Vitamin D3 is better than D2 for most people. When taken at the same dose, D3 raises blood levels of vitamin D more than twice as effectively as D2, and it stays in your system longer. Both forms can correct a deficiency, but D3 does it faster and more reliably.
How D3 Outperforms D2 in Your Blood
The clearest evidence comes from comparing what happens to blood levels after taking equal doses of each form. In a head-to-head clinical trial published in the Journal of Clinical Endocrinology & Metabolism, participants taking high-dose D3 saw their total vitamin D levels rise by 27.6 ng/mL, while those taking D2 saw only a 12.2 ng/mL increase. By the end of the study, the D3 group reached an average blood level of 50.9 ng/mL compared to 34.3 ng/mL in the D2 group. That’s a meaningful gap, especially if you’re trying to climb out of deficiency.
The “free” form of vitamin D in the blood, the portion actually available for your cells to use, also rose significantly more with D3. This matters because total levels alone don’t tell the full story. D3 delivered roughly 70% more free vitamin D than D2 at the same dose.
Why Your Body Handles D3 Differently
Vitamin D2 and D3 have slightly different molecular structures, and that small difference changes how your body processes them at every step. The liver enzyme responsible for converting vitamin D into its usable form works about five times faster on D3 than on D2 in laboratory studies. D2 also binds less tightly to the carrier protein that transports vitamin D through your bloodstream, which means it gets cleared and broken down more quickly.
This shows up in half-life measurements. The active form of D2 circulates for an average of about 13.9 days before your body eliminates half of it, while D3’s active form lasts roughly 15.1 days. A day or two might not sound like much, but over weeks and months of supplementation, this difference compounds. D3 builds up a more stable reserve in your body, which is why people taking D3 tend to maintain higher levels between doses.
Where Each Form Comes From
Vitamin D3 (cholecalciferol) is the form your skin naturally produces when exposed to sunlight. It’s also found in animal-based foods like fatty fish, egg yolks, and liver. Most over-the-counter supplements contain D3 derived from lanolin (sheep’s wool oil), though vegan D3 sourced from lichen has become widely available.
Vitamin D2 (ergocalciferol) comes from plants and fungi. Mushrooms exposed to UV light are the most common food source. D2 has historically been the form used in prescription-strength supplements, partly because it was easier and cheaper to manufacture at high doses. Many pharmacies still dispense 50,000 IU D2 capsules for weekly deficiency treatment, even though D3 at the same dose would raise levels more effectively. If your doctor prescribes D2, it will still work, but you may want to ask whether a D3 option is available.
When D2 Still Makes Sense
For vegans and vegetarians who want to avoid animal-derived products, D2 has long been the default choice. It’s entirely plant-based and widely available. That said, lichen-sourced D3 supplements are now sold by most major brands, giving plant-based eaters access to the more effective form. If you can find vegan D3, it’s the better option.
D2 also remains useful in countries or healthcare settings where D3 isn’t available in prescription strength. Taking D2 is far better than taking nothing. It does raise vitamin D levels, just not as efficiently.
How Much You Actually Need
The recommended daily intake for most adults aged 19 to 70 is 600 IU (15 mcg). Adults over 70 need 800 IU (20 mcg). Infants up to 12 months need 400 IU. These recommendations apply to both D2 and D3, though given D3’s superior absorption, meeting these targets is easier with D3.
Many clinicians suggest higher amounts for people who are already deficient, have darker skin, live at northern latitudes, or spend little time outdoors. Blood levels between 20 and 50 ng/mL are generally considered adequate, while levels above 150 ng/mL signal toxicity. Reaching toxic levels from standard supplements is extremely unlikely, as it typically requires sustained daily intake above 10,000 IU. Still, if you’re taking high doses to correct a deficiency, periodic blood testing helps ensure you’re in the right range.
Storing Your Supplements Properly
Vitamin D is sensitive to light, heat, and moisture. In aqueous solutions at room temperature, D3 can lose over 90% of its potency within a single day if conditions are poor. Standard supplement capsules and tablets are far more stable than liquid forms, but you should still store them in a cool, dry, dark place. A medicine cabinet away from the bathroom (where humidity runs high) is ideal. Keeping liquid vitamin D drops refrigerated can slow degradation significantly, reducing the breakdown rate by roughly fivefold compared to room temperature storage.
The Bottom Line on Choosing a Form
If you’re picking a supplement off the shelf, D3 is the stronger choice. It raises blood levels more effectively at the same dose, stays active in your body longer, and mirrors the form your skin produces naturally. D2 works, but you’re getting roughly half the bang for your buck. Check the label for “cholecalciferol” (D3) rather than “ergocalciferol” (D2), and if you follow a plant-based diet, look specifically for vegan D3 sourced from lichen.