CHARLOTTESVILLE, Va. -- Zinc lozenges do nothing to prevent or treat the common cold, a systematic review of studies of the over-the-counter treatment suggested.
CHARLOTTESVILLE, Va., Aug. 3 -- Zinc lozenges do nothing to prevent or treat the common cold, a systematic review of studies of the over-the-counter treatment suggested.
The benefits of zinc lozenges "have yet to be established," according to Jack Gwaltney, Jr., M.D., of the University of Virginia School of Medicine, here.
On the other hand, there is some limited evidence that a zinc-based nasal gel may be beneficial, Dr. Gwaltney and colleagues reported in the Sept. 1 issue of Clinical Infectious Diseases.
Over the past 40 years, there have been 105 published reports on the cold-fighting effects of zinc, but only 14 randomized controlled trials, the researchers found.
Overall, seven of the randomized studies reported a benefit for zinc and seven reported no effect.
However, all but four of the randomized trials had significant design flaws that may invalidate their results, Dr. Gwaltney and colleagues said, and of the remaining four, only one found a beneficial effect.
Two of the well-designed studies looked at zinc lozenges, one at zinc nasal spray, and one at zinc nasal gel.
The first three found that zinc had no effect, either on preventing a cold, decreasing its symptoms, or shortening its duration, but the gel study found that the product was beneficial.
The studies were considered on the basis of 11 design characteristics, including such elements as double-blinding, an intent-to-treat analysis, a quantifiable hypothesis, and measurements of compliance and drop-out rates.
"Failure to meet just one of these predetermined essential criteria may potentially invalidate a study," the researchers said.
Of the 14 studies, four met all of the standards, two (both reporting a positive effect) met 10, and the remaining eight (evenly divided among benefit and no effect) met eight or fewer.
The most common design flaw was lack of an intent-to-treat analysis, which - if participants finding no effect or disliking the product tended to drop out of the study - could lead to a significant but spurious positive effect, the researchers said.
"When examining a treatment such as zinc, which is known to have adverse effects, such as sore mouth, upset stomach, and loss of smell," the researchers said, participants "may have a greater likelihood of dropping out or not complying with protocol because of such adverse effects."
An intent-to-treat analysis counters such effects, they said, but only six of the studies included one.
Effective blinding is also essential, but difficult to achieve in zinc studies because of the distinctive taste of the lozenges, the researchers said.
The studies in this review were all aimed at naturally occurring colds, but other studies have shown that zinc has no effect in preventing or treating colds acquired after challenge with rhinoviruses, the researchers noted.
But it may be too soon to throw zinc on the scrap heap, the researchers said, because one of the well-designed studies did show a benefit for a zinc nasal gel.
"Further work is necessary to clarify zinc's effectiveness with intranasal application," they concluded.