Vitamin D vs Vitamin D3
"Vitamin D" on a supplement label is almost always vitamin D3 (cholecalciferol), but "vitamin D" as a chemistry term is an umbrella including both D3 and D2 (ergocalciferol). Here's what actually differs.
| Attribute | Vitamin D3 (cholecalciferol) | Vitamin D2 (ergocalciferol) |
|---|---|---|
| Origin | Animal skin from 7-DHC + UVB; fatty fish; egg yolk | Yeasts/fungi from ergosterol + UV; UV-exposed mushrooms |
| Chemistry | Side chain has H at C24 | Side chain has methyl at C24; double bond at C22–C23 |
| Effect on serum 25(OH)D | Raises by ~1 ng/mL per 100 IU/day | ~60–90% less effective per equivalent dose (Tripkovic 2012) |
| Half-life of the metabolite | 25(OH)D3 ~15–25 days apparent; ~60 d terminal | 25(OH)D2 shorter |
| Common supplement source | Lanolin (sheep wool) or lichen (vegan) | Yeast or fungi (vegan) |
| Prescription forms | Most European & ROW prescription vitamin D | US prescription 50,000 IU capsules (Drisdol) |
| Recommendation | Preferred by most modern guidelines | Reasonable for vegans who avoid lichen-D3; ask about substitution |
Why the effectiveness gap?
Both forms are 25-hydroxylated by CYP2R1 in the liver. But D2 is less avidly bound by vitamin D-binding protein (DBP), cleared faster, and its metabolites are catabolised more rapidly. Net result: D2 produces smaller and shorter-lived rises in serum 25(OH)D.
If your prescription is ergocalciferol (Drisdol, "vitamin D2"), it's worth asking your prescriber whether cholecalciferol at the same IU dose would suit you better — most will say yes.
Reading a supplement label
- "Vitamin D3" or "cholecalciferol" — the effective form.
- "Vitamin D2" or "ergocalciferol" — less potent but still works; usually cheaper and vegan.
- "Vitamin D" without a number — check the ingredients panel. Almost always D3 in modern supplements.
- Doses are given in IU or µg (1 µg = 40 IU). See our unit converter.