Vitamin D3 supplements: what the evidence says

Vitamin D promotes calcium absorption and helps keep bones healthy. The body makes it from sunlight on the skin, and small amounts come from foods such as oily fish, egg yolks and fortified foods. Plenty of sun doesn't guarantee good levels: according to the NIH, older people, people with darker skin, people who spend most of their time indoors and people who cover most of their skin are less able to make enough vitamin D from sunlight. Vitamin D3 (cholecalciferol) is the form the skin makes and is widely sold in South African pharmacies. People take it for immunity, bones and general health. The evidence supports some of those reasons far better than others.

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.

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 context, the US National Institutes of Health (NIH) recommends 600 IU (15 µg) a day for most adults and 800 IU (20 µg) over 70, with an upper limit of 4,000 IU (100 µg) a day from age 9. The UK's NHS and the European Food Safety Authority (EFSA) give the same 100 µg (4,000 IU) upper limit 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 an ambulance on 10177, or 112 from a cell phone.

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 your doctor or pharmacist before taking more than a standard daily dose if you take any of these medicines or have a kidney or parathyroid condition.

South African regulatory status

In South Africa, vitamin D supplements are regulated by the South African Health Products Regulatory Authority (SAHPRA) as complementary medicines in the "health supplements" group (Category D) under the Medicines and Related Substances Act. Labels must carry the words "Complementary Medicine" and "Health Supplement", and any product SAHPRA has not registered must carry this exact disclaimer:

This unregistered medicine has not been evaluated by the SAHPRA for its quality, safety or intended use.

A product claiming to treat or prevent a disease would need registration as a medicine. The framework continues to be updated, so details can change.

What to look for when buying

• Strength in IU and µg. 1,000 IU equals 25 µg. Products above 4,000 IU (100 µg) exceed the adult upper limit used by the NIH, NHS and EFSA, so take them only on medical advice.
• %NRV. Labels may show vitamin D as a percentage of the nutrient reference value. Biogen's Vitamin D3 1000IU, for example, lists 1,000 IU as 167% NRV. A figure above 100% isn't a warning in itself.
• Capsule and source. Some capsules use bovine gelatine; Biogen's Vitamin D3, for example, lists a bovine gelatine capsule shell. According to the NIH, D3 is typically made from lanolin (sheep's wool), with an animal-free lichen version also available.
• The SAHPRA disclaimer. The unregistered-medicine disclaimer and the words "Complementary Medicine" and "Health Supplement" show the product is labelled under the current rules. Biogen's label, for example, carries "Category D 34.11 Vitamins".
• Your total intake. Vitamin D is often combined with calcium, magnesium or vitamin K2, and it's in most multivitamins. Add up your total.

FAQ

Do South Africans need vitamin D supplements with all our sun?

Not necessarily. The NIH lists older adults, people with darker skin, people who are mostly indoors and people who cover most of their skin as more likely to run low.

Should I get my vitamin D level checked?

The Endocrine Society suggests against routine testing in the populations it considered. Your doctor may test you if you have symptoms or a condition that affects absorption.

Why does my label say "unregistered medicine"?

The regulations require that disclaimer on any complementary medicine SAHPRA hasn't registered. It doesn't mean the product is illegal or unsafe; it means SAHPRA hasn't assessed it.

Is vitamin D3 better than D2?

The NIH says most evidence shows D3 raises blood levels more and keeps them up longer. In a 2012 meta-analysis, the advantage was clear with large, infrequent doses but not with daily dosing.

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