Colds arrived just as often on vitamin C as on placebo
Across 24 comparisons, at least 0.2 grams a day left the chance of catching a cold essentially unchanged, a 3% difference whose range includes no difference at all.
What the study found
Daily vitamin C did not stop 10,708 people catching colds
Across 24 comparisons, taking at least 0.2 grams of vitamin C every day left the chance of catching a cold essentially unchanged, a 3% difference whose range includes no difference at all. Two research groups re-ran the same primary trials and reached the same answer.
The caveat · Largely the same trials, read twice
The trials pooled here run from the 1960s onward, defined a cold in different ways, and mostly counted them by asking people. The two groups that agree worked from largely the same trial set, so this is one body of evidence read twice rather than two independent verdicts.