The benefits of zinc lozenges as a treatment for the common cold are unproven, according to an analysis of 20 years of research on the lozenges.

A new study, published online in advance of the Sept. 1 issue of Clinical Infectious Diseases, reviews 14 placebo-controlled studies from the past two decades and finds significant fault with 10 of the studies. Of the four other studies, three reported no therapeutic effect from zinc lozenges 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, a retired professor of internal medicine from the University of Virginia’s School of Medicine and one of the authors of the new study.

The first study reporting that zinc lozenges successfully reduced the length of the common cold was published in 1984. Many studies followed, backing the claim, according to the most recent analysis.

But Gwaltney and researchers from Stanford University found a number of flaws in the placebo-controlled studies.

The most frequently found problem was the lack of an “intent to treat” analysis, which ensures that data for all subjects will be used regardless of whether or not they complete the trial.

Other problems found in the studies included the lack of a quantifiable hypothesis or the use of sample sizes that were too small to produce statistically valid findings.

Of the four studies that met the authors’ criteria, two studies reported that zinc lozenges had no effect on the symptom severity or duration of a cold, one study reported no effect of zinc nasal spray, and one study reported a positive effect of zinc nasal gel in lessening symptoms and length of a cold.

“Since less information is available on the intranasal approach, additional well-designed studies of intranasal zinc spray or zinc-treated nasal swabs should be performed,” said Gwaltney.