Vitamin D: do you need 400 IU or 4,000 IU?

Many best-selling vitamin D tablets are 4,000 IU, ten times the NHS recommendation. Here's what the official advice says, who needs more, and whether adding K2 helps.

By Carol Standing · Published 4 November 2026 · 2 min read

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Search Amazon for vitamin D and many of the best-sellers are 4,000 IU, often with vitamin K2 added. That's ten times what the NHS recommends. Is it better?

What the official advice says

The NHS advises that adults consider taking 10 micrograms (400 IU) of vitamin D a day in autumn and winter, when sunlight in the UK isn't strong enough for your skin to make enough.

Some people should consider taking it all year round, including people who:

aren't often outdoors, for example if they're frail or housebound
live in a care home
usually wear clothes that cover most of their skin outdoors

The NHS also says people with dark skin may not get enough vitamin D from sunlight in summer.

The NHS's safety limit: don't take more than 100 micrograms (4,000 IU) a day, as it could be harmful. Over time, too much vitamin D can cause calcium to build up in the body, which can weaken bones and damage the kidneys and heart.

So a 4,000 IU tablet is at the maximum safe daily amount. It's not a "better" version of the recommended dose.

Why are high-dose tablets so popular?

Higher numbers look stronger on a product listing. Some doctors also prescribe higher doses to people with a confirmed deficiency, usually for a set period, and that advice can get passed around online as if it applies to everyone.

If you think you might be deficient, ask your doctor about a test rather than guessing at a dose.

What about vitamin K2?

Many high-dose tablets add K2 (MK-7). The approved claim for vitamin K in the UK is that it contributes to normal blood clotting and to the maintenance of normal bones. We haven't found strong evidence that healthy adults taking a standard vitamin D dose need K2 as well.

Important: if you take warfarin or another blood thinner, speak to your doctor before taking anything containing vitamin K.

What vitamin D does

Approved health claims for vitamin D include that it contributes to the maintenance of normal bones and teeth, normal muscle function and the normal function of the immune system. Be wary of products that promise more, such as boosting energy, mood or weight loss.

Our picks

Best for: Most adults in autumn and winter (UK advice)

Vitamin D3 10 micrograms (400 IU) daily tablets

Matches the NHS recommendation exactly. Usually the cheapest option too. A year's supply often costs very little.

See options on Amazon

Best for: People who want a middle dose, after checking with a pharmacist

Vitamin D3 25 micrograms (1,000 IU) daily

Still well below the NHS maximum of 100 micrograms. Some people choose this if they're rarely outdoors. Check with a pharmacist or doctor if you're unsure.

See options on Amazon

Our verdict

Fiction. Most adults don't need 4,000 IU a day. The NHS recommends 400 IU (10 micrograms) in autumn and winter, and 4,000 IU is the upper safe limit. A simple, inexpensive 10 microgram tablet does the job. Take higher doses only if a doctor advises it.

Common questions

What's the difference between micrograms and IU?

They're two ways of measuring the same thing. The NHS says 1 microgram of vitamin D equals 40 IU, so 10 micrograms = 400 IU and 100 micrograms = 4,000 IU.

Is vitamin D3 better than D2?

Both raise vitamin D levels. Most over-the-counter supplements use D3. Vegans may prefer D2 or a plant-based (lichen) D3.

Should I take vitamin K2 with vitamin D?

There isn't strong evidence that most healthy adults need K2 alongside vitamin D. If you take a blood thinner such as warfarin, don't take vitamin K supplements without speaking to your doctor first, because vitamin K affects how those medicines work.

Can I take vitamin D all year?

Yes. The NHS says people at higher risk of low vitamin D, such as those who are rarely outdoors, live in a care home or usually cover their skin outdoors, should consider taking it all year round.

This article is general information, not medical advice. Speak to a doctor, pharmacist or other qualified clinician about your own health, especially before starting a supplement or if symptoms are new, severe or worrying.

Got a question about this, or a claim you want checked? Ask us.

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