The benefits of zinc also appear to be modest, and don't extend to children. But they could add up considering there are about 62 million cases of the common cold in the United States every year that result in 22 million missed days of work, according to the U.S. National Institutes of Health.

For their review, published in the Canadian Medical Association Journal, Dr. Michelle Science from the Hospital for Sick Children in Toronto and her colleagues compiled data from clinical trials comparing people who took zinc orally to another group that either took a placebo or received no treatment.

The researchers included only randomized controlled trials -- considered the "gold standard" of medical research -- but cautioned that the quality of the results of those previous trials varied widely.

Overall, the researchers looked at 17 trials that included 2,121 people from one to 65 years old. In those taking zinc, colds were shortened by a little more than one and a half days, on average, compared to participants in the placebo group.

That changed when the researchers analyzed zinc-takers by age. In adults, zinc shortened the common cold by a little more than two and a half days compared to the placebo. In children, zinc didn't seem to make much of a difference in cold duration.

One possible reason for the difference between adults and kids, according to the authors, is that the adults tended to use a different form of zinc than the children.

Peoples' cold symptoms also seemed to clear up faster if they took a higher dosage of zinc compared to those who took the least. The various studies used different dosages, said Science.

The authors cannot say why zinc stops the rhinovirus, a frequent cause of the common cold, from reproducing. But some believe it acts as an astringent on important facial nerves where viruses tend to congregate.

Although adults who didn't take zinc tended to have colds lasting a week or more, there was no difference in the severity of cold symptoms on day three in any of the groups.

Side effects were more common in the people taking zinc, however. They were 64 percent more likely to experience nausea and 65 percent more likely to detect an unpleasant taste.

Zinc can lead to a person having a metal taste in their mouth, said Dr. Meenu Singh, a pediatrician at the Post Graduate Institute of Medical Education and Research in Chandigarh, India, who was not involved in the new analysis.

Singh led another review published last year by the Cochrane Collaboration -- an international organization that evaluates medical research -- which came to similar conclusions, though (see Reuters Health story of 02/16/2011).

"The findings are more or less the same. The side effects are the same," said Singh, who added that the new review included other studies that were excluded from hers because they did not meet Cochrane standards.

Singh told Reuters Health that people shouldn't be afraid to take zinc, but should always consult their doctor first.

Science and her colleagues also warn that their results are based on studies that varied from one another in their methods and quality. Also, they did not look at zinc nasal sprays that have been linked to the loss of smell or zinc in combination with other vitamins.

"Until further evidence becomes available," they conclude, "there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold."

"Otherwise, for healthy adults… it's probably an individual decision and it's something they can talk to their physician about," Science told Reuters Health.

SOURCE: bit.ly/JX1Fwr Canadian Medical Association Journal, online May 7, 2012.

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