Don’t rely on vitamins C to prevent colds

Taking vitamin C every day will basically not make you catch fewer colds, and starting only after you already have a cold works only sometimes. Daily use can shorten one cold by about 8%; whether 300 to 500 a year is worth it is your call. Marathon runners and soldiers marching in cold regions are the exception: among them, those who take it get about half as many colds.

Cost

Each effervescent tablet costs roughly 1–2 RMB; annual expense ranges from 300 to 700 RMB.

Benefit

Cochrane reviewed 29 randomized controlled trials involving 11,306 participants. For ordinary people, regular vitamin C consumption does not meaningfully reduce overall cold incidence; the risk ratio here is 0.97, meaning there is virtually no difference between groups. The 95% confidence interval for this figure is 0.94–1.00. Trials where vitamin C was taken only after cold symptoms appeared showed mixed results: some indicated a benefit, others did not. For the general population, long-term daily intake does shorten cold duration by about 8% in adults (95% CI 3%–12%) and 14% in children (95% CI 7%–21%). Marathon runners, skiers, and soldiers in cold climates again form an exception: 5 trials with 598 participants showed their cold incidence dropped by roughly half, with a risk ratio of 0.48 (95% CI 0.35–0.64). The perceived value of vitamin C for cold prevention is largely driven by its low cost, pleasant sweet-and-sour taste, and the fact that this claim has been widely circulated for half a century.

Notes

Controversy note: The same Cochrane review confirms that regular vitamin C intake does shorten the duration of cold symptoms. For adults, this reduction is about 8% (95% CI 3%–12%); for children, it is 14% (95% CI 7%–21%). This translates to roughly half a day of reduced illness per cold episode. Marathon runners, skiers, and soldiers in cold climates are again the exception: 5 trials with 598 participants showed their cold incidence fell by about half (risk ratio 0.48, 95% CI 0.35–0.64). The vast majority of people do not fall into these special groups. 〔500〕

Sources · 1

1. Hemilä H, Chalker E (2013). Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. doi.org/10.1002/14651858.CD000980.pub4