Lozenges containing zinc acetate in sufficient quantity work to shorten the common cold from the usual seven days to about four, according to a new analysis of clinical trial results.
While some past research also suggested that zinc lozenges work better for people with allergies, the new review found similar benefits for people without allergies.
In common colds, the virus may grow in the pharyngeal, or throat, region, which is why using lozenges may work better than swallowing pills, the study team notes in British Journal of Clinical Pharmacology.
"Ordinary tablets that enter directly into the stomach, without releasing zinc in the pharyngeal region, are not effective," said lead author Dr. Harri Hemila of the University of Helsinki in Finland.
Since clinical studies using low-dose zinc lozenges had found no effect on common colds, Hemila and his team looked only at trials that used lozenges delivering a total of 75 milligrams of zinc or more per day.
They found three trials comparing these high-dose zinc lozenges against a placebo in common cold sufferers who were randomly assigned to one treatment or the other. One trial also looked at differences in results for people with and without allergies.
The trials had a total of 199 participants, who were were mostly female and between 20 and 50 years old. One third had allergies, including allergies to grasses, trees and pets.
Participants were instructed to have a lozenge every two to three hours and the lozenges took about 15 to 30 minutes to dissolve.
Overall, the average dose of zinc was between 80 and 92 mg of zinc per day.
The review found that patients who used zinc lozenges experienced colds that were 2.94 days shorter, compared with the seven-day average of colds in the comparison group.
The effect of the zinc lozenges was not altered by a person's allergies, smoking or how severe the cold was. The results were also similar across age, sex and ethnic groups.
"This is a very important subject to study because the common cold is a very common problem and it leads to loss of work as well as disability because people are not able to focus," said Dr. Meenu Singh, a professor of pediatrics at the Postgraduate Institute of Medical Education and Research in Chandigarh, India.
Singh, who was not involved in the study, told Reuters Health by email that while these results are promising, there are some side effects of zinc that need to be explored before high doses of zinc are available over the counter.
For example, in previous studies, using zinc as an inhalant caused people to temporarily lose their sense of smell, Singh said, noting that lozenges could dampen taste as well.
Some patients also complain that the lozenges leave a metallic taste in the mouth, though the participants in trials reviewed did not find this to be a problem, the study team points out.
"There is information now available that zinc in a higher dose could reduce your cold by a certain duration, but there is an added discomfort of taking lozenges," Singh said by email. The tablets are large and need to be taken continually throughout the cold to be effective, he said.
Hemila cautioned that common cold patients should look for lozenges that do not contain citric acid, which can prevent the release of zinc into the body.
"Common cold patients should try zinc lozenges with doses about 80 to 100 milligrams per day very soon after the onset of the common cold," Hemila advised, adding that the lozenges can be used for one to two weeks of treatment.