Vitamin C for preventing and treating the common cold.

Systematic review and meta-analysis of randomized comparisons of oral vitamin C for prevention or treatment of the common cold. Participants included adults and children from the general community, school and military groups, and people exposed to brief periods of severe physical exercise such as marathon runners, skiers, and soldiers; some laboratory transmission studies in volunteers were also included.

Oral vitamin C was evaluated as continuous prophylaxis and, in some trials, as treatment started after cold onset. Prophylaxis doses ranged from 0.2 g/day to 3 g/day, with one trial using 4 g/day on the first day of illness; comparisons were generally against placebo.

Routine vitamin C prophylaxis did not reduce common cold incidence in the general community, but it did in people under brief severe physical stress. In general community trials, the pooled risk ratio for developing a cold was 0.97 (95% CI 0.94 to 1.00), whereas five heavy-exercise trials showed a risk ratio of 0.48 (95% CI 0.35 to 0.64). Prophylaxis modestly shortened cold duration, by 8% in adults and 13% in children, and reduced severity. Therapeutic vitamin C started after symptom onset showed no consistent benefit for duration or severity. Reported adverse effects were uncommon and similar to placebo: 5.8% vs 6.0% in seven large prophylaxis studies, with no serious symptoms reported.

The evidence was heterogeneous across populations, doses, and outcome definitions, with many older trials and several small studies. Therapeutic evidence was limited and inconsistent, and the apparent benefit was concentrated in a small subgroup exposed to severe physical stress, which limits generalizability. Baseline arm-level characteristics and standardized severity measures were often not reported.