It is used as a dietary supplement[3] to prevent and treat vitamin D deficiency[4] due to poor absorption by the intestines or liver disease.

[15] Ergocalciferol (D2) and cholecalciferol (D3) are considered to be equivalent for vitamin D production, as both forms appear to have similar efficacy in ameliorating rickets[16] and reducing the incidence of falls in elderly patients.

[17] Conflicting reports exist, however, concerning the relative effectiveness, with some studies suggesting that ergocalciferol has less efficacy based on limitations in absorption, binding, and inactivation.

[18] A meta-analysis concluded that evidence usually favors cholecalciferol in raising vitamin D levels in blood, although it stated more research is needed.

[23] Fungus, from USDA nutrient database (per 100g), D2 + D3:[24][25] Lichen Plantae The vitamin D2 content in mushrooms and C. arbuscula increase with exposure to ultraviolet light.

[29][33] Vitamin D2 from UV-irradiated yeast baked into bread or mushrooms is bioavailable and increases blood levels of 25(OH)D.[29] Viosterol, the name given to early preparations of irradiated ergosterol, is essentially synonymous with ergocalciferol.

[36][37] Ergocalciferol is manufactured and sold under various brand names, including Deltalin (Eli Lilly and Company), Drisdol (Sanofi-Synthelabo), and Calcidol (Patrin Pharma).