individuals, may be more likely to have a vitamin D deficiency. Increased need for vitamin D should be determined by your health care professional. Alfacalcidol, calcifediol, Gemcal, and dihydrotachysterol are forms of vitamin D used to treat hypocalcemia (not enough calcium in the blood). Alfacalcidol, calcifediol, and Gemcal are also used to treat certain types of bone disease that may occur with kidney disease in patients who are undergoing kidney dailysis. Claims that vitamin D is effective for treatment of arthritis and prevention of nearsightedness or nerve problems have not been proven. Some psoriasis patients may benefit from vitamin D supplements; however, controlled studies have not been performed.