Placebo-controlled trails have shown that vitamin C supplementation decreases the duration and severity of common cold infections. However, the magnitude of the benefit had substantially varied, hampering conclusions about the clinical significance of the vitamin. 23 studies with regular vitamin C supplementation were analyzed to find out factors that may explain some part of the variation in the results. It was found that an average, vitamin C produces greater benefit for children than for adults. The dose may also affect the magnitude of the benefit, there being on average greater benefit from > 2g/day compared to 1g/day of the vitamin. In five studies with adults administered 2g/day the median decrease was four times higher, 26%. The trials analyzed in this work used regular vitamin C supplementation, but it is conceivable that therapeutic supplementation starting early at the onset of the cold episode could produce comparable benefits. Since few trials have examined the effects of the therapeutic supplementation and their results have been variable, further therapeutic trials are required to examine the role of vitamin C in the treatment of cold.