Ergocalciferol

Ergocalciferol is one of the two major forms of vitamin D, distinguished from cholecalciferol (vitamin D3) by small differences in its side chain ^[1][2]. Chemically it is a secosteroid, a steroid-like molecule in which one of the rings has been broken open ^[1]. It occurs naturally in certain fungi and is formed when ultraviolet light strikes ergosterol, a provitamin present in yeast and mushrooms; irradiated ergosterol was historically marketed under the name viosterol ^[1][4].

Like other forms of vitamin D, ergocalciferol is not active as taken and must be chemically modified within the body ^[3]. The liver adds a hydroxyl group to form 25-hydroxyvitamin D, the circulating form measured in blood tests, and the kidney adds a second to produce the hormonally active 1,25-dihydroxy form, which acts on the vitamin D receptor ^[3]. Working through this receptor, it increases the absorption of calcium and phosphate from the gut and helps regulate their levels in blood and bone ^[3].

Ergocalciferol was first characterized in the 1930s, during the period when the structures of the vitamins were being worked out ^[1]. It is used to prevent and treat vitamin D deficiency, including deficiency arising from poor dietary intake, malabsorption, or liver disease, and to manage rickets and the low blood calcium of hypoparathyroidism ^[1]. It remains a widely used medicine, and in the United States it ranks among the more frequently prescribed drugs ^[1].

Both vitamin D2 and vitamin D3 can correct deficiency, and studies suggest they are broadly comparable for outcomes such as healing rickets, but the evidence generally indicates that vitamin D3 is somewhat more effective at raising and maintaining blood levels of 25-hydroxyvitamin D ^[2]. Dietary sources of vitamin D2 are limited, with ultraviolet-treated mushrooms being a notable source outside the animal kingdom ^[4]. Ergocalciferol is available both without prescription and in prescription strengths, sold under names such as Drisdol ^[1]. Because vitamin D is fat-soluble and stored in the body, excessive intake can cause toxicity, marked by high blood calcium with symptoms such as increased urination, kidney stones, weakness, and, over the long term, deposition of calcium in tissues ^[1].

Vitamin D is safe within sensible doses, but because it's fat soluble it can accumulate and cause toxicity at very high intakes, leading to high calcium, nausea, and kidney issues. Occasional monitoring of blood levels is wise for anyone on high doses. Most people tolerate replacement doses without any problem.

Ergocalciferol is vitamin D2, and almost every meaningful interaction runs through calcium. Thiazide diuretics reduce urinary calcium excretion, so pairing them with vitamin D supplementation can tip serum calcium high, particularly in people with hyperparathyroidism. Hypercalcemia in turn sensitizes the myocardium to digoxin, and arrhythmias have been reported on that footing. Calcium supplements and other vitamin D analogues such as calcitriol stack in the same direction.

Absorption is the other axis. Ergocalciferol is fat soluble, so bile acid sequestrants like cholestyramine and colestipol, along with orlistat and mineral oil, cut how much of a dose is actually taken up.

Enzyme inducers speed its breakdown. Phenytoin, phenobarbital, carbamazepine and rifampin accelerate vitamin D catabolism, which is one reason long term anticonvulsant therapy is associated with low 25-hydroxyvitamin D and metabolic bone disease.

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