Ergocalciferol (vitamin D2) is a form of vitamin D. Ergocalciferol is a secosteroid formed by a photochemical bond breaking of a steroid, specifically, by the action of ultraviolet light on ergosterol. Viosterol, the name given to early preparations of irradiated ergosterol, is essentially synonymous with ergocalciferol. Ergocalciferol may be used as a vitamin D supplement, and a 2011 clinical guideline considered ergocalciferol to be as effective as cholecalciferol (vitamin D3) which is produced naturally by the skin when exposed to ultraviolet light. Ergocalciferol is manufactured and marketed under various names, including Deltalin (Eli Lilly and Company), Drisdol (Sanofi-Synthelabo) and Calcidol (Patrin Pharma).