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 .
Large-scale trials of vitamin C.
Among the many cl aims made about the value of large doses of vitamin C , those relating to the prevention and treatment of " colds " have aroused most interest , and the first question to be answered is whether there is any real evidence that such an effect exists , or whether the cl aims are based entirely on wishful thinking . If there is no good evidence of an effect , then there is little point in pursuing the matter . If , on the other h and , the evidence suggests that there is some sort of effect , a host of secondary questions present themselves : ( 1 ) Is the effect specific for the common-cold group of viruses , or is it a more general anti-infective ( or even antistress ) effect ? ( 2 ) Is it a reflection of an increased metabolic need in certain individuals or at certain times , or is it a pharmacological action unrelated to the normal metabolic function of ascorbic acid ? ( 3 ) Is the beneficial effect related to a regular daily intake ( " prophylactic " ) or to extra intake during the course of an illness ( " therapeutic " ) or to a com bination of both ? ( 4 ) Are large doses really necessary , or could equally good results be ob tained with a more moderate supplementation of normal dietary intake ? ( 5 ) If large doses are necessary , does the risk of side effects outweigh the possible benefits ? From a practical point of view it is this last question that most urgently needs to be answered . Further , since it seems reasonable to suppose that side effects are more likely to appear with higher doses and with prolonged intake , the first point to establish is whether there is any value to the regular " prophylactic " use of large doses of vitamin C , and whether in fact the size of the daily dose is as important as has been cl aim ed . In passing , it may be noted that although we have seen no symptomatic evidence of toxicity result ing from doses of up to 2000 mg daily over three or four months in healthy persons , this does not mean that this dose level is necessarily safe for longer periods , particularly in individuals with preexisting disease , or that the occasional individual might not show some unusual and undesirable reaction . Furthermore , while perhaps not a side effect in the ordinary sense of the word , the depression in blood ascorbate levels that occurs on sudden withdrawal of a chronic high intake should be recognized as a potentially harmful reaction . For example , an individual admitted to the hospital with an acute medical or surgical condition might be at a physiological disadvantage if this period of unusual stress coincided with an acute  hypoascorbemia  due to sudden withdrawal of a regular high intake . What then , is the evidence regarding the prophylactic use of large doses of vitamin C ? There is , of course , a great deal of anecdotal evidence , but this is of dubious value because of the well-recognized interaction between mental
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 .