Doses of vitamin C in excess of 1 g daily taken shortly after onset of a cold did not reduce the duration or severity of cold symptoms in healthy adult volunteers when compared with a vitamin C dose less than the minimum recommended daily intake.
Bibliography
Audera, C., R. V. Patulny, B. H. Sander, and R. M. Douglas. 2001. “Mega-Dose Vitamin C in Treatment of the Common Cold: A Randomised Controlled Trial.” The Medical Journal of Australia 175 (7): 359–62. https://doi.org/10.5694/j.1326-5377.2001.tb143618.x.