Of 14 studies published over the last 25 years, about half have shown benefits. Others have shown none. Some have even found zinc to make colds worse.
On the positive side, a 2008 study of 50 people in the Journal of Infectious Diseases found that people who took zinc lozenges every few hours within the first 24 hours of developing a cold had symptoms that lasted half as long as people who sucked on placebo lozenges. With zinc, they were also only half as miserable.
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Compared with the placebo group, those in the zinc group had colds that lasted for an average of four days instead of seven. Coughs lasted for two days instead of five. And runny noses lasted for three days instead of 4 1/2 . The lozenges contained 13.3 milligrams of zinc, and people took them five or six times a day while awake.
A review study, soon to be published in the journal Medical Hypotheses, concluded that allowing zinc lozenges to dissolve slowly in your mouth over a period of 20 to 30 minutes every two hours beginning with the first symptoms could shorten the common cold by as much as seven days, in essence curing the illness.
But getting those kinds of results from zinc lozenges depends on what kind you use, says study author Ananda Prasad, an internist at the Wayne State University School of Medicine in Detroit and author of the 2008 study. The lozenges in his study contained zinc acetate, which the body absorbs well.
Many brands of lozenges do nothing. They may contain ingredients such as citric acid or flavorings that block zinc's immune-boosting powers and even make things worse. Zinc oxide is another form that's fairly ineffective. You could take hundreds of milligrams of some of those supplements and not notice any effects at all -- good or bad.
"A lot of confusion has arisen because there are many preparations of lozenges that are not very effective," Prasad says. "They have added a lot of things to flavor the lozenges in such a way that it affects the release of zinc ions."