Vitamin D tablets: how much vitamin D3 to take, which form, and whether you need D3 with K2

NHS vitamin D advice: who needs tablets and when

Your skin makes vitamin D from UVB sunlight. In the UK, the NHS says most people can make all they need from late March or early April to the end of September. Between October and early March the sun is too low, and we don’t make enough.

That’s why the NHS advises everyone to consider 10 mcg (400 IU) of vitamin D a day in autumn and winter. Some people should take it all year round:

• anyone who is rarely outdoors, for example because they’re frail or housebound;
• people living in a care home or another institution;
• those who usually cover most of their skin when outside;
• people with dark skin, such as those of African, African-Caribbean or south Asian background;
• all children aged 1 to 4.

Babies under 1 need 8.5–10 mcg a day unless they drink more than 500 ml of infant formula. In pregnancy and breastfeeding, the NHS advice is also 10 mcg. Ask your midwife if you’re unsure.

Which months the sun works for you depends on where you live. Our vitamin D sunlight calendar shows them by city. For other winter nutrients, see vitamins to take in winter.

Can you get enough vitamin D from food?

Rarely. Only oily fish, cod liver oil, egg yolks, liver and fortified foods contain useful amounts. In the UK, some fat spreads and breakfast cereals are fortified, but the amount varies by brand. Compare a normal portion with the NHS daily amount:

One portion of oily fish can cover a day or more. Most other foods give a fraction of 10 mcg, and few people eat mackerel every day, so a supplement is the simplest option in winter.

How much vitamin D3 to take: IU to mcg

UK labels give vitamin D in micrograms (mcg), and many iHerb products show international units (IU) too. The conversion is simple: 1 mcg = 40 IU. For other nutrients, use our IU to mcg converter.

Many popular bottles on iHerb are 5,000 IU. They look just like the 400 IU ones, so read the number, not the packaging. 5,000 IU is above the upper limit for long-term daily use. Doctors prescribe doses like this for a confirmed deficiency, for a limited time, with blood tests.

For children, the NHS limits are lower: no more than 50 mcg (2,000 IU) a day at ages 1 to 10 and 25 mcg (1,000 IU) under 12 months. Follow the label and your health visitor or GP.

Vitamin D vs D3 (and D2)

D3 (cholecalciferol) is the same form your skin makes. A 2012 meta-analysis found D3 raises blood levels more effectively than D2 (ergocalciferol). So choose D3. Standard D3 is made from lanolin, the grease in sheep’s wool, so it isn’t vegan. If you’re vegan, look for D3 from lichen, labelled “vegan D3” or “from lichen”.

Vitamin D3 and K2: should you take them together?

Vitamin K2 (usually as MK-7) is involved in calcium metabolism, and brands often pair it with D3. It sounds logical, but no large trials show that the pair adds benefit at ordinary D3 doses. For most people a combined product does no harm; it simply costs more. For the evidence on K2 and calcium, see our guide to vitamin D3 and K2 with calcium.

If you take warfarin or another vitamin K antagonist, don’t take K2 without your doctor: it changes how the drug works.

Vitamin D and immunity

In a large 2021 meta-analysis, daily vitamin D slightly cut the risk of acute respiratory infections. It was clearer with daily moderate doses (400–1,000 IU) than with large, infrequent “bolus” doses. The benefit was small and showed mainly in children. Vitamin D won’t stop you catching colds, and it doesn’t replace vaccination or treatment. For other nutrients, see the immunity section and our zinc supplement guide.

How and when to take vitamin D3

Vitamin D is fat-soluble, so take it with a meal that has some fat in it: eggs on toast, cheese, oily fish or porridge made with milk. The time of day doesn’t matter. Keeping the bottle next to the kettle and taking it with breakfast works for many people.

• Take it daily: for prevention, daily doses look better than large, infrequent ones.
• Count every source. Vitamin D is also in multivitamins, cod liver oil and fortified foods.

Vitamin D side effects: how much is too much

Too much vitamin D raises blood calcium (hypercalcaemia). Signs include nausea, thirst, passing urine often and weakness. In severe cases it damages the kidneys. This usually follows months of very high doses, not the NHS 10 mcg.

Don’t take vitamin D without a doctor if you have sarcoidosis or another granulomatous disease, kidney stones, high calcium or a parathyroid condition. Ask first, too, if you take thiazide diuretics, digoxin or high-dose calcium. Anti-seizure drugs and steroids can lower vitamin D levels, so ask your GP about the dose. If you want to know your level, read about vitamin D test results first.

How to choose vitamin D tablets on iHerb

1. Choose D3, or D3 from lichen if you’re vegan.
2. Choose the dose: 10 mcg (400 IU) is the NHS amount, and up to 25 mcg (1,000 IU) stays well below the limit. The Endocrine Society (2024) found no extra benefit from routinely going above the recommended intake in healthy adults under 75.
3. Pick the format: tablets, softgels, drops in oil or chewables.
4. Compare the price per daily dose and check the expiry date. Not sure you need vitamin D at all? Try our quick vitamin check.