Vitamin D Supplements: D3 vs D2, Units and Dosage

Vitamin D is the clearest example of why supplement labels need reading twice. The same dose appears as international units on one bottle and micrograms on the next, and the two forms sold are not interchangeable in the way the shelf implies.

IU and micrograms are the same thing

1,000 IU and 25 mcg describe an identical amount of vitamin D. Products across this category span roughly 400 IU to 10,000 IU per softgel, and because both units are in use, two bottles can look wildly different while being the same strength. Our conversion guide covers the arithmetic and the common labelling patterns.

D3 and D2

D3 (cholecalciferol) is the more commonly sold form and is usually derived from lanolin, though lichen-derived D3 exists for anyone avoiding animal sources. D2 (ergocalciferol) is plant-derived. They are not equivalent at the same number, and which one a product uses is stated on the panel rather than the front. See vitamin D3 vs D2.

Combination products

Many vitamin D supplements now include vitamin K2, and some include magnesium. Where they do, each nutrient has its own dose on the panel and each needs reading — a product marketed on its vitamin D content may carry a token amount of the others. Our guide to D and K2 together covers what to look for.

Dose, upper limits and testing

Vitamin D is fat-soluble and accumulates, which is why it has an established upper intake level and why high-dose products deserve more care than most. Deficiency is diagnosed with a blood test rather than inferred from symptoms, and a test is the right first step if you suspect you are low. Our dosage guide sets out what the guidelines say; it is not a substitute for advice from your doctor.

Sunlight, food and supplements

Vitamin D is unusual among nutrients in being synthesised in skin exposed to UVB, which is why intake advice varies with latitude and season — an important point in Ireland and the UK, where several health authorities issue seasonal guidance. Dietary sources are limited. See food and sun vs supplements.

Comparing products

Our vitamin D supplement comparison filters by strength, form, format, brand and pack size, so you can line up doses in the same units before choosing.

References

• NIH Office of Dietary Supplements — Vitamin D (Health Professional Fact Sheet)
• NHS — Vitamin D
• Health Service Executive (Ireland) — Vitamin D