It turns out the subject is quite controversial, with many people preaching the value of vitamin C in cold prevention. The science, however, is less clear. Several studies have been done to examine the effects of vitamin C on the cold prevention and treatment. One meta-analysis of these studies showed that vitamin C has no effect on the incidence of the common cold and slight but variable effects on duration and severity. From the summary:
"It reduced the duration and severity of common cold symptoms slightly, although the magnitude of the effect was so small its clinical usefulness is doubtful."One of the authors of this review has since argued that therapeutic vitamin C treatment (i.e. treatment after onset of symptoms) may be beneficial in children. Some studies have shown similar modest (but statistically insignificant) effects, while others none at all. Overall the evidence for therapeutic vitamin C use is unconvincing. The NIH has a detailed page describing vitamin C usage, that has the following to say about cold treatment:
"More than 30 clinical trials including more than 10,000 participants have examined the effects of taking daily vitamin C on cold prevention. Overall, no significant reduction in the risk of developing colds has been observed. [...] Numerous studies have examined the effects of starting vitamin C after the onset of cold symptoms. Overall, no significant benefits have been observed. Initial evidence from one study reports possible benefits with high doses of vitamin C taken at the onset of symptoms, but without additional evidence this remains indeterminate. At this time, the scientific evidence does not support this use of vitamin C."On both cold prevention and cold treatment, the NIH gives vitamin C a grade of D meaning "fair scientific evidence against this use." It is interesting to note, however, that vitamin C does seem to have an effect in people living in extreme circumstaces such as soldiers in sub-arctic exercises or marathon runners. Overall, for the general public, vitamin C is important for health but the evidence does not support its use in cold prevention or treatment.