Vitamin C does not prevent the common cold for most people.

Despite decades of popular belief, regular vitamin C supplementation does not reduce the risk of catching a cold in the general population. A large Cochrane systematic review pooling dozens of trials found that taking vitamin C daily made no meaningful difference in how often ordinary adults and children got sick, compared with those taking a placebo.

The one exception was a narrow group: marathon runners, skiers, and soldiers in subarctic conditions who saw their cold risk roughly halved with regular supplementation, likely because extreme physical stress and cold temperatures suppress immune function in ways vitamin C partially offsets. For everyone else, the review did find a small benefit in cold duration and severity once symptoms started — colds ran about 8% shorter in adults — but starting vitamin C only after you’re already sick showed no benefit at all. The popular idea that megadosing vitamin C at the first sniffle will shorten or stop a cold isn’t supported by the evidence.