Vitamin C and colds: what the Cochrane review really says
Regular use barely changes your chance of catching a cold, but shortens it slightly. Starting once your nose is running does close to nothing.
Vitamin C is a case study in how a loud mid-century hypothesis survives longer in culture than in the data.
The headline findings
The Cochrane review pooled dozens of randomised trials. Regular preventive use did not reduce how often people caught colds in the general population. The exception was people under extreme physical stress — marathon runners, soldiers in harsh conditions — where colds became noticeably less frequent.
Regular use did shorten episodes a little: roughly eight per cent in adults and around fourteen per cent in children. That is hours, not days.
And starting at the first sniffle?
Trials of treatment started after symptoms appear have not shown a consistent effect.
Doses and safety
The tolerable upper intake level for adults is usually given as about 2,000 mg a day. Excess is mostly excreted, but large doses can cause diarrhoea and raise the risk of oxalate stones in susceptible people.
What to do instead
Sleep, flu vaccination, hand washing and humidified air give more predictable results than loading up on ascorbic acid mid-illness.
Sources
- Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev, 2013.
- NIH Office of Dietary Supplements. Vitamin C — Health Professional Fact Sheet.