Vitamin D3 vs D2: which form actually raises your levels (and why it matters)

If you've ever stared at two vitamin D bottles — one labeled D3, one D2 — and assumed they're the same thing, this is the article for you. They're not interchangeable, and the difference shows up directly in your blood test.

The two forms

Both are "vitamin D," but they come from different places and behave differently in the body:

• Vitamin D3 (cholecalciferol) — the form your skin makes from sunlight, and the form in animal foods. Supplements source it from lanolin (sheep wool oil), fish, or, for vegans, lichen.
• Vitamin D2 (ergocalciferol) — made by fungi/yeast exposed to UV light. It's in UV-treated mushrooms, some fortified foods, and the traditional US high-dose prescription (50,000 IU).

The key difference: D3 raises your level better

This is the part that matters. Once in your body, vitamin D is converted to 25-hydroxyvitamin D [25(OH)D] — the storage form your blood test measures. The evidence is consistent: D3 raises 25(OH)D more, and keeps it up longer, than an equivalent dose of D2.

• Head-to-head trials and meta-analyses repeatedly show D3 is more potent per IU at increasing 25(OH)D.
• D2 also rises levels, but less efficiently and with a shorter duration — its effect fades faster between doses.
• Mechanistically, D3 appears to be metabolized and maintained more effectively.

So for the same number on the label, D3 does more of the job. If your goal is to correct or maintain your level (see vitamin D deficiency: signs, testing and correction), D3 is the efficient pick.

Why D2 still exists

If D3 is better, why is D2 everywhere in prescriptions? Mostly history and cost: D2 is cheap to mass-produce and was the established high-dose Rx form in the US. It's not wrong — a 50,000 IU D2 capsule will correct a deficiency — it's just less efficient than the same effort with D3, and many clinicians have shifted to high-dose D3.

Is the vitamin D from sunlight the same as D3?

Yes — the vitamin D your skin makes from UVB is D3 (cholecalciferol), the exact molecule in a D3 supplement. That's part of why D3 is the sensible default: you're topping up the same form your body already makes and knows how to use. D2 never appears in human skin synthesis; it's a fungal-origin form we adopted for fortification and cheap high-dose pills.

Does "D3" mean you can take fewer IU?

Not on the label — an IU is an IU, and you should dose to your blood test, not to the form. But because D3 is more efficient at raising and holding 25(OH)D, the same IU of D3 generally does more real-world work than D2. So switching D2 → D3 is a free upgrade in effectiveness at the same number, not a reason to slash the dose — confirm where you land with a 25(OH)D test.

Vegans: you don't need to compromise

For years, plant-based eaters were pushed toward D2 because most D3 came from sheep wool or fish. That's outdated: lichen-derived vegan D3 is now widely available and gives plant-based eaters the more effective form. Look for "vegan D3" or "lichen" on the label.

Dosing is the same idea

Choosing D3 doesn't change the amount you need. The NIH RDA is 600–800 IU/day; a common practical supplement range is 1,000–2,000 IU/day (still well under the 4,000 IU adult upper limit), and the "take it with a fatty meal" rule is the same. The full picture is in vitamin D3 dosing, and at higher doses it's worth reading vitamin D3 and K2 together.

How to buy

Pick D3 (vegan/lichen if you need it), choose your dose, and compare on real cost per serving — vitamin D is cheap, so don't overpay. Current value picks: best-value vitamin D3 and the vitamin D3 hub. We rank by cost per effective dose, not sticker price (methodology).

Bottom line

D3 and D2 are both real vitamin D, but D3 raises and holds your blood level more effectively — so it's the better everyday choice. D2 isn't a mistake (especially as a prescription), just less efficient. Vegans can use lichen-derived D3 and skip the compromise. General references from the NIH and published trials, not medical advice.