Ergocalciferol

Vitamin D analog, Fat-soluble vitamin

Ergocalciferol (vitamin D2)

Drisdol, Calcidol; also many OTC preparations

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Summary

Classes

Vitamin D analog, Fat-soluble vitamin

Pharmacy

Starting dose

Deficiency: 50,000 IU PO weekly for 8-12 weeks, then maintenance 800-2,000 IU/day; maintenance OTC dosing typically 800-2,000 IU/d

Preparations

50,000 IU capsules (Rx); 8,000 IU/mL oral solution (Rx); 400-5,000 IU OTC tablets and softgels

US FDA Max

Not strictly fixed; long-term Upper Limit ~4,000 IU/d in adults (Institute of Medicine)

Pharmacology

Routes

Oral

Onset

Days to weeks to raise 25(OH)D; clinical effect on bone over months

Duration

Long; fat-soluble, stored in adipose

Half-life

~24 hours (parent); ~15 days (25(OH)D); 25(OH)D stores persist for months

Bioavailability

~75% (oral; fat-soluble, absorption requires intact biliary/lipid digestion)

Pregnancy

Safe at replacement doses; deficiency is itself a risk in pregnancy and lactation.^{[citation needed]}

Legal status

OTC (low-dose) and Rx-only (50,000 IU and concentrated solutions) in US

Purported mechanism

Ergocalciferol (D2) and cholecalciferol (D3) are hydroxylated in the liver by CYP2R1 to 25-hydroxyvitamin D (the storage form measured clinically) and then in the kidney by CYP27B1 to 1,25-dihydroxyvitamin D (calcitriol), the active hormone that binds the vitamin D receptor (VDR) to drive transcription of genes governing intestinal calcium absorption, bone mineralization, and broad immunomodulation.▲0▼ D3 (cholecalciferol) is more potent at raising serum 25(OH)D per dose; D2 remains widely prescribed in the US Rx 50,000 IU formulation.