The New York Times reports that the Cochrane Database of Systematic Reviews compiled a literature review that claims zinc can reduce the duration and severity of colds if taken within 24 hours of initial symptoms. Some of the studies cited offered significant results. A March 2008 report in the Journal of Infectious Diseases found that patients who used zinc lozenges reduced cold duration from seven to four days and reduced coughing from five to two days. The authors, however, offered no suggestions as to what products to purchase for cold sufferers. Over-the-counter zinc supplements may not be as effective as the researched products because commercial products often alter zinc concentrations or add flavors that detriment product effectiveness.
Many previous zinc studies have been discarded due to the inability of researchers to mask the taste, smell, etc. of the zinc supplement. The Cochrane review includes 15 studies with a combined 1,360 participants. All studies were believed to have solid methodologies; each compared the use of zinc with a placebo. The studies used various forms of zinc and ranged in doses from 30 to 160 milligrams. Some concern has been raised in the past over the use of zinc after the FDA warned consumers to stop using Zicam nasal spray (which contains zinc) after some users lost their sense of smell.
It is uncertain how zinc reduces the duration or severity of colds but it is believed that it has antiviral properties that inhibit viral replication or adherence to nasal membranes.