• R M Douglas, E B Chalker, and B Treacy.
• National Centre for Epidemiology and Population Health, The Australian National University, Canberra, ACT, Australia, 0200. Bob.Douglas@anu.edu.au
• Cochrane Db Syst Rev. 2000 Jan 1 (2): CD000980.

BackgroundThe role of oral ascorbic acid (vitamin C) in the prevention and treatment of colds remains controversial despite many controlled trials. There have also been a number of efforts to synthesize and/or overview the results of these trials, and controversy over what these overviews tell us.ObjectivesThe objective of this review was to answer the following two questions: (1) Does regular high dosage supplementation with vitamin C reduce the incidence of colds? (2) Does taking vitamin C in high doses at the onset of a cold have a therapeutic effect?Search StrategyThis review currently deals only with published trials from two previously published reviews by Kleijnen 1989 and Hemila 1992.Selection CriteriaRandomised and non-randomised trials of vitamin C taken to prevent or treat the common cold.Data Collection And AnalysisTwo reviewers independently extracted data and assessed trial quality.Main ResultsThirty trials were included. The quality of the included trials was variable. Vitamin C in doses as high as one gram daily for several winter months, had no consistent beneficial effect on incidence of the common cold. For both preventive and therapeutic trials, there was a consistently beneficial but generally modest therapeutic effect on duration of cold symptoms. This effect was variable, ranging from -0.07% to a 39% reduction in symptom days. The weighted difference across all of the studies revealed a reduction of a little less than half a symptom day per cold episode, representing an 8% to 9% reduction in symptom days. There was no clear indication of the relative benefits of different regimes or vitamin C doses. However in trials that tested vitamin C after cold symptoms occurred, there was some evidence that a large dose produced greater benefits than lower doses.Reviewer's ConclusionsLong term daily supplementation with vitamin C in large doses daily does not appear to prevent colds. There appears to be a modest benefit in reducing duration of cold symptoms from ingestion of relatively high doses of vitamin C. The relation of dose to therapeutic benefit needs further exploration.

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