Zinc lozenges may shorten the duration of a cold. (Karen Tapia / Los Angeles…)
People who begin using zinc lozenges, tablets or syrup at the first signs of a cold are more likely to get well faster, researchers reported Tuesday. But the new findings probably won't be the last word on the issue, which has been the subject of debate since the idea was first proposed in 1984.
Since that time, 18 studies have examined zinc in preventing or treating colds. Some found zinc supplements were modestly helpful, others failed to turn up any benefits.
One analysis of 14 studies, published in 2007, concluded that many of the studies were too flawed to draw any conclusions.
In the latest report, published by the Cochrane Library, an international network of experts who conduct systematic reviews of research, scientists in India evaluated 15 studies, including four published since 2000.
Two of the studies evaluated focused on zinc's effectiveness in preventing colds and the rest on its ability to shorten the duration of colds. The 15 studies involved 1,360 participants ranging in age from 1 to 65 with good overall health.
Pooling the data, researchers found that people who took zinc within 24 hours of the start of symptoms were over their colds about one day sooner than people who took placebos. The analysis also found that the severity of cold symptoms was somewhat milder among people who took zinc.
Whether these results will be considered meaningful depends on whom you ask, said Dr. Kay Dickersin, a professor of epidemiology at Johns Hopkins University's Bloomberg School of Public Health and director of the U.S. Cochrane Center, one of the 12 centers around the world that facilitate the work of the Cochrane reviews. Dickersin was not involved in the research.
"I might say, 'A day less of symptoms is good; I'll do it.' But you might say, 'A day is nothing; it's not worth driving to the drugstore,' " she said.
Moreover, since the study designs varied widely, it's impossible to make recommendations on what doses are optimal, what formulations are best and how long to use the products, said the authors of the analysis, Meenu Singh and Rashmi R. Das of the Postgraduate Institute of Medical Education and Research in Chandigarh, India.
"I think there is a need for more research so we can get a sense of how well zinc works or if it even does work," Dickersin said.
Zinc lozenges and syrup, commonly available in drugstores, are typically taken every two to three hours during waking hours for at least five days. Most products recommend a standard daily dose for cold treatment is about 30 milligrams of syrup per day or about 60 milligrams in lozenges.
When a zinc acetate formulation is taken in a high enough dose and started early in the onset of a cold, it's likely to be effective, said Dr. Ananda Prasad, an expert on zinc at Wayne State University in Detroit who conducted two studies, both of which showed a positive effect.
"In our studies, we only included patients who had [begun treatment] within 24 hours" of the start of symptoms, he said. "If you don't take zinc within 24 hours, it does not have much effect."
But an examination of only the most scientifically rigorous of the zinc studies shows it probably doesn't shorten colds, said Dr. Terence M. Davidson, the director of the UC San Diego Nasal Dysfunction Clinic.
"The more rigorously scientific studies, where you took a group of people and gave half of them zinc and half a placebo and inoculated their nose with a cold virus, found there were no differences," Davidson said. "I think enough research has been done to show if there is some benefit, it's not going to be very significant."
There may also be risks from some of these products, said Davidson, who was the first to identify harmful side effects from zinc nasal spray.
In 2009, the Food and Drug Administration warned consumers to stop using three zinc-containing Zicam nasal products after receiving 130 reports about loss of smell associated with the products. The Cochrane Library analysis did not investigate zinc nasal sprays.
Researchers don't know why zinc may affect the common cold. It could be that zinc prevents rhinoviruses from attacking nasal cells, slows the replication of the virus or prevents histamine release (which causes sneezing, runny nose and rash).
In the United States, colds contribute to 75 to 100 million visits to doctors each year at a cost of about $7.7 billion. Colds are among the most common reasons for absenteeism from work and school.
"Any medication that is only partially effective in the treatment and prevention of the common cold could markedly reduce morbidity and economic losses due to this illness," Singh and Das wrote.