Ergocalciferol

Ergocalciferol, also known as vitamin D2, is a secosteroid vitamin produced by ultraviolet irradiation of ergosterol, a sterol found in fungi and yeasts. It is used as a dietary supplement and as a pharmacologic treatment for vitamin D deficiency and to support bone health. Ergocalciferol is commonly chosen when vegan or plant-derived sources are preferred, though some guidelines note that cholecalciferol (vitamin D3) may be more potent at raising serum 25-hydroxyvitamin D levels.

Mechanism of action and metabolism: After ingestion, ergocalciferol is converted in the liver to 25-hydroxy ergocalciferol and then in the kidneys to the active metabolite 1,25-dihydroxy ergocalciferol (calcitriol). Calcitriol acts as a hormone that increases intestinal absorption of calcium and phosphate, supports bone mineralization, and modulates immune function and cell growth.

Forms and dosing: Ergocalciferol is available in oral capsules, tablets, liquids, and drops. Dosing varies by age, deficiency status, and clinical goals; common over-the-counter products provide 400–1000 IU per day, with higher-dose regimens used under medical supervision for deficiency. Maintenance dosing is typically tailored to achieve and sustain adequate serum 25-hydroxyvitamin D levels, with monitoring of calcium and vitamin D status.

Safety and interactions: Hypercalcemia and hypercalciuria are risks with excessive intake. Ergocalciferol should be used with caution in individuals with kidney disease, granulomatous disorders, or a history of vitamin D toxicity. It can interact with bile acid sequestrants, certain anticonvulsants and steroids (which can alter metabolism), and thiazide diuretics (which can raise calcium levels). Pregnancy and lactation should follow clinician guidance based on deficiency status.

See also: vitamin D, cholecalciferol, bone health.