Does vitamin C actually help against a cold?

Vitamin C does not prevent a cold if you simply take a daily pill. Data from 29 trials involving more than 10,000 participants show that the risk of catching a cold remains virtually unchanged. For the average healthy adult eating a normal diet, extra supplementation provides no measurable benefit.

There is one exception: people who perform heavy short-term physical exertion, such as marathon runners, skiers, or military personnel in extreme cold. In that group, daily vitamin C halves the risk of catching a cold. That is a remarkably large and consistent effect. However, the evidence involves only 598 participants across 5 trials, so the evidence for this group is moderate.

People who take vitamin C daily recover from a cold somewhat faster on average. In adults, a cold lasts about 8% less time; in children, about 14% less. In absolute terms, that amounts to roughly half a day. Children taking 1 to 2 grams per day achieve an 18% shorter duration. The severity of symptoms also decreases by an average of 15% at doses of 1 gram or more per day. Modest, but consistent.

Taking vitamin C after you are already ill does not show consistent measurable benefit at doses of 3 to 4 grams per day. Two individual trials suggest that 6 to 8 grams per day shortens the cold noticeably more, but the evidence is too thin to support a recommendation.

No serious side effects are reported at the doses studied. This makes it reasonable to try vitamin C on an individual basis, especially if you exercise intensively or have a low vitamin C level. In older adults and people with diabetes there are limited indications that supplementation has a favourable effect on the inflammatory response, but that evidence is also still small.