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Myth check

Does taking vitamin C every day keep colds away?

Across 24 comparisons, daily vitamin C left the chance of catching a cold essentially unchanged. Colds fell by about half only in five small, old trials of marathon runners, skiers and soldiers.

Vitamin C

Colds arrived just as often on daily vitamin C as on placebo. Across 24 comparisons, at least 0.2 grams a day left the chance of catching one essentially unchanged, in trials from the 1960s onward that mostly counted colds by asking people. The one exception was five small trials of marathon runners, skiers and soldiers on subarctic exercises, 598 people, where colds roughly halved; none tested ordinary gym-goers. For the wider community, the daily dose did not change the odds.

The findings behind this

1–
ImmuneColds arrived just as often on vitamin C as on placebo

Across 24 comparisons, at least 0.2 grams a day left the chance of catching a cold essentially unchanged, a 3% difference whose range includes no difference at all.

★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.The trials pooled here run from the 1960s onward, defined a cold in different ways, and mostly counted them by asking people. The two groups that agree worked from largely the same trial set, so this is one body of evidence read twice rather than two independent verdicts.Click to keep this openSupport immunityVitamin C
2↓
ImmuneUnder extreme physical stress, colds fell by about half

Pooling five trials in marathon runners, skiers and soldiers, 598 people in all, daily vitamin C roughly halved the chance of a cold. In the wider community, no change.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.CautionFive small trials and 598 people between them, in settings almost nobody lives in: a subarctic military exercise, a 90 kilometre race, an Alpine ski camp. None of them tested ordinary gym-goers. The underlying trials date from the 1960s to the early 1990s and counted colds largely by asking people.Click to keep this openSupport immunityVitamin C

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