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 .
Co-twin control studies: vitamin C and the common cold.
Jtending to make one pair different from another , so providing a more efficient test of the treatment effect . The increase in efficiency of the matched pair over the r and om group design is dependent upon the extent to which the matching has removed extraneous factors influencing the effect of the drug . The ultimate matched pair in human experimentation is a pair of monozygotic ( MZ ) twins who have been reared together . Not only is there a perfect match for age , sex , and family background but , perhaps most importantly , genetic constitution is identical , at least for nuclear genes . The paired and unpaired design s are shown in Table I and have been compared extensively by Christian and Kang [ 1972 ] . The paired analysis allows the partition of variance between individuals ( 0111 ) into that between pairs ( oip ) and that within pairs ( a2 wp ) . For sufficiently large N , if oip > 0 ( ie , if the between-pairs mean square is significantly greater than the pairs x treatments mean square ) , then the paired design will give a more powerful test of the between-treatments effect than the unpaired design . For the num­ bers we have here , it may be shown that oi1/a2 wp is the multiplier to estimate how many more pairs of unrelated individuals would be needed to provide a test of the between-treatments effect of the same power as is provided by a given number of pairs of twins
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 .