The active ingredient in vitamin D3 supplements is cholecalciferol, according to Everyday Health. Other ingredients may include gelatin, cornstarch, cellulose gel and maltodextrin, according to Nature Made. Ingredients vary based on manufacturer.
Vitamin D3, often called the sunshine vitamin, is commonly used as a supplement to support general health or help treat osteoporosis, notes Everyday Health. It is also used to treat conditions in which the body's vitamin D levels may be low, such as underactive parathyroid glands or other hereditary parathyroid conditions. Vitamin D3 supplements are available for purchase without a prescription, and come in various dosages, such as 1000, 2000 or 5000 international units.