Do not rely on vitamin C to prevent colds
Taking vitamin C every day does little to prevent colds in the general population. Starting it after a cold begins has inconsistent effects. Regular use can shorten an adult cold by about 8%, roughly half a day; decide whether that is worth several hundred RMB a year. Marathon runners and soldiers exercising in cold environments are exceptions: their cold incidence was approximately halved.
What it takes
Effervescent tablets cost roughly RMB 1–2 each, or 300–700 a year.
What you may gain
A Cochrane review combined 29 controlled trials involving 11,306 people. In the general population, regular vitamin C did not meaningfully reduce cold incidence: RR 0.97 (95% CI 0.94–1.00). A risk ratio of 1 means no difference. Trials starting treatment after symptoms began showed no consistent effect: some found a benefit and others did not. Regular supplementation shortened cold duration by 8% in adults (95% CI 3%–12%) and 14% in children (7%–21%). Marathon runners, skiers, and soldiers exercising in cold environments were a distinct group: five trials involving 598 people found an incidence RR of 0.48 (0.35–0.64), about half as many colds. The product remains appealing because it is inexpensive, tastes pleasantly tart and sweet, and the claim that vitamin C prevents colds has circulated for half a century.
Context & considerations
The modest reduction in duration—8% for adults and 14% for children—comes from taking it regularly, not reliably from starting once ill. The reduction in incidence under extreme physical stress is also real evidence, but most people are not marathon runners or soldiers training in freezing conditions. Do not apply that exception to everyone.
Research & references
Hemilä H, Chalker E (2013). Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. https://doi.org/10.1002/14651858.CD000980.pub4.