Walk into any pharmacy and you'll find vitamin D supplements in two forms: D3 (usually labelled "cholecalciferol") and D2 ("ergocalciferol"). Both are sold as vitamin D. Both are used in fortified foods. The difference matters more than most packaging lets on.
What D3 and D2 Are
Vitamin D3 (cholecalciferol) is the form your skin produces when exposed to UVB radiation from sunlight. It's also found in animal foods: oily fish (salmon, mackerel, sardines), egg yolks, beef liver, and fortified dairy products. Most supplement manufacturers use D3 derived from lanolin (sheep's wool) or from lichen (for vegan-friendly versions).
Vitamin D2 (ergocalciferol) is produced by plants and fungi when exposed to UV light. UV-treated mushrooms are a natural food source. D2 is the form historically used in fortification (particularly in the US for fortified milk) and was the original prescription-dose vitamin D.
Neither form is active in the body until converted. Both go through the same two-step process: the liver converts them to 25-hydroxyvitamin D (25(OH)D - the form measured in blood tests), then the kidneys convert that to 1,25-dihydroxyvitamin D (calcitriol - the active hormone form).
What the Research Shows
The comparison between D3 and D2 has been studied directly in multiple clinical trials. The consistent finding: D3 is more effective at raising and maintaining blood 25(OH)D levels.
A 2012 meta-analysis in the American Journal of Clinical Nutrition - analysing seven randomised controlled trials - found that D3 supplementation raised serum 25(OH)D levels significantly more than D2 at equivalent doses. The effect was clearest in studies lasting 14 days or more, suggesting D3 maintains higher levels over time even after supplementation stops.
The proposed mechanisms: D3 is converted to 25(OH)D in the liver more efficiently than D2, and 25(OH)D3 (the D3 metabolite) has greater affinity for vitamin D binding proteins in the blood, giving it a longer half-life. D2's metabolites are cleared faster.
A 2013 study in Bone found that D3 was approximately 87% more potent than D2 in raising blood 25(OH)D levels. Other studies show a less dramatic difference (25-50% more effective), but the direction is consistent: D3 wins.
When D2 Is Used
D2 isn't inferior in all contexts. High-dose prescription vitamin D (50,000 IU doses used to correct severe deficiency) is often dispensed as D2 - the D3 equivalent prescription forms are less standardised in some markets. D2 does raise levels; it's just less efficient per mg.
For vegans who prefer to avoid animal-derived products: traditional D3 from lanolin is animal-derived. Lichen-based D3 supplements offer a plant-sourced D3 alternative with the same efficacy as standard D3. D2 from UV-treated yeast or mushrooms is another vegan option, though less effective per dose.
Vitamin D and Magnesium: The Overlooked Relationship
This is frequently missed in the D3 vs D2 debate: magnesium is essential for vitamin D metabolism. The enzymes that convert vitamin D (both D2 and D3) to its active forms are magnesium-dependent.
People who are magnesium-deficient may have impaired vitamin D conversion - meaning supplementing D3 without adequate magnesium may not raise active vitamin D levels as effectively. Signs low in magnesium are common, and the two deficiencies often co-occur.
Taking magnesium alongside vitamin D supplementation is well-reasoned for anyone who suspects their diet is magnesium-poor.
Vitamin D3 and K2: Should They Be Combined?
Vitamin D increases calcium absorption from the gut. Vitamin K2 directs calcium into bones and away from soft tissues (arteries, kidneys). The concern is that high-dose vitamin D supplementation without adequate K2 may increase calcium in places you don't want it.
The research on this combination is not yet conclusive enough to say K2 is definitively necessary alongside vitamin D. But it's plausible, and K2 supplementation at 90-200mcg daily (as MK-7 form, which is better absorbed than MK-4) is used in many quality vitamin D products.
Many D3 supplements now come combined with K2 for this reason. If you're taking doses above 2,000 IU daily, co-supplementation with K2 is a reasonable precaution.
What Deficiency Looks Like (and Who's at Risk)
Vitamin D deficiency symptoms covers this in full detail. Brief summary: fatigue, bone pain, muscle weakness, frequent illness, and mood changes are the most common presentations. Depression, particularly seasonal, has consistent associations with low vitamin D in research.
Risk groups: people who spend little time outdoors, people with darker skin (melanin reduces UV conversion efficiency), people in northern latitudes (UK, Canada, Scandinavia - UVB barely reaches skin from October to March), older adults (skin conversion efficiency decreases with age), people with obesity (vitamin D is fat-soluble and can be sequestered in adipose tissue), people wearing full-body covering.
How to Take D3 Effectively
With fat. Vitamin D is fat-soluble - it absorbs significantly better when taken with a fat-containing meal. A 2010 study in the Journal of Bone and Mineral Research found that taking vitamin D with a high-fat meal increased absorption by 32% compared to a fat-free meal.
Timing. Most people take it with breakfast or lunch. Some evidence suggests avoiding nighttime supplementation if sleep disturbance is a concern (vitamin D may affect melatonin production at high doses), though this is a minor consideration at standard doses.
Dosage guidance is covered in detail in how much vitamin D per day. The short version: 1,000-2,000 IU daily is a reasonable maintenance dose for most adults in northern latitudes. Get your blood 25(OH)D tested to see where you actually sit - deficiency (under 50 nmol/L in the UK, under 20 ng/mL in the US) is surprisingly common.
Supplement Comparison Table
| Feature | Vitamin D3 (Cholecalciferol) | Vitamin D2 (Ergocalciferol) |
|---|---|---|
| Source | Animal (lanolin) or lichen | Plant/fungi |
| Blood level effectiveness | Higher | Lower per dose |
| Half-life in blood | Longer | Shorter |
| Vegan options | Yes (lichen-based D3) | Yes |
| Typical supplement doses | 400-4000 IU | 400-50,000 IU (prescription) |
| With K2? | Often combined | Rarely |
| Cost | Low | Low |
What to Buy
For most people, a standard D3 supplement of 1,000-2,000 IU is the practical choice. If you're vegan, look for "lichen-sourced D3" rather than D2 for equivalent efficacy.
If you're taking more than 2,000 IU daily, a D3+K2 combination product is worth the minimal additional cost.
If you've been prescribed high-dose vitamin D by a doctor and received D2 prescription tablets - that's fine, it will raise your levels. D3 is preferable for self-purchased supplementation because of better per-dose efficiency.