Physicians reporting in the February 2011 issue of "The Cochrane Review" analyzed data from clinical trials dating back to 1984, which investigated zinc as a cold symptom treatment. The researchers, Drs. Singh and Das, independently reviewed the data and reported that zinc reduced the average duration of cold symptoms by nearly one day when taken within 24 hours of onset of symptoms. Zinc also eased symptom severity. However, the researchers concluded that side effects will most likely limit the usefulness of zinc in treating cold symptoms
The Studies
The clinical trials reviewed evaluated zinc efficacy in lozenge, tablet and syrup formulations. Inclusion criteria stipulated that the trials be randomized, double-blind and placebo controlled. Trials were assessed with regard both to treatment and prevention. Treatment trials followed participants for at least five days, and prevention trials followed participants taking zinc or a placebo for at least five months. Side effects were determined by comparing complaints from the placebo groups with complaints from the zinc-supplementing groups.
The Side Effects
Adverse events were more frequent in the zinc groups than the placebo groups. Lozenge formulations were most problematic, and the most frequent complaints were bad taste and nausea. No standard zinc formulations were used, so it was difficult to determine severity of symptoms relative to dose. Other reported side effects included loss of the sense of smell. Singh and Das wrote that because some formulations produced side effects and not all formulations are effective, that the use of zinc to treat common cold symptoms should be advised with caution.
Nausea
One case of nausea from taking a zinc supplement resulted in severe vomiting within 30 minutes of ingestion. Other study participants felt ill immediately after supplementing. Nausea was most frequently experienced when participants supplemented on an empty stomach and in some cases, nausea was resolved entirely by supplementing after a meal. Severity appeared to be dose dependent, although the data weren't conclusive. Nausea also resulted from sucking on lozenges, and Singh cautioned that zinc interferes with other metals in the body and may contribute to unpredictable consequences.
Loss of Sense of Smell
ScienceDaily reported in June 2009 that the U.S. Food and Drug Administration advised consumers to stop using three zinc products sold as over-the-counter cold remedies because they can cause loss of sense of smell. The loss, known as anosmia, can be long lasting or permanent. The FDA issued the warning after 130 users reported anosmia. The July 2010 issue of "Archives of Otolaryngology" also reported that intranasal zinc therapy is linked to potentially permanent loss of smell.
References
- "The Cochrane Review"; Zinc for the Common Cold; M. Singh, D. Rashmi; February 2011
- ScienceDaily; Certain Zinc-Based Cold Remedies Linked to Loss of Smell; June 20, 2009
- "Archives of Otolaryngology: Head & Neck Surgery"; The Bradford Hill Criteria and Zinc-Induced Anosmia; T. Davidson, W. Smith; July 2010
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