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Myth check

Vitamin C when you feel a cold coming on: does it do anything?

Started once symptoms begin, vitamin C showed no consistent effect across seven comparisons. Colds ran slightly shorter only in people already taking it daily, and even that is disputed.

Vitamin C

Many reach for vitamin C once a cold starts. Seven comparisons covering 3,249 colds, in people who started it after symptoms appeared, found no consistent effect on how long or how bad the cold was, though none went past 3 grams a day. The half day saved belongs to people already taking it daily before they fell ill, and an independent review calls even that minimal or none. Whether timing or dose explains the gap is untested; started at the first sniffle, it showed nothing consistent.

The findings behind this

1–
ImmuneStarting vitamin C once you feel a cold coming did nothing

Seven comparisons covering 3,249 cold episodes found no consistent effect on duration or severity, including a trial that began dosing within four hours of symptoms.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.Conflicting evidenceThe strongest single trial here lost 251 of its 400 volunteers before any diary came back, and how ill someone felt could plausibly decide whether they filled one in. Its comparison group still took 0.03 grams of vitamin C rather than nothing. None of these trials went past 3 grams a day.Click to keep this openSupport immunityVitamin C
2↓
ImmuneDaily vitamin C took about half a day off a cold

Across 31 comparisons covering 9,745 cold episodes, people already taking it daily recovered about 8% sooner. A later pooling put the drop in severity at 15%.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.Conflicting evidenceEarlier versions of this review called half a day of doubtful clinical usefulness. The pooled trials span six decades, with doses from 0.2 grams a day upward and no shared definition of a cold. One researcher has authored the review across its updates and argued this finding publicly for years.Click to keep this openSupport immunityVitamin C

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