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 prophylaxis in marine recruits.
A prospect i ve , r and omized , double-blind study was carried out to determine whether vitamin C prophylaxis , 2.0 g/day , vs placebo prophylaxis would reduce the incidence or morbidity of the common cold  and other respiratory illnesses in 674 marine recruits during an eight-week period . Whole-blood ascorbic acid levels measured six weeks after initiation of the study were significantly higher in the vitamin C group . There was no difference between the two groups in the incidence or duration of colds . The vitamin C group rated their colds as being less severe , but this was not reflected in different symptom complexes or in fewer sick-call visits or training days lost . This study and the literature do not support the prophylactic use of vitamin C to prevent the common cold
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 .