Vitamin D3, also known cholecalciferol, is a type of vitamin D that is made by the skin when exposed to sunlight; it is found in some foods and can be taken as a dietary supplement. 

Cholecalciferol can be taken as an oral dietary supplement to prevent vitamin D deficiency or as a medication to treat associated diseases, including rickets. It is also used for familial hypophosphatemia, hypoparathyroidism that is causing low blood calcium, and Fanconi syndrome. Vitamin-D supplements may not be effective in people with severe kidney disease. Excessive doses in humans can result in vomiting, constipation, weakness, and confusion. Other risks include kidney stones.