Vitamin D3 supplements: what the evidence says

Vitamin D helps regulate calcium and phosphate in the body, which are needed to keep bones, teeth and muscles healthy. The body makes it from direct sunlight on the skin, but between October and early March people in the UK don't make enough from sunlight. It's found in a small number of foods, including oily fish, red meat, liver, egg yolks and fortified foods. Vitamin D3 (cholecalciferol) is the form the skin makes and the one most supplements use. People also take it hoping for fewer colds or protection against heart disease and cancer. The evidence for those is much weaker.

What the evidence shows

This review only counts human studies. Grades mean: strong (consistent results from several good-quality trials or meta-analyses), moderate (good trials exist, but effects are small, trials are few or results vary), weak (small or low-quality studies, mixed results) and insufficient (too little human research to judge). For some outcomes the strong evidence points to no benefit, and the table says so.

• Raising and maintaining blood vitamin D. Strong. Supplements reliably raise blood levels of 25-hydroxyvitamin D, the standard marker. Vitamin D sufficiency prevents rickets in children and osteomalacia (soft bones) in adults.
• Vitamin D3 vs D2 at raising blood levels. Moderate. A 2012 meta-analysis found D3 raised blood levels more than D2 overall. The advantage was clear with large, infrequent (bolus) doses but was lost with daily dosing.
• Fractures in generally healthy adults (vitamin D alone). Strong, no benefit. In the US VITAL trial (25,871 midlife and older adults, median 5.3 years), 2,000 IU a day did not lower total, hip or non-vertebral fractures. A Cochrane review found high-quality evidence that vitamin D alone is unlikely to prevent hip or any new fracture in older people.
• Fractures in older people (vitamin D plus calcium). Moderate. The same Cochrane review found that vitamin D with calcium produced a small reduction in hip fractures. The absolute benefit was larger in people living in institutions (about nine fewer hip fractures per 1,000 a year) than in the community (about one fewer).
• Respiratory infections such as colds. Weak. A 2021 meta-analysis found a small reduction, mainly with daily doses. A 2025 update (40 trials, 61,589 people) found the overall effect was no longer statistically significant.
• Heart disease and cancer in healthy adults. Strong, no benefit. VITAL found 2,000 IU a day did not reduce invasive cancer or major cardiovascular events over a median of 5.3 years.
• Progression from prediabetes to type 2 diabetes. Weak. The D2d trial (2,423 adults, 4,000 IU a day) found no statistically significant reduction in diabetes. The Endocrine Society's 2024 guideline nonetheless suggests supplementation for adults with high-risk prediabetes, based on pooled trial data.

What this means in practice

The clearest case for vitamin D is preventing deficiency. For people who already have adequate levels, large trials have mostly found no extra benefit for bones, heart, cancer or infections. The Endocrine Society's 2024 guideline suggests against routine supplementation above the current dietary reference intakes for healthy adults under 75, and against routine blood testing for vitamin D in the populations it considered. It suggests supplementation for children and teenagers aged 1 to 18, people who are pregnant, adults aged 75 and over and people with high-risk prediabetes. For non-pregnant people over 50 who take vitamin D, it favours daily doses over large intermittent ones. That US guideline does not replace NHS advice.

Doses used in studies

These are the amounts researchers gave participants. They aren't recommendations. One microgram (µg) of vitamin D equals 40 IU.

The VITAL trial used 2,000 IU (50 µg) a day. The D2d prediabetes trial used 4,000 IU (100 µg) a day. In the 2021 respiratory infection meta-analysis, the protective effect was seen in trials that gave daily doses equivalent to 400 to 1,000 IU (10 to 25 µg). The trial that found more falls and fractures used a single 500,000 IU dose once a year.

For comparison, the NHS says everyone should consider a daily 10 µg (400 IU) supplement during autumn and winter. It advises 10 µg a day all year for adults and children over 4 who are not often outdoors, live in an institution such as a care home, or usually wear clothes that cover up most of their skin. People with dark skin, for example of African, African-Caribbean or south Asian background, may not make enough vitamin D from sunlight and should also consider 10 µg a day all year. Children aged 1 to 4 should have a daily 10 µg supplement all year, and babies have separate advice. The NHS says adults (including during pregnancy and breastfeeding) and children aged 11 to 17 should not take more than 100 µg (4,000 IU) a day, children aged 1 to 10 no more than 50 µg (2,000 IU), and infants under 12 months no more than 25 µg (1,000 IU). The European Food Safety Authority (EFSA) also sets 100 µg a day as the upper level for adults.

Safety, side effects and interactions

Too much vitamin D over time can raise calcium in the blood (hypercalcaemia). According to the NIH, this can cause nausea, vomiting, muscle weakness, confusion or other neuropsychiatric symptoms, pain, loss of appetite, dehydration, passing a lot of urine, excessive thirst and kidney stones; in extreme cases it can lead to kidney failure, irregular heart rhythm and death. The NHS notes it can weaken bones and damage the kidneys and heart. The NIH says toxicity is almost always caused by supplements, for example products with manufacturing errors, products taken inappropriately or in excessive amounts, or doses prescribed incorrectly. You can't overdose on vitamin D from sunlight.

Dosing pattern may matter too. In a randomised trial of older women, a single 500,000 IU dose once a year led to more falls and fractures than placebo.

If someone has taken a large amount of vitamin D, or has severe vomiting, confusion, extreme thirst or muscle weakness after taking high-dose vitamin D, call 999. For urgent advice that isn't an emergency, call NHS 111.

Medicines the US NIH describes as interacting with vitamin D:

• Orlistat (a weight-loss medicine), taken with a reduced-fat diet, can reduce absorption of vitamin D.
• Statins: because vitamin D is made from cholesterol, statins may reduce vitamin D synthesis. High intakes of vitamin D, especially from supplements, might reduce the potency of atorvastatin, lovastatin and simvastatin.
• Steroid medicines such as prednisolone and prednisone can reduce calcium absorption and impair vitamin D metabolism.
• Thiazide diuretics reduce calcium loss in urine. Combined with vitamin D supplements they might lead to hypercalcaemia, especially in older adults and people with reduced kidney function or hyperparathyroidism.

Speak to a GP or pharmacist before taking more than a standard daily dose if you take any of these medicines or have a kidney or parathyroid condition. In pregnancy and breastfeeding, the NHS advises considering 10 µg a day.

UK regulatory status

Vitamin D supplements are sold in the UK as food supplements, under the Food Supplements (England) Regulations 2003 (as amended) and equivalent rules in Scotland, Wales and Northern Ireland. Cholecalciferol (D3) and ergocalciferol (D2) are both permitted forms. Products don't need approval before sale; local authorities enforce the rules. Labels may only use authorised health claims from the GB Nutrition and Health Claims Register. Women and children who qualify for the Healthy Start scheme can get free supplements containing vitamin D.

What to look for when buying

• Strength in µg and IU. The NHS says 10 µg (400 IU) a day is enough for most people. Higher-strength products are within the adult upper limit up to 100 µg (4,000 IU), but children's limits are lower.
• D3 vs D2. According to the NIH, vitamin D3 is typically made from lanolin (sheep's wool), so it isn't vegan; an animal-free D3 from lichen is also available, and D2 is made from yeast.
• Your total intake. Check vitamin D in multivitamins and combination products so your total stays within NHS limits.
• Claims. Only authorised health claims are allowed on UK food supplements.

FAQ

How much vitamin D should I take in the UK?

The NHS advises considering 10 µg (400 IU) a day in autumn and winter, and all year if you rarely get sun on your skin, usually cover up outdoors, live in a care home, or have dark skin. It advises adults not to exceed 100 µg (4,000 IU) a day.

Is vitamin D3 better than D2?

D3 raised blood levels more than D2 in a 2012 meta-analysis, mainly with large, infrequent doses. With daily dosing, that study found no clear difference.

Does vitamin D stop colds?

The 2025 pooled analysis of trials found no statistically significant protection overall.

Should I get my vitamin D level checked?

The Endocrine Society suggests against routine testing in the general population. If you have symptoms or a medical condition, speak to your GP.

Can I take too much vitamin D?

Yes. Taking too much over a long period can cause hypercalcaemia. Stay within NHS limits unless a doctor has advised a different amount.

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