Ergocalciferol is a form of vitamin D, also called vitamin D2. It is marketed under various names including Deltalin (Eli Lilly and Company), discontinued Drisdol (Sanofi-Synthelabo), and Calcidol (Patrin Pharma). Created from viosterol when ultraviolet light activates ergosterol (found in fungi, it is named as a sterol from ergot), with fungi as the primary plant source.
Ergocalciferol may be used as a vitamin D supplement, and a 2011 clinical guideline considered it to be as effective as cholecalciferol (vitamin D3), which is produced naturally by the skin when exposed to ultraviolet light. Conflicting evidence exists for how similarly D2 and D3 behave in the body and whether they are equally active, with some studies suggesting D3 is more potent, while others report equal efficacy. Cholecalciferol (vitamin D3) is sensitive to UV radiation and rapidly, but reversibly, forms other sterols which can further irreversibly convert to ergosterol.