A. Vitamin D supplements generally contain either cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2). Ergocalciferol is a vegetarian form of vitamin D; it is derived from plant or fungal sources and is not produced in the body. Cholecalciferol, on the other hand, is obtained from animal sources (sheep wool oil). It is also the form of vitamin D that is made by the body. Although ergocalciferol may be preferred by vegetarians, research has established that cholecalciferol is the more potent form of vitamin D. Clinical trials have noted that cholecalciferol raises serum levels of vitamin D3 2-3 times higher than equivalent doses of ergocalciferol.*