Vitamin D3 vs D2: Why the Form on Your Label Matters

Origins and chemistry

D2 (ergocalciferol) comes from UV-irradiated yeast or fungal sterols. D3 (cholecalciferol) comes from UV-irradiated lanolin or, increasingly, from algae or lichen for vegan formulations. The two molecules differ by a methyl group on the side chain. Both convert to 25-hydroxyvitamin D in the liver and then to calcitriol in the kidney, but D2 metabolites have lower affinity for vitamin D-binding protein. This shorter circulation time is the leading mechanistic explanation for why D2 produces lower steady-state levels.

The Tripkovic meta-analysis

The 2012 meta-analysis by Tripkovic and colleagues pooled seven RCTs comparing equivalent IU doses of D2 and D3 and concluded that D3 was significantly more effective at raising serum 25(OH)D. The effect was largest with bolus dosing — large infrequent doses (e.g. 50,000 IU monthly) showed D3 raising blood levels nearly twice as much as D2. With daily dosing the gap narrowed to roughly 30 percent. A 2017 update by the same group, with twenty-four RCTs included, confirmed the original finding.

Hammami et al. (2017) added rigor by running a head-to-head trial with 50,000 IU/week of either D2 or D3 in older adults: D3 produced 8.3 ng/mL higher mean 25(OH)D after 8 weeks. Logan et al. found similar results in pregnant women. There is essentially no high-quality trial in which D2 outperformed D3 for raising the standard biomarker.

Why bolus dosing exposes the gap most clearly

Pharmacy-dispensed weekly D2 (the 50,000 IU green capsule, ergocalciferol) is the U.S. standard for treating deficiency because it has historically been the only high-dose prescription form. The bolus protocol exploits the slow conversion to 25(OH)D, but D2's shorter half-life means a higher fraction is cleared between doses. Daily D3 mostly closes this gap because steady intake compensates for clearance kinetics.

The clinical consequence: a patient prescribed 50,000 IU D2 weekly may show a 25(OH)D rise from 18 to 24 ng/mL — still deficient. The same patient on 5,000 IU daily D3 will typically reach 35–45 ng/mL. The first regimen looks like treatment failure when the actual problem is form choice.

When D2 still has a place

D2 is appropriate for vegans whose only D3 source is algae- or lichen-derived (some object to lanolin-derived D3 even though most rabbinic and halal authorities accept it). D2 also costs less per IU when sourced as a prescription. For correcting deficiency in a patient with the renal capacity to handle daily dosing, prescribed D3 (which most insurers will cover when written explicitly as cholecalciferol) is generally preferable. For maintenance supplementation, D3 is the dominant choice based on both potency per IU and shelf stability.

Form recommendations

Choose D3 (cholecalciferol) by default, in oily-base softgels or oil drops, taken with a fat-containing meal. Powder/dry tablets are absorbed less reliably, particularly in patients on bariatric surgery, cystic fibrosis, or fat malabsorption. Vegan D3 from lichen or algae has equivalent activity to lanolin-derived D3 — confirmed in head-to-head trials.

Standard adult maintenance is 1,000–2,000 IU/day. Deficiency repletion typically uses 4,000–5,000 IU/day for 8–12 weeks, then re-test 25(OH)D. Avoid weekly or monthly bolus regimens unless adherence is a documented problem; daily dosing produces more stable serum levels and lower fall and fracture risk than the high-bolus regimens that were briefly popular in the 2010s.

The bottom line: when your prescription says "ergocalciferol 50,000 IU," ask whether 5,000 IU/day cholecalciferol from the pharmacy aisle would be a more reliable way to reach an adequate level. The answer is usually yes, and the price difference is trivial.