Vitamin D dosing is surprisingly confusing. The NHS says 400 IU. Many supplements come in 1000 IU or 2000 IU. Some wellness influencers recommend 5000 IU or higher. Your GP might suggest something different again. So what’s actually right?
The short answer: it depends on who you are, where you live, how much sun you get, and whether you’re already deficient. The longer answer — with the actual numbers and the reasoning behind them — is below.
What the official guidelines say
Different bodies recommend different amounts, which is part of the confusion:
NHS / SACN (UK): 10 micrograms (400 IU) per day for everyone over the age of 1, including during pregnancy and breastfeeding. This is framed as a baseline to prevent deficiency in the general population, not as an optimal dose for someone who’s already low.
EFSA (EU): 15 micrograms (600 IU) per day as an Adequate Intake for adults. Slightly higher than the UK figure.
Tolerable Upper Limit (EFSA): 100 micrograms (4000 IU) per day for adults — the maximum considered safe for long-term daily use without medical supervision.
Endocrine Society (clinical guidance): Has historically suggested 1000–2000 IU daily for adults at risk of deficiency, and higher therapeutic doses (sometimes 50,000 IU weekly for 8 weeks) for people with confirmed deficiency — but always under medical supervision.
Why the numbers vary so much
The UK’s 400 IU recommendation is deliberately conservative — it’s the amount estimated to keep 97.5% of the population above a minimum threshold (25 nmol/L blood level) even with zero sun exposure. It’s a floor, not a target.
Many researchers and clinicians argue that the optimal blood level is higher (50–75 nmol/L or even 75–125 nmol/L depending on who you ask), and that reaching those levels requires more than 400 IU daily, particularly for people with limited sun exposure, darker skin, or higher body weight.
This doesn’t mean 400 IU is “wrong” — it means it’s the minimum public health recommendation, and your individual needs might be higher depending on your circumstances.
How much do YOU probably need?
Rather than one number for everyone, here’s a more practical breakdown:
400 IU (10mcg) — the baseline
Suitable if you eat oily fish regularly, get reasonable sun exposure in summer, are under 50, and have no specific risk factors. This is the NHS-recommended daily minimum.
1000–2000 IU (25–50mcg) — common maintenance dose
More appropriate if you have limited sun exposure (office worker, northern latitude, autumn/winter in the UK), have darker skin, are over 50, or follow a diet low in oily fish. This is the range most standalone vitamin D supplements are formulated in, and it sits well within EFSA’s safe upper limit.
2000–4000 IU (50–100mcg) — higher maintenance
Sometimes recommended for people with obesity (vitamin D is sequestered in fat tissue, reducing bioavailability), malabsorption conditions, or those whose blood tests show levels consistently on the low side despite standard supplementation. Best guided by a blood test rather than guesswork.
Above 4000 IU — medical supervision only
Doses above EFSA’s tolerable upper limit should only be taken on the advice of a healthcare professional, typically to correct a confirmed, significant deficiency. Vitamin D is fat-soluble and can accumulate to toxic levels (hypervitaminosis D), causing nausea, kidney problems, and dangerously high calcium levels — though this is rare at sensible doses.
When does sun exposure count?
Your skin produces vitamin D when exposed to UVB radiation — but only under specific conditions:
- In the UK, UVB rays are only strong enough for vitamin D synthesis roughly from late March to September, and primarily between 11am and 3pm
- 10–15 minutes of midday sun on exposed forearms and face (without sunscreen) is often cited as sufficient for lighter-skinned individuals during summer months
- Darker skin requires significantly longer exposure to produce the same amount
- Sunscreen with SPF 15+ blocks roughly 93% of UVB — necessary for skin cancer prevention, but it does reduce vitamin D synthesis
- Glass blocks UVB entirely — sitting by a window doesn’t count
- From October to March in the UK and most of Northern Europe, the sun is too low in the sky for any meaningful vitamin D production, regardless of time spent outdoors
This is why the NHS specifically recommends that everyone in the UK consider supplementing during autumn and winter, and that higher-risk groups supplement year-round.
Can you take too much?
Yes, but it’s harder than the internet suggests. Toxicity is extremely rare below 10,000 IU daily taken over extended periods, and the vast majority of reported cases involve accidental mega-dosing (manufacturing errors) or deliberate very-high-dose supplementation without medical guidance.
At standard supplement doses (1000–2000 IU daily), the risk of toxicity is essentially zero. The more realistic risk isn’t toxicity but wasted money — taking 4000 IU daily when your levels are already adequate doesn’t provide additional benefit, it just raises blood levels into a range where there’s no further health return.
A blood test (25-hydroxyvitamin D) removes the guesswork entirely and costs very little through your GP.
Vitamin D3 vs D2 — does it matter?
D3 (cholecalciferol) is the form produced by human skin and is generally more effective at raising and maintaining blood levels. Most evidence and most supplement formulations favour D3.
D2 (ergocalciferol) is plant/fungus-derived and was historically used in prescription supplements. It works, but is less potent per unit and clears from the body faster.
For most people, D3 is the better choice. For strict vegans, D3 derived from lichen (rather than lanolin) is available, or D2 remains a functional alternative.
What about vitamin K2?
Some higher-strength vitamin D supplements include vitamin K2 (typically as MK-7). The rationale: vitamin D increases calcium absorption, and K2 helps direct that calcium toward bones and teeth rather than soft tissues and arteries. This is a plausible and increasingly popular combination, particularly at D3 doses of 1000 IU and above, though the clinical evidence for the specific pairing is still developing.
It’s not essential — plenty of people supplement D3 alone without any issue — but it’s a reasonable addition, not a marketing gimmick.
Our pick: Terranova Vitamin D3 2000 IU + K2 Complex — a higher-strength D3/K2 combination for people with limited sun exposure.
→ Full guide: Vitamin D Deficiency Signs, Sources & How Much You Need
The bottom line
For most UK/EU adults: 1000–2000 IU of vitamin D3 daily from October to March is a sensible, well-supported baseline. Year-round if you’re in a higher-risk group. A blood test is the only way to know your actual level and tailor accordingly.
400 IU is a minimum, not a goal. 4000 IU is a ceiling, not a target. Somewhere in between is where most people will land.
Not sure where you fall? Our free AI analysis factors in your sun exposure, diet, and lifestyle to recommend a personalised plan.
FAQ
Should I take vitamin D every day or once a week? Daily is generally preferred for maintaining steady blood levels. Some GPs prescribe high weekly doses (e.g. 20,000–50,000 IU once a week) to correct an existing deficiency, but for ongoing maintenance, a daily dose of 1000–2000 IU is simpler and better studied.
Can I get enough vitamin D from food alone? It’s very difficult. The richest dietary source is oily fish — you’d need roughly 2–3 portions per day to reach 1000 IU from food alone. Most people in the UK/EU rely on a combination of summer sun exposure, modest dietary intake, and supplementation.
Do I need a blood test before supplementing? At standard doses (1000–2000 IU), supplementation is considered safe for the general population without testing — the NHS recommends it broadly. A blood test becomes more useful if you want to know your actual level, if you’re considering higher doses, or if you’ve been supplementing and still feel symptomatic.
I take a multivitamin with 400 IU of vitamin D — is that enough? It meets the UK minimum recommendation, but if you’re in a higher-risk group (limited sun, darker skin, over 50, overweight), it may not be sufficient to maintain optimal levels. A standalone vitamin D supplement at 1000–2000 IU is a common next step.
Can I get vitamin D from a sunbed? Sunbeds do produce UVB, but the skin cancer risk far outweighs any vitamin D benefit. NHS, WHO, and cancer research organisations all advise against sunbed use for any purpose. A supplement is safer, cheaper, and more reliable.
Sources & Further Reading
- NHS — Vitamin D
- SACN — Vitamin D and Health report (2016)
- NIH Office of Dietary Supplements — Vitamin D Fact Sheet
- EFSA — Dietary Reference Values for vitamin D
- Linus Pauling Institute — Vitamin D Micronutrient Information
- Examine.com — Vitamin D
This article is for general educational purposes only and is not medical advice. If you suspect a deficiency or are considering doses above 4000 IU daily, please consult your GP.
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