In addition , numerous trials have shown no difference regarding the most commonly studied endpoint : frequency of symptoms .
In 5 trials with participants under heavy acute physical stress ( 3 with marathon runners )  , vitamin C  reduced the incidence of colds by 52 % ( 95 % CI : 36 % to 65 % ) , but there is no evidence that vitamin C prevents colds in the general population .
Proponents of evidence -based medicine ( EBM ) emphasize that conclusions about medical interventions should be based on controlled trials with clinical ly relevant outcomes , so that the greatest weight should be given to the systematic review s of the trials .
So far , therapeutic supplementation has not been shown to be effective .
Vitamin C and the common cold: a double-blind trial.
A large scale double-blind trial was conducted to test the cl aim that the intake of one gram of vitamin C per day substantially reduces the frequency and duration of " colds " . It was found that in terms of the average number of colds and days of sickness per subject the vitamin group experienced less illness than the placebo group , but the differences were smaller than have been cl aim ed and were statistically not significant . However , there was a statistically significant difference ( P < 0.05 ) between the two groups in the number of subjects who remained free of illness throughout the study period . Furthermore the subjects receiving the vitamin experienced approximately 30 % fewer total days of disability ( confined to the house or off work ) than those receiving the placebo , and this difference was statistically highly significant ( P < 0.001 ) . The reduction in disability appeared to be due to a lower incidence of constitutional symptoms such as chills and severe malaise , and was seen in all types of acute illness , including those which did not involve the upper respiratory tract
In addition , numerous trials have shown no difference regarding the most commonly studied endpoint : frequency of symptoms .
In 5 trials with participants under heavy acute physical stress ( 3 with marathon runners )  , vitamin C  reduced the incidence of colds by 52 % ( 95 % CI : 36 % to 65 % ) , but there is no evidence that vitamin C prevents colds in the general population .
Proponents of evidence -based medicine ( EBM ) emphasize that conclusions about medical interventions should be based on controlled trials with clinical ly relevant outcomes , so that the greatest weight should be given to the systematic review s of the trials .
So far , therapeutic supplementation has not been shown to be effective .