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 .
Ascorbic acid and the common cold. Evaluation of its efficacy and toxicity.
We review ed the clinical data relating to the efficacy and safety of pharmacologic doses of ascorbic acid in the prevention and treatment of the common cold .  Although one study tentatively supports the hypothesis that such doses of ascorbic acid may be efficacious , a second study by the same group did not confirm the significant findings , and no clear , reproducible pattern of efficacy has emerged from the review of all the evidence . Similarly , there is currently little adequate evidence on either the presence or the absence of serious adverse reactions to such doses of ascorbic acid , although many such reactions have been hypothesized . The unrestricted use of ascorbic acid for these purpose s can not be advocated on the basis of the evidence currently available
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 .