Does vitamin C stop you catching colds?

What the evidence says

Cochrane's systematic reviews of placebo-controlled trials, updated in 2000, 2004, 2007 and 2013, repeatedly pooled data on regular vitamin C supplementation (0.2g or more daily) versus placebo for preventing colds. Across the general community, the combined risk of catching a cold sat close to 1: a risk ratio of 0.96 in the 2007 update (11,350 participants, 95% CI 0.92–1.00) and 0.97 in the 2013 update (10,708 general-community participants). In plain terms, this means no consistent, meaningful reduction in the chance of catching a cold for typical adults.

Where a clearer benefit did show up, it was in a very different population: people under brief but extreme physical or cold stress. A subgroup of 642 marathon runners, skiers and soldiers on sub-arctic exercises had roughly half the risk of colds while taking vitamin C (risk ratio 0.50, 95% CI 0.38–0.66). A separate 2004 review of military and exertion-based trials found five small, short studies reporting 45–91% reductions in cold incidence, alongside some reductions in pneumonia incidence in a few trials. These are not the circumstances of someone taking a vitamin C tablet through an ordinary winter.

On duration, once a cold has started, evidence is more consistently positive but modest. The original 2000 Cochrane review described a "consistently beneficial but generally modest" shortening of symptoms with regular supplementation, on average a little less than half a symptom-day per cold, though individual trial results ranged from almost no effect to a 39% reduction in symptom days. A 2020 review of single micronutrients in healthy adults likewise found vitamin C did not measurably prevent colds, in contrast with zinc, which reduced duration by around two days in the same review.

What is still uncertain

The size of the duration benefit varies widely between trials, and the older reviews describe included trial quality as variable, which limits confidence in the precise numbers. It is not established which people, doses or circumstances (beyond intense short-term physical exertion) might see a genuine preventive effect, nor whether the modest shortening of illness is large enough to matter to most people day to day. No review in this set reports vitamin C causing meaningful harm.

This is an evidence summary, not medical advice.