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 .
Vitamin C and acute illness in Navajo school children.
To evaluate earlier observations , including our own , showing usefulness of vitamin C for managing the common cold , we performed a double-blind trial of vitamin C versus placebo in 868 children . There was no difference in number becoming ill ( 133 versus 129 ) , number of episodes ( 166 versus 159 ) or mean illness duration ( 5.5 versus 5.8 days ) between the groups . Children receiving vitamin C  had fewer throat cultures yielding beta-hemolytic streptococcus ( six versus 13 , P less than 0.10 ) , but no difference in overall complicated illness rate ( 24 versus 25 ) . Plasma ascorbic acid levels were higher in the vitamin group 24 to 26 hours after supplementation ( 1.28 versus 1.04 mg per 100 ml , P less than 0.01 ) . Children with high plasma ascorbic acid concentrations had longer mean illness ( 6.8 versus 4.0 days , P less than 0.05 ) than those with low levels . Vitamin C does not seem to be an effective prophylactic or therapeutic agent for upper respiratory illness
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 .