Vitamin C for colds: what the studies show

General information, not medical advice. If you take medication, have kidney problems or are unsure about a supplement, talk to your GP or pharmacist.

If you reach for vitamin C for colds in autumn, you want to know what a daily pill really does. Two sources together give a sober answer: the international Cochrane review by Hemilä and Chalker, and the Belgian website Gezondheid en Wetenschap (Health and Science), which summarises health research in plain language. You will also read how to simply get vitamin C from your plate.

Vitamin C for colds: what a daily pill delivers

The 2013 Cochrane review is a meta-analysis, an overview study that pools the results of several trials. These are placebo-controlled trials: one group gets vitamin C, the other a placebo, a dummy pill without an active ingredient.

In the trials among people from the general population, 10,708 participants in total, the risk ratio comes out at 0.97. That ratio compares the chance of catching a cold with and without vitamin C. A result of 1 means the same chance, so 0.97 means practically the same chance. The range in which the true result most likely lies runs from 0.94 to 1.00. A daily pill therefore hardly makes you catch colds less often.

Slightly shorter colds, or no noticeable effect?

If you take vitamin C every day, your colds are on average a little shorter according to Cochrane: duration drops by 8 percent in adults and by 14 percent in children. If you only start once symptoms appear, the same review finds no consistent effect on duration or severity.

In 2021, Gezondheid en Wetenschap looked at 37 systematic literature reviews in total, 16 of them Cochrane reviews. Its conclusion: there is no convincing evidence of positive effects of vitamin C supplements on health complaints, and no positive effects were found on preventing colds or on how long they last. So Cochrane measures a small difference in duration, while Gezondheid en Wetenschap weighs the whole body of evidence and does not find it convincing. In practice both come down to the same thing: a daily pill changes little about your colds.

A later meta-analysis by the same Cochrane authors, from 2023, looks at severity. At doses of at least 1 gram a day, symptoms are 15 percent less severe. That is about how bad a cold feels, not about how often you catch one.

Why extreme exertion is a different story

In five trials with 598 marathon runners, skiers and soldiers on subarctic exercises, in the cold regions just below the Arctic Circle, the risk ratio is 0.48. That comes down to roughly half as many colds. This is, however, a small group of people facing short periods of very heavy exertion, not an ordinary training week. The authors conclude that a routine daily pill for everyone is not justified, but that vitamin C may be useful for people facing short bouts of very heavy exertion.

Two runners just after a marathon finish, with a foil blanket and a bottle of water

How to simply get vitamin C from your plate

According to Gezondheid en Wetenschap, vitamin C deficiency is virtually non-existent in Belgium. People who smoke, are older or chronically drink too much alcohol run the greatest risk. For adults the site gives a daily requirement of 110 milligrams, based on the 2016 dietary recommendations of the Belgian Superior Health Council, which gives scientific advice to Belgian policymakers. Per 100 grams you find roughly this much vitamin C:

A market trader shows a customer kiwi, peppers and Brussels sprouts as sources of vitamin C

citrus fruit: 50 to 60 milligrams;
strawberries and raspberries: 30 to 60 milligrams;
apples and pears: around 3 to 10 milligrams.

Potatoes are an important source too, although their content varies with the seasons. Preparation matters. When blanching, briefly cooking in boiling water, a third of the vitamin C is lost, when steaming a fifth. In a pressure cooker or microwave the amount stays the same. According to Gezondheid en Wetenschap, the advice of the Flemish Institute for Healthy Living to eat fruit and vegetables every day amply covers your needs. Preferably choose fresh products over ultra-processed food.

Still considering a supplement?

Gezondheid en Wetenschap points out that studies show a higher risk of kidney stones with a daily intake of 1000 to 2000 milligrams of vitamin C. The dose from the 2023 meta-analysis, at least 1 gram a day, sits right at the start of that range. If you still choose a supplement, check how to read a supplement label and ask your pharmacist if in doubt. Flu, by the way, is a different illness from a common cold, and there the flu vaccine after 65 is what mainly protects you.