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 .
The effect on winter illness of large doses of vitamin C.
Between December 1972 and February 1973 , 2349 volunteers participated in a double-blind trial to assess the effect of large doses of vitamin C on the  incidence and severity of winter illness .  In addition , records were kept but no tablets taken during March . Subjects were r and omly allocated to eight treatment regimens : three prophylactic-only ( daily dose 0.25 , 1 or 2 g ) , two therapeutic-only ( 4 or 8 g on the first day of illness ) , one combination ( 1 g daily and 4 g on the first day of illness ) , and two all-placebo . None of the groups receiving vitamin C showed a difference in sickness experience that was statistically significant from that of the placebo groups , but the results obtained were compatible with an effect of small magnitude from both the prophylactic and therapeutic regimens , and an effect of somewhat greater magnitude from the combination regimen . The combination regimen was associated more with a reduction in severity than frequency of illness , although the extra dosage was limited to the first day of illness . In spite of the eightfold range in daily dose , the three prophylactic-only regimens showed no evidence of a dose-related effect , but the 8 g therapeutic dose was associated with less illness than the 4 g therapeutic dose . There was no evidence of side effects from the 1 and 2 g prophylactic doses of vitamin C , and no evidence of a rebound increase in illness during the month following withdrawal of the daily vitamin supplements . On the basis of this and other studies it is suggested that the optimum daily dose of vitamin C is less than 250 mg , except possibly at the time of acute illness , when a larger daily intake may be beneficial
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 .