Vitamin D Supplements: Forms, Strengths & How to Choose

Vitamin D supplements look deceptively simple until you start comparing the labels. One product says 400 IU, another says 25µg, another says 3,000 IU. The range includes capsules, tablets, sprays, liquids, gummies, vegan options and combinations containing Vitamin K2.

Most of that complexity disappears once you know what the numbers mean. The useful question isn't "which Vitamin D is strongest?" but which form it contains, how much Vitamin D a serving actually provides, how IU converts to micrograms, and whether other nutrients are included.

This guide explains how to compare Vitamin D supplements using those details, alongside current UK guidance on Vitamin D intake.

What does Vitamin D contribute to?

Vitamin D has several recognised nutritional roles. Among other things, Vitamin D contributes to:

• the maintenance of normal bones and teeth
• the maintenance of normal muscle function
• the normal function of the immune system
• normal absorption and utilisation of calcium and phosphorus
• normal blood calcium levels
• the process of cell division

These are recognised nutritional functions, not claims that Vitamin D treats or prevents any medical condition.

Vitamin D is unusual among nutrients because the body can produce it when skin is exposed to sunlight. In the UK, however, sunlight isn't strong enough between roughly October and early March for the body to make enough this way — which is why supplementation comes up so often in UK guidance.

Vitamin D2 vs D3: what's the difference?

The two main forms you'll encounter are Vitamin D2 and Vitamin D3.

• Vitamin D2 — ergocalciferol — found in some fortified foods and supplements, and can be produced from sources such as fungi and yeast. It appears less often on supplement shelves than D3, though products such as Veganicity Vitamin D 800iu use a plant-based D2.
• Vitamin D3 — cholecalciferol — the form found across the majority of Vitamin D supplements, including capsules, tablets, liquids, gummies and oral sprays.

Both forms raise Vitamin D status. The evidence comparing them is genuinely mixed: a widely cited meta-analysis of randomised controlled trials led by researchers at the University of Surrey concluded that D3 was more efficacious than D2 at raising blood 25(OH)D concentrations, while other trials have found the two broadly comparable at equivalent doses.

The sensible reading is that D2 shouldn't be dismissed as ineffective, and D3 shouldn't be assumed superior in every situation. Form is one consideration alongside strength, source, format and serving size.

Is Vitamin D3 vegan?

Sometimes — but not always.

Traditionally, much supplemental Vitamin D3 has been derived from lanolin, obtained from sheep's wool. That typically makes a product suitable for vegetarians without being appropriate for a strict vegan diet.

Plant-origin Vitamin D3 — usually derived from algae or lichen — is now widely available too, so the letters "D3" on their own no longer tell you whether a supplement is vegan.

Two real examples from our range show the difference clearly:

• BetterYou DLux 3000 states that its D3 is sourced from lanolin — approved by the Vegetarian Society, but noted by the manufacturer as possibly unsuitable for those following a strict vegan diet.
• Cytoplan Daily D3 + K2 states that its cholecalciferol is derived from a vegan source.

If this matters to you, check whether the product is specifically labelled vegan, the stated source of the D3, and the capsule shell and other ingredients.

Vitamin D IU vs µg: what do the numbers mean?

This is the single most useful piece of label knowledge in the category.

Vitamin D strength is expressed two ways: micrograms (µg or mcg) and International Units (IU). These aren't two different ingredients — they're two units describing the same thing.

Common label strengths convert as follows:

• 10µg = 400 IU
• 15µg = 600 IU
• 25µg = 1,000 IU
• 50µg = 2,000 IU
• 75µg = 3,000 IU
• 100µg = 4,000 IU

If you're comparing two products, convert both into the same unit before judging their strength. Many labels helpfully show both — BetterYou DLux 3000, for instance, states "3000IU / 75μg" per spray.

Why can 25µg say 500% NRV when the NHS recommends 10µg?

This trips people up because the NRV and the UK dietary recommendation are two different reference figures.

The adult Nutrient Reference Value used for food labelling is 5µg. That's why a supplement containing 25µg can display 500% NRV, and why Lindens Vitamin D3 1100 IU states 560% NRV on a tablet providing 1,100 IU.

UK public-health guidance, meanwhile, uses a Vitamin D reference intake of 10µg per day for adults and children aged four and over.

%NRV is useful for comparing labels. It isn't a personalised dosage recommendation.

How much Vitamin D do you need in the UK?

Current NHS guidance says children from age one and adults need 10µg (400 IU) of Vitamin D a day.

For adults and children over four, the NHS advises everyone to consider taking a daily supplement containing 10µg during autumn and winter, when UK sunlight isn't strong enough for the body to make sufficient Vitamin D.

Who should consider Vitamin D all year round?

The NHS advises some people to consider a 10µg daily supplement throughout the year, because they may make little or no Vitamin D from sunlight. This includes people who:

• are not often outdoors, including people who are frail or housebound
• live in an institution such as a care home
• usually cover most of their skin when outdoors
• have darker skin, including people of African, African-Caribbean or South Asian backgrounds

Advice for babies and young children differs, so adult products shouldn't automatically be used for children.

How to compare Vitamin D supplements

• Strength per recommended serving — compare µg or IU using the same unit.
• D2 or D3 — check which form the product provides.
• Source — particularly important if you require vegan Vitamin D3.
• Serving size — one capsule, one spray, several drops or a measured liquid dose. Some sprays state a per-spray strength and a different recommended daily number of sprays.
• Format — tablets, capsules, sprays, liquids or gummies.
• Additional nutrients — some formulas include Vitamin K2 or other vitamins and minerals.
• Age suitability — adult, infant and children's products have very different directions.
• Number of servings — compare how long the product lasts based on its recommended serving.
• Dietary suitability — check vegan, vegetarian and allergen information rather than inferring it from the Vitamin D form.

Real examples from our range

Four real products show how strength, format, source and combination formulas differ in practice.

These shouldn't be ranked by which has the largest number. They differ in strength, format, source, age suitability and whether Vitamin D is supplied alone or alongside another nutrient.

Capsules, tablets, sprays or liquid Vitamin D?

Format is largely a practical decision rather than evidence that one delivery method is universally superior.

• Capsules and softgels — a fixed, pre-measured serving, easy to fit into an existing routine.
• Tablets — another measured format, used for both standalone Vitamin D and combination formulas.
• Oral sprays — deliver Vitamin D through a measured number of sprays; useful if you'd rather not swallow a tablet.
• Liquid drops — allow flexible dosing, but the number of drops or volume per dose needs checking carefully.
• Gummies — an alternative format, available in both adult and children's products.

Whatever the format, compare the Vitamin D amount in the recommended serving. A spray isn't directly comparable with a capsule until both are expressed in the same unit — and with sprays and liquids, check whether the headline figure is per spray or per recommended daily dose.

What about Vitamin D3 + K2 supplements?

Some Vitamin D products combine D3 with Vitamin K2.

Vitamin D and Vitamin K are separate nutrients with separate recognised functions. Vitamin D contributes to normal absorption and utilisation of calcium and phosphorus and to the maintenance of normal bones; Vitamin K contributes to normal blood clotting and the maintenance of normal bones.

A D3 + K2 supplement therefore supplies two nutrients in one formula — but that does not mean everybody taking Vitamin D needs a combined product.

When comparing one, check both amounts separately: the Vitamin D3 (in µg and/or IU), the Vitamin K2 (usually µg), the form of K2 (MK-7 where specified) and the serving size.

Vitamin D from food and sunlight

Supplements are only one source of Vitamin D.

From roughly late March until the end of September, most people in the UK can make Vitamin D through sunlight on the skin when outdoors.

Dietary sources include oily fish such as salmon, sardines and mackerel, egg yolks, and some fortified foods such as certain breakfast cereals and fat spreads.

Vitamin D occurs naturally in relatively few foods, which is one reason UK public-health advice recommends considering supplementation during autumn and winter.

Higher-strength Vitamin D and safety

A higher number on the front of the bottle shouldn't automatically be treated as better.

The NHS states that 10µg a day will be enough for most people who choose to take a supplement, and advises adults not to take more than 100µg (4,000 IU) a day, because excessive long-term intake can cause calcium to build up in the body.

That upper amount is a safety limit, not a target to aim for.

Vitamin D supplements: the practical takeaway

Most of the apparent complexity comes down to a handful of genuine differences. When comparing products, ask:

• How many micrograms does the recommended serving provide?
• What is that amount in IU?
• Is it Vitamin D2 or D3?
• If D3, is the source suitable for my dietary requirements?
• Is it a capsule, tablet, spray, liquid or gummy?
• Does it contain Vitamin D alone or additional nutrients such as K2?
• What do the manufacturer's directions say?

Once you're comparing the same units and the same serving size, a Vitamin D shelf becomes far easier to read. The highest IU figure isn't automatically the most appropriate choice — the more useful goal is a clearly labelled product whose strength, format and ingredients match what you're actually looking for.

Sources & further reading

• NHS — Vitamin D
• GOV.UK — Great Britain Nutrition and Health Claims (NHC) Register
• Tripkovic L, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status. American Journal of Clinical Nutrition, 95(6), 1357–1364.

Last reviewed: September 2026.