The claim that vitamin C "prevents colds" is only partly true. There is a kernel of truth, but the "prevents" part is wrong for most people.
What we checked
A popular claim says vitamin C prevents colds. We reviewed the best available scientific evidence.
Evidence
The Cochrane systematic review and the US NIH fact sheet conclude that, in the general population, regular vitamin C intake does not prevent colds. However, regular intake may slightly shorten the duration and ease symptoms. A reduction in cold risk appears only in groups under extreme physical stress, such as marathon runners or people doing heavy exercise in cold environments.
Conclusion
Vitamin C does not prevent colds for most people, but it may slightly shorten them. The claim is therefore partly true.