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.