Oral zinc if taken within 24 hours of the onset of cold symptoms does seem to reduce the duration of a cold by 1-2 days. However, zinc did not reduce the incidence nor the severity of a cold. This report is available in the Journal of the American Medical Association this week. It is a summary of the results of 14 trials including 1780 patients given zinc or placebo in controlled studies. In prevention trials in children, daily zinc did reduce the incidence of colds in children when it was used for 3 months during the cold/flu season.
JAMA April 9, 2014; pages 1440-1441.