Oral zinc treatments may shorten the duration of symptoms of the common cold in adults, although adverse effects are common, according to a study published in CMAJ (Canadian Medical Association Journal).
Canadian researchers looked at 17 randomized controlled trials with 2121 participants between 1 and 65 years of age to determine the efficacy and safety of zinc in treating the common cold. All trials were double-blinded and used placebos as well as oral zinc preparations. The authors found that, compared with placebos, zinc significantly reduced the duration of cold symptoms, although the quality of evidence was moderate. High doses of ionic zinc were more effective than lower doses at shortening the duration of cold symptoms.
"We found that orally administered zinc shortened the duration of cold symptoms," writes Dr. Michelle Science, The Hospital for Sick Children (SickKids), Toronto, with coauthors at McMaster University. "These findings, however, are tempered by significant heterogeneity and quality of evidence."
There was weak evidence that people taking zinc were less likely to have symptoms after one week, although there was no difference in symptoms between the two groups at three days. While zinc appeared to reduce the duration of symptoms in adults, there was no apparent effect in children. Participants taking zinc treatment were more likely to experience adverse effects including bad taste and nausea.
Previous studies have shown conflicting effects of zinc in reducing cold symptom severity and the duration of symptoms.
"Until further evidence becomes available, there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold," conclude the authors. "The questionable benefits must be balanced against the potential adverse effects."