Randomised evidence covering more than 64,000 people finds little sign that broad-spectrum supplements prevent coughs, colds or infections. Where supplementation clearly works, it is a single nutrient correcting a documented shortfall — and in the UK, that mostly means vitamin D.
Supplement packaging rarely says a multivitamin will stop you catching a cold. It says the product "supports immune function", a phrase with no agreed clinical meaning. The testable version of the claim is narrower and much easier to check: does taking the pill change how often you get ill, or how long it lasts?
That question has been put to randomised controlled trials — studies in which participants are assigned by chance to the supplement or to a dummy pill, so that the two groups differ only in what they swallow. Dozens of such trials now exist, and their results have been pooled in meta-analyses, which combine trials statistically to produce a single estimate.
Those pooled estimates come with a 95 per cent confidence interval: the range of values compatible with the data. Because these results are expressed as ratios comparing the supplement group with the dummy group, a value of 1.00 means no difference at all. If the interval spans 1.00, the trials have not ruled out "no effect". That single test is what Exhibit 01 shows.
Six of the eight intervals cross the no-effect line. The two that do not are highly specific. Vitamin C halves the risk of a cold in people undergoing short bursts of extreme physical stress — the trials were in marathon runners, skiers and soldiers on subarctic exercises, a total of 598 people.2 In the general population, across 10,708 participants, regular vitamin C makes no difference to how often you catch a cold.2
It does shorten colds, modestly and consistently: by 8 per cent in adults and 14 per cent in children.2 On a cold lasting a week — an illustrative figure, not a measured average — 8 per cent is roughly half a day. Taken only once symptoms start, rather than every day beforehand, vitamin C showed no consistent effect at all.2
Vitamin D has the strongest theoretical case of any micronutrient: laboratory work shows its metabolites support the innate immune response to respiratory viruses, and people with low blood levels get more infections.1 In 2021 a meta-analysis of 37 trials found a modest protective effect, with an odds ratio of 0.92 and an interval of 0.86 to 0.99 — just clear of 1.00.1
Six further trials were completed after that, including one enrolling 15,804 people. The 2025 update pooled 46 trials and 64,086 participants. The point estimate barely moved, to 0.94, but the interval now runs to 1.00 exactly, with a p-value of 0.057.1 The authors' own conclusion is that there is no statistically significant protection.
The intuitive escape route — that supplements only help people who are actually short of the nutrient — does not survive the data. Among participants who began the trials profoundly deficient, with blood levels below 25 nanomoles per litre, the odds ratio was 0.98 with an interval of 0.80 to 1.20.1 The 3,806 people in that subgroup show no benefit.
One subgroup did: trials giving 400 to 1,000 international units daily. Exhibit 02 shows the underlying percentages rather than the ratios, because the ratios hide something important.
In the 400 to 1,000 unit band, 53.1 per cent of the vitamin D group reported an infection against 58.7 per cent on placebo: a gap of 5.6 percentage points across 2,305 people in ten trials. In the other three bands the gap is under one percentage point, and above 2,000 units a day the vitamin D group did marginally worse.
A dose window with nothing either side of it is a pattern that should invite suspicion, and the researchers treated it that way. They ran a meta-regression — a test of whether the differences between subgroups are larger than chance would produce — and found no statistically significant interaction for dose size, dosing frequency, trial length or age.1 Their funnel plot also showed asymmetry consistent with small negative trials never being published, which led them to downgrade their own confidence in the evidence.1
"Energy support" is the second big claim on the label. Mechanically it cannot work the way the packaging implies: vitamins supply no energy. Several B vitamins are cofactors in the reactions that release energy from food, but adding more of a cofactor to a system that already has enough does nothing.
The trial evidence matches. A randomised trial in 228 adults over 70 found no effect of a multivitamin-mineral supplement on its primary wellbeing outcome, with scattered secondary findings that the authors themselves treated cautiously.15 Larger reviews of supplementation in healthy adults have not produced a reliable effect on fatigue.
Where fatigue does respond, a specific deficiency is being corrected. In a Swiss trial of 136 women aged 18 to 55 with unexplained tiredness but no anaemia, four weeks of iron reduced fatigue scores by 29 per cent against 13 per cent on placebo, a difference of 0.95 points on a ten-point scale.16 The benefit was confined to women whose ferritin — the blood marker of iron stores — was at or below 50 micrograms per litre.16 Iron did nothing for the women who were not short of iron.
That is the shape of the whole field. Persistent fatigue is a reason to have blood taken, not a reason to buy a pill: iron, vitamin B12, folate and thyroid function are all testable, and guessing wrong costs months.
None of this means UK diets are adequate. The National Diet and Nutrition Survey takes blood samples from a nationally representative sample; its 2019 to 2023 round found that 4 per cent of adults meet the fibre recommendation and 17 per cent manage five portions of fruit and vegetables a day.4
But dietary shortfall and clinical deficiency are different things, and only the second is measured in blood.
Vitamin D is the standout, and it is seasonal. Between January and March the share below the deficiency threshold ran from 21 to 38 per cent across age and sex groups aged 11 and over, falling to between zero and 10 per cent in July to September.4 It is also unequal: in the most deprived fifth of England, 34 per cent of adults aged 19 to 64 were below the threshold.4
This is why UK government advice singles out one nutrient rather than recommending a multivitamin: everyone should consider 10 micrograms — 400 international units — of vitamin D daily from October to March, and year-round for people with darker skin, those who cover up and those rarely outdoors.5 That advice rests on bone and muscle health, not on infection.5
One more figure deserves separating out because it uses a different threshold. Among women of childbearing age, 83 per cent had red blood cell folate below 748 nanomoles per litre — the level below which risk of a neural tube defect in pregnancy rises.4 That is not a deficiency measure: only 7 per cent of the same group were below the clinical deficiency threshold.4 It is the reason folic acid is advised before and during early pregnancy, and it is another single nutrient aimed at a single documented risk.
If a multivitamin helps anyone fight infection, the strongest candidates are people whose immune systems are suppressed — by HIV, by transplant medication, by drugs for autoimmune disease, or simply by age. These groups also have higher rates of genuine micronutrient deficiency. The reasoning is sound. The trial results are not encouraging.
In adults living with HIV, a Cochrane review of seven trials and 2,897 participants found routine multiple-micronutrient supplementation had little or no effect on death, with a risk ratio of 0.91 and an interval of 0.72 to 1.15, rated low-certainty.7 A trial of 3,418 Tanzanian adults starting antiretroviral therapy tested high-dose against standard-dose multivitamins; it was stopped early because liver enzymes rose in the high-dose group, and it showed no benefit on disease progression or death.13
In older adults, the MAVIS trial randomised 910 people aged 65 or over in Grampian to a year of multivitamin and multimineral supplementation or placebo. It found no effect on GP contacts for infection, self-reported days of infection, or quality of life.6 An earlier Dutch trial of 15 months found the same for infection rates, and reported that its vitamin E arm had longer illnesses and worse symptoms than placebo.14
Now the contrast. People taking methotrexate — an immunosuppressant used for rheumatoid arthritis and other conditions — are routinely co-prescribed folic acid, because methotrexate works partly by blocking folate. A Cochrane review of six trials found folic acid cut abnormal liver enzyme elevation by 77 per cent in relative terms, with a risk ratio of 0.23 and an interval of 0.15 to 0.34, and cut discontinuation of methotrexate for any reason by 61 per cent, risk ratio 0.39.8 It did not blunt the drug's effect.8 UK practice reflects this: the BNF and NICE guidance recommend 5 mg of folic acid weekly, on a different day to the methotrexate.19
The same pattern appears in the one place where broad supplementation in HIV did work. In African children living with HIV, vitamin A roughly halved all-cause mortality across three trials — populations where vitamin A deficiency was widespread and the supplement corrected a specific, measurable, dangerous shortfall.10
There is also a hazard here that does not apply to the general population. Immunosuppressants such as tacrolimus and ciclosporin have a narrow therapeutic window and are metabolised through the CYP3A4 enzyme pathway, which several supplements and herbal products influence in both directions — too much drug means toxicity, too little means rejection.20 Anyone on these medicines should tell their transplant team or specialist about every supplement they take.
The idea that multivitamins strengthen immunity in older people has a traceable origin. In 1992 The Lancet published a trial by Ranjit Kumar Chandra reporting that a micronutrient supplement improved immune measures and roughly halved infection time in elderly subjects. It was cited hundreds of times.
A 2001 companion paper in the journal Nutrition, using the same subjects, was retracted in 2005. Memorial University investigated; The Lancet reopened its own inquiry in 2015 and retracted the 1992 paper in January 2016, stating that its reliability could no longer be assured.11
In the meantime a 2005 meta-analysis in the BMJ, pooling three trials including Chandra's, had reported that multivitamins and minerals cut days spent with infection by around 17.5 days a year.17 That figure circulated widely. The MAVIS trial, published in the same journal three months later, was larger than all three pooled trials combined and found nothing.6
This is not a story about one dishonest researcher. It is a demonstration of how a field's headline number can outlive its evidence, and why the current estimates — built on tens of thousands of participants across dozens of independent trials — deserve more weight than a striking result from a single study.
Beyond infection, the broader question of whether multivitamins improve health has clearer answers. The US Preventive Services Task Force reviewed the evidence in 2022 and issued an "I statement" for multivitamins — the evidence is insufficient to weigh benefits against harms — while recommending against beta-carotene and vitamin E supplements for preventing cardiovascular disease or cancer, with beta-carotene judged to increase lung cancer risk in people already at high risk.9
On mortality, a 2024 analysis of 390,124 US adults with no history of chronic disease, followed for over twenty years and recording 164,762 deaths, found no lower risk of death among daily multivitamin users. In the early years of follow-up their mortality was 4 per cent higher, a hazard ratio of 1.04.12 That is an observational finding and the authors are careful about it: people who start taking supplements when they feel unwell can generate exactly this pattern without the pills causing anything.
Almost no trial in this field was designed around measured nutrient status: participants were recruited by age or diagnosis, not by a blood test showing a deficiency, so the trials answer "does supplementing a general population help?" rather than "does correcting a documented shortfall help?" The vitamin D analysis is the exception and found nothing even in its deficient subgroup — but that subgroup was defined by a single baseline reading and was not the trials' recruitment criterion. The 2025 vitamin D funnel plot shows asymmetry consistent with publication bias, meaning the true effect could be smaller than reported, not larger. Formulations differ enormously between trials and few compared one against another, so nothing here ranks specific products. The multivitamin mortality data are observational and cannot establish cause. Almost no trial has tested a supermarket multivitamin in a UK immunosuppressed population; the HIV evidence comes largely from low-income settings with much higher background deficiency, and may not transfer. And no trial has adequately tested whether a supplement helps someone with a diagnosed deficiency and a suppressed immune system, which is the group most people asking this question actually want to know about.
One area is genuinely unsettled among competent researchers, and the disagreement is about how to measure the effect rather than about the underlying trials.
The 2024 review pooled eight treatment trials and found a mean reduction of 2.37 days, with an interval running from 4.21 days to 0.53 days and near-total heterogeneity between studies. It rates the evidence low to very low certainty and concludes that it is insufficient to recommend zinc. It also found more non-serious side effects — taste disturbance and stomach upset — with a risk ratio of 1.34.3
Averaging in days is the wrong scale, because a one-day cold cannot be shortened by 2.37 days; effects on illness duration should be measured proportionally. On a relative scale, seven trials of zinc acetate or gluconate lozenges delivering more than 75 mg a day shortened colds by 33 per cent, with an interval of 21 to 45 per cent. They argue the wide Cochrane interval is an artefact of the chosen scale.18
Both sides are working from largely the same trials. Note that Hemilä and Chalker are also the authors of the Cochrane vitamin C review cited throughout this piece, and are long-standing critics of how supplement trials are analysed — an interested party in a methodological argument, not a commercial one. On prevention the two camps agree: zinc does not stop you catching colds.3
Every ratio plotted in Exhibits 01 and 04, with its outcome, population and source. A ratio of 1.00 means no difference from placebo.
| Intervention | Population | Outcome | Estimate (95% CI) | Source |
|---|---|---|---|---|
| Vitamin C, regular | Extreme exertion, 598 | Catching a cold | 0.48 (0.35–0.64) | Cochrane 2013 |
| Vitamin C, regular | General public, 10,708 | Catching a cold | 0.97 (0.94–1.00) | Cochrane 2013 |
| Vitamin D, 400–1,000 IU | 10 trials, 2,305 | Any respiratory infection | 0.70 (0.55–0.89) | Lancet D&E 2025 |
| Vitamin D, under 400 IU | 2 trials, 2,133 | Any respiratory infection | 0.76 (0.41–1.41) | Lancet D&E 2025 |
| Vitamin D, over 2,000 IU | 7 trials, 6,906 | Any respiratory infection | 1.05 (0.84–1.31) | Lancet D&E 2025 |
| Vitamin D, any dose | Deficient at baseline, 3,806 | Any respiratory infection | 0.98 (0.80–1.20) | Lancet D&E 2025 |
| Vitamin D, any dose | 40 trials, 61,589 | Any respiratory infection | 0.94 (0.88–1.00) | Lancet D&E 2025 |
| Zinc, preventive | 9 trials, 1,449 | Catching a cold | 0.93 (0.85–1.01) | Cochrane 2024 |
| Multivitamin | HIV adults, 2,897 | Death | 0.91 (0.72–1.15) | Cochrane 2017 |
| Multivitamin + mineral | UK over-65s, 910 | GP contacts for infection | 0.96 (0.78–1.19) | MAVIS, BMJ 2005 |
| Multivitamin + mineral | UK over-65s, 910 | Days of infection | 1.07 (0.90–1.27) | MAVIS, BMJ 2005 |
| Folic acid | On methotrexate, 624 | Liver enzyme elevation | 0.23 (0.15–0.34) | Cochrane 2013 |
| Folic acid | On methotrexate, 624 | Stopping treatment | 0.39 (0.28–0.53) | Cochrane 2013 |
This is not medical advice. It summarises published trial evidence at population level. Decisions about supplements, particularly for anyone taking immunosuppressant medication or being treated for a diagnosed deficiency, belong with a GP, pharmacist or specialist team.