Vitamin C for preventing and treating the common cold.

Systematic review/meta-analysis of randomized trials in adults and children from the general community, with some studies in participants under short-term extreme physical stress and some laboratory viral-challenge studies. The review compared preventive daily vitamin C and therapeutic vitamin C started after symptom onset.

Vitamin C was evaluated as regular daily supplementation for prevention and as short-course therapy started at the onset of cold symptoms. Regimens varied widely across trials, commonly from 0.2 g/day to 1 g/day for prevention and from 3 g/day to 8 g on the first day or during early illness for treatment, generally compared with placebo.

Regular vitamin C supplementation did not reduce the incidence of the common cold in the general population, but it shortened cold duration and may have reduced symptom severity. Duration benefits were greater in children and in some studies of people under brief extreme physical stress. Therapeutic vitamin C started after symptoms began showed inconsistent benefit, although a few high-dose trials suggested possible improvement. Overall, routine vitamin C for cold prevention is not supported, but it may modestly reduce duration once a cold occurs.

The evidence was heterogeneous across populations, doses, treatment timing, and outcome definitions, limiting direct comparability. Therapeutic trials were especially inconsistent, and many included studies were old, small, or indirect for modern practice.