Daily 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%.
What the study found
Daily vitamin C shortens a cold by 8% in adults, 14% in children
Eight percent of a cold is about half a day. Across 31 comparisons covering 9,745 cold episodes, people already taking vitamin C daily before they fell ill recovered a little sooner, and a later pooling of 10 double-blind trials put the drop in severity at 15%.
The caveat · One author across every update
Earlier 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.
What we don't know
Whether daily vitamin C shortens colds turns on which evidence counts. Every trial showing a benefit dosed people before the cold began, and a stricter overview of 8 primary studies found little or no effect.