We reviewed the clinical data relating to the efficacy and safety of pharmacologic doses of ascorbic acid in the prevention and treatment of the common cold. Although one study tentatively supports the hypothesis that such doses of ascorbic acid may be efficacious, a second study by the same group did not confirm the significant findings, and no clear, reproducible pattern of efficacy has emerged from the review of all the evidence. Similarly, there is currently little adequate evidence on either the presence or the absence of serious adverse reactions to such doses of ascorbic acid, although many such reactions have been hypothesized. The unrestricted use of ascorbic acid for these purposes cannot be advocated on the basis of the evidence currently available.
(JAMA 231:1073-1079, 1975)
Dykes MHM, Meier P. Ascorbic Acid and the Common ColdEvaluation of Its Efficacy and Toxicity. JAMA. 1975;231(10):1073–1079. doi:10.1001/jama.1975.03240220051025