America's Compounding Center says the following about Vitamin D3:
"Vitamon D3, also known as cholecalciferol, is a fat-soluble product which is naturally produced by the skin when a patient is exposed to sunlight. Fat-soluble vitamins are stored by the body and do not need to be replaced daily through dietary consumption. Dietary sources of vitamin D include dairy products and fish. Vitamin D is necessary for calcium absorption and is involved in bone formation. Adequate vitamin D intake reduces the risk of bone fractures, and may reduce the risk of heart disease and certain types of cancer. Severe vitamin D deficiency may cause rickets, a condition involving softening of the bones and joint malformation due to a combination of calcium, phosphate, and vitamin D deficiency. Causes of vitamin D deficiency include malabsorption or genetic disorders, lack of exposure to sunlight, poor dietary intake, and kidney failure. Because vitamin D is a fat-soluble vitamin, doses are represented in international unitls (IU), rather than milligrams (mg). Vitamin D is commonly given orally in doses of 400-2000IU daily. Side effects are rare, and may include weakness, fatigue, headache, and loss of appetite."