Vitamin C prevents the common cold.

Linus Pauling's advocacy made high-dose vitamin C a household defence against colds. The testable question splits into two: does it prevent colds, and does it shorten them?

The Cochrane review by Hemilä & Chalker (2013), covering dozens of trials, found that regular vitamin C did not reduce cold incidence in the general population — with one exception: people under extreme physical stress, such as marathon runners and soldiers, roughly halved their risk. For everyone, regular supplementation modestly shortened colds (about 8% in adults) and eased severity, but taking vitamin C only after symptoms start did little. So the blanket 'vitamin C prevents colds' is not supported; the accurate picture is a small, real benefit on duration and a special case for extreme exertion.