Vitamin C won't stop your cold, unless you're running marathons

Whether vitamin C prevents or treats the common cold has been argued over for 70 years, ever since it became one of the world's most popular supplements. Colds are the most frequent illness humans catch, so even a small genuine effect would matter at population scale, and an ineffective ritual wastes money and trust. This Cochrane review pooled the placebo-controlled trial evidence to settle three separate questions: does daily vitamin C stop colds occurring, does it shorten them, and does taking it once symptoms start do anything at all.

The reviewers restricted their analysis to placebo-controlled trials using at least 0.2 g of vitamin C per day. For prevention, 29 trial comparisons involving 11,306 participants contributed data on the risk of catching a cold during regular daily supplementation. For duration, 31 comparisons covering 9745 cold episodes were analysed. Seven comparisons involving 3249 episodes tested therapeutic use, meaning vitamin C started at the onset of symptoms. Most included trials were randomised and double-blind, and excluding those that were not did not change the conclusions.

In the general community, spanning 10,708 people, regular vitamin C did not reduce the chance of catching a cold. The exception was striking: in five trials of 598 marathon runners, skiers and soldiers on subarctic exercises, cold risk roughly halved. Regular supplementation did shorten colds, by 8% in adults and 14% in children, and reduced their severity; in children, 1 to 2 g per day shortened colds by 18%. Therapeutic vitamin C, started once symptoms appeared, showed no consistent effect on either duration or severity.

This review draws a clean line through decades of controversy: prevention fails in ordinary life but appears to work under brief, severe physical stress, while modest reductions in duration require taking vitamin C continuously, not reactively. The authors conclude that routine supplementation is not justified for the general population. They note that given vitamin C's low cost and safety, individuals with colds could reasonably test for themselves whether it helps them, and they call for further therapeutic trials.

If you take vitamin C hoping to dodge winter colds, the pooled evidence says the average person gets no protection, which is a reason to rethink the habit. The picture changes at the extremes: people undergoing heavy exertion in harsh conditions, like endurance racers and soldiers in subarctic training, saw their cold risk roughly halved. And the timing detail matters: the modest shortening of colds appeared only in people already supplementing daily, not in those who reached for vitamin C after the sniffles began.