REVIEW ERS ' CONCLUSIONS The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use .
But evidence shows that it could be justified in persons exposed to brief periods of severe physical exercise and /or cold environments .
Also , the consistent and statistically significant small benefits on duration and severity for those using regular vitamin C prophylaxis indicates that vitamin C plays some role in respiratory defence mechanisms .
BACKGROUND The role of oral vitamin C ( ascorbic acid ) in the prevention and treatment of the  common cold  has been a subject of controversy for at least sixty years .
Public interest in the topic continues to be high and vitamin C continues to be widely sold and used as a preventive and therapeutic agent for this common ailment .
OBJECTIVES To discover whether oral vitamin C in doses of 200 mg or more daily , reduces the incidence , duration or severity of the common cold when used either as continuous prophylaxis or after the onset of cold symptoms .
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
REVIEW ERS ' CONCLUSIONS The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use .
But evidence shows that it could be justified in persons exposed to brief periods of severe physical exercise and /or cold environments .
Also , the consistent and statistically significant small benefits on duration and severity for those using regular vitamin C prophylaxis indicates that vitamin C plays some role in respiratory defence mechanisms .
BACKGROUND The role of oral vitamin C ( ascorbic acid ) in the prevention and treatment of the  common cold  has been a subject of controversy for at least sixty years .
Public interest in the topic continues to be high and vitamin C continues to be widely sold and used as a preventive and therapeutic agent for this common ailment .
OBJECTIVES To discover whether oral vitamin C in doses of 200 mg or more daily , reduces the incidence , duration or severity of the common cold when used either as continuous prophylaxis or after the onset of cold symptoms .