Vitamin D3 and vitamin D2 are both forms of vitamin D, but they come from different sources, have slightly different molecular structures, and differ meaningfully in how well they raise your blood levels. The most important practical difference: D3 is roughly 40% more effective than D2 at raising your overall vitamin D levels when taken as a daily supplement.
Where Each Form Comes From
Vitamin D2 (ergocalciferol) is a plant-derived form. It’s manufactured by exposing yeast to ultraviolet light, and it occurs naturally in mushrooms, especially those that have been treated with UV light to boost their vitamin D content. This makes D2 the form most commonly found in fortified plant-based foods.
Vitamin D3 (cholecalciferol) is the form your skin produces when exposed to sunlight. In supplements, it’s typically made from lanolin, a waxy substance extracted from sheep’s wool. For people who avoid animal products, a plant-based version of D3 sourced from lichen is also available. D3 is found naturally in fatty fish, egg yolks, and liver.
How They Differ at the Molecular Level
The two forms are nearly identical in structure, with one small but consequential difference in their side chains: D2 has an extra double bond and an additional small carbon branch that D3 lacks. This might sound trivial, but it changes how each molecule binds to the carrier protein that transports vitamin D through your bloodstream, and it affects how your body processes each form.
How Your Body Processes Both Forms
Once you swallow either form or your skin makes D3 from sunlight, the same basic pathway kicks in. Your liver converts the vitamin into its main circulating form, which is what doctors measure in a blood test. Your kidneys then convert that into the fully active hormone your body actually uses to regulate calcium, support immune function, and carry out dozens of other jobs.
Both D2 and D3 follow this same two-step activation process and produce active hormones with comparable biological activity. The difference isn’t in what the end product does. It’s in how efficiently each form gets you there.
D3 Raises Blood Levels More Effectively
A 2023 systematic review and meta-analysis of 20 comparative studies found that daily D2 supplementation raised total blood vitamin D levels about 40% less than the same dose of D3. That’s a substantial gap. If you took 1,000 IU of D3 and someone else took 1,000 IU of D2, your blood levels would climb meaningfully higher.
Interestingly, body weight plays a big role in this difference. The gap between D2 and D3 was most pronounced in people with a normal BMI. In people with a BMI over 25, the advantage of D3 over D2 largely disappeared. Researchers identified BMI as the strongest factor influencing the relative response to either form, even more than the starting vitamin D level.
One nuance worth noting: when researchers measured each form’s specific metabolite individually rather than total vitamin D, both D2 and D3 raised their own corresponding blood markers equally well. The total level difference comes from the fact that taking D2 tends to slightly reduce your existing D3 levels, dragging down the overall number.
Which Form You’ll Find in Supplements
Most over-the-counter vitamin D supplements use D3 because of its superior ability to raise blood levels. Prescription vitamin D, however, is often D2. This is partly historical, as D2 was the first form available in high-dose pharmaceutical preparations.
If you follow a vegan diet, you have two options. D2 supplements are inherently plant-based since they come from yeast. Lichen-sourced D3 supplements are also vegan and offer the added benefit of being the more potent form. Either will raise your vitamin D levels, but you may need a higher dose of D2 to achieve the same result as D3.
Stability During Storage and Cooking
The two forms also behave a bit differently when exposed to heat and moisture. In powder form, D2 breaks down faster than D3 when stored in dry air at room temperature. D3 holds up better under dry conditions but degrades quickly at high temperatures.
During cooking, both forms lose some potency, but the losses depend more on the cooking method than on which form you’re dealing with. Baking in an oven for 40 minutes retains only about 39 to 45% of the vitamin D in foods like eggs and margarine, while frying preserves 82 to 84%. Boiling eggs keeps 86 to 88% intact. In baked bread, retention ranges from about 69 to 89% depending on the flour type, with D2 actually holding up slightly better than D3 in that context.
Choosing Between D2 and D3
For most people, D3 is the better choice for supplementation simply because it raises blood levels more efficiently at the same dose. If your doctor has prescribed D2, it will still work, but you may need to take more of it or take it more consistently to reach the same target blood level.
If you’re vegan or vegetarian, lichen-derived D3 gives you the potency advantage without animal-derived ingredients. D2 from UV-treated mushrooms or yeast-based supplements is a solid alternative, especially if cost or availability is a factor. Regardless of which form you choose, taking it with a meal that contains some fat improves absorption, since vitamin D is fat-soluble.