Clinical trails with varying doses of ascorbic acid showed that ascorbic acid does not have significant prophylactic effect, but reduced the severity and duration of symptoms of cold during the period of infection.

Consumption of ascorbic acid as high as 1 g a day for several winter months, had no consistent beneficial effect on the incidence of common cold.

There was a consistent beneficial but generally modest therapeutic effect on duration of cold symptoms. 

In trials that tested vitamin C after cold symptoms occurred, there was some evidence of greater benefits with large dose than with lower doses.