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 the common cold: a double-blind trial.
A large scale double-blind trial was conducted to test the cl aim that the intake of one gram of vitamin C per day substantially reduces the frequency and duration of " colds " . It was found that in terms of the average number of colds and days of sickness per subject the vitamin group experienced less illness than the placebo group , but the differences were smaller than have been cl aim ed and were statistically not significant . However , there was a statistically significant difference ( P < 0.05 ) between the two groups in the number of subjects who remained free of illness throughout the study period . Furthermore the subjects receiving the vitamin experienced approximately 30 % fewer total days of disability ( confined to the house or off work ) than those receiving the placebo , and this difference was statistically highly significant ( P < 0.001 ) . The reduction in disability appeared to be due to a lower incidence of constitutional symptoms such as chills and severe malaise , and was seen in all types of acute illness , including those which did not involve the upper respiratory tract
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 .