Vitamin D3 and D2 are both forms of vitamin D, but they come from different sources, behave differently in your body, and are not equally effective at raising your blood levels. Vitamin D3 (cholecalciferol) is the form your skin produces naturally in sunlight and is generally the better choice for supplementation. Vitamin D2 (ergocalciferol) comes primarily from fungi and is less potent at building and maintaining your vitamin D stores.
Where Each Form Comes From
Vitamin D3 is the animal and human form. Your skin manufactures it when ultraviolet B light hits a cholesterol compound called 7-dehydrocholesterol. It’s also found naturally in fatty fish, egg yolks, and liver. The D3 in fish originates from planktonic microalgae at the base of the ocean food chain, which is why fish are the richest natural dietary source. Interestingly, D3 has also been identified in the leaves of certain plant families, though in very small amounts.
Vitamin D2 is produced when UV light hits a different precursor compound in fungi and yeast. Wild mushrooms are one of the only significant food sources. Small amounts show up in plants contaminated with fungi, and even cow’s milk contains trace D2 picked up from grass and hay. Because of its fungal origin, D2 has traditionally been the go-to form for vegan supplements.
How They Differ in Your Body
Both D2 and D3 must be processed by your liver before they become useful. The liver converts them into the circulating form your doctor measures on a blood test (25-hydroxyvitamin D). But the two forms don’t go through this process equally well. D2 has a slightly different molecular shape: an extra double bond and a methyl group on its side chain. These structural tweaks matter more than they sound.
The shape differences mean D2 binds less tightly to the transport protein that carries vitamin D through your bloodstream, so it gets cleared faster. One of the liver’s key processing enzymes can convert D3 but cannot process D2 at all. Another liver enzyme handles both forms at similar rates, but with only partial processing available, D2 starts at a disadvantage. The result is that D2 is converted less efficiently, clears from your blood faster, and delivers less usable vitamin D per dose.
D3 Raises Blood Levels Significantly More
Head-to-head studies make the potency gap clear. In a clinical trial comparing equal high doses of D2 and D3, participants taking D3 reached final blood levels of about 51 ng/mL, while those taking D2 reached only about 34 ng/mL. That’s a roughly 50% higher endpoint with D3. The increase from baseline was also telling: D3 boosted levels by 27.6 ng/mL compared to just 12.2 ng/mL for D2, more than double the rise.
D3 also holds those levels longer. The circulating form of D3 has a half-life of about 15 days, while D2’s half-life is about 14 days. That one-day difference compounds over time, especially with weekly or monthly dosing. The NIH notes that D3 both raises blood levels to a greater extent and maintains them longer than D2, even though both forms are well absorbed in the gut. Both can cure rickets, but D3 is simply more efficient at keeping your stores topped up.
What This Means for Supplements
Most over-the-counter vitamin D supplements sold today contain D3, and this is the form most experts prefer. The recommended daily intake is the same regardless of form: 600 IU (15 mcg) for adults up to age 70, and 800 IU (20 mcg) for those over 70. Infants need 400 IU daily. These recommendations are designed to maintain bone health and normal calcium metabolism.
If you’re taking D2, you’re not wasting your time. It does raise blood levels and it does work. But you may need higher or more frequent doses to achieve the same result as D3. Prescription vitamin D in the United States has historically been D2, partly because of how pharmaceutical regulations developed. If your doctor prescribes D2 specifically, there’s usually a reason, but for general supplementation, D3 is the more effective option.
D3 Options for Vegans
Conventional D3 supplements are made from lanolin, a waxy substance extracted from sheep’s wool after shearing. The lanolin contains the same cholesterol precursor found in human skin, which is then exposed to UV light to produce D3. This makes standard D3 an animal-derived product.
For vegans, D2 from mushrooms or yeast was long the only option. That’s changed. Lichen, a symbiotic organism made up of fungi and algae, naturally contains the same precursor compound found in human skin. When exposed to UV light at controlled temperatures, lichen produces genuine cholecalciferol, identical to the D3 your body makes. Vegan D3 supplements sourced from lichen are now widely available, giving plant-based eaters access to the more effective form without compromising on ethics.
Cooking and Storage Stability
If you’re getting vitamin D from food, you might wonder whether cooking destroys one form faster than the other. Research on this is reassuring: there’s no meaningful difference in how D2 and D3 hold up during heat treatment. Both survive pasteurization, boiling, and pan-frying with minimal losses. D3 in fortified cheese showed no degradation over nine months of storage at either room temperature or refrigerator temperature, and D2 in fortified milk held steady through pasteurization, boiling, and sterilization. You don’t need to worry about losing your vitamin D to everyday cooking.
Safety at High Doses
Because D3 is more potent at raising blood levels, it also carries a slightly higher risk of pushing levels too high if you take excessive doses over long periods. The tolerable upper intake level set by nutrition authorities is 4,000 IU per day for adults, and this applies to both forms combined. Vitamin D toxicity is rare and almost always the result of megadose supplementation, not food or sun exposure. The key safety concern with any vitamin D excess is a dangerous buildup of calcium in the blood, which can affect the kidneys and heart.
At standard supplement doses (600 to 2,000 IU daily), neither form poses a safety concern for most people. The practical takeaway is straightforward: D3 does more per unit than D2, so you need less of it to get where you want to be.