From Wikipedia: Cholecalciferol, also known as vitamin D3 and colecalciferol, 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 is made in the skin following UVB light exposure. It is converted in the liver to calcifediol (25-hydroxyvitamin D) which is then converted in the kidney to calcitriol (1,25-dihydroxyvitamin D). One of its actions is to increase calcium uptake by the intestines. It is found in food such as some fish, beef liver, eggs, and cheese. Plants, cow milk, fruit juice, yogurt, and margarine also may have cholecalciferol added to them in some countries, including the United States.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. Doses greater than 40,000 IU (1,000 μg) per day are generally required before high blood calcium occurs. Normal doses, 800–2000 IU per day, are safe in pregnancy.Cholecalciferol was first described in 1936. It is on the World Health Organization's List of Essential Medicines. In 2020, it was the 60th most commonly prescribed medication in the United States, with more than 11 million prescriptions. Cholecalciferol is available as a generic medication and over the counter.