Despite 20 years of research, the benefits of zinc lozenges as a therapy for the common cold have not been proven.
A new study, published in the Sept. 1 issue of Clinical Infectious Diseases, currently available online, reviews the 14 placebo-controlled studies from the past two decades and finds significant fault with 10 of the studies. Of the four remaining studies, three reported no therapeutic effect from zinc lozenge or nasal spray, and one study reported positive results from zinc nasal gel.
"The best scientific evidence available indicates that zinc lozenges are not effective in treating colds," said Jack M. Gwaltney, Jr., MD, one of the authors.
With colds affecting virtually everybody (one study estimates that adults experience an average of three colds each year and children may experience as many as eight or 10), people are eager to alleviate the discomfort that accompanies a cold. In 1984, the first study reporting that zinc lozenges effectively reduced the duration of the common cold was published. Many other studies followed, some seeming to support the idea of zinc either lessening symptoms or length of illness and some finding no effect.
In this new research, the authors have sorted through 105 studies of zinc and the common cold. From this, they extracted the 14 randomized, placebo-controlled studies, the type of study that might provide the strongest evidence for or against zinc's usefulness in cold-relief. They then checked each study for 11 features of experimental design that needed to be met in order for the study to produce valid results.
The research was performed by medical student Thomas Caruso of Stanford University School of Medicine with the direction of Dr. Gwaltney, a professor of internal medicine, emeritus, at the University of Virginia School of Medicine, and with the assistance of Charles Prober, MD, also at Stanford.