A cure for the common cold has been the holy grail for medical researchers, drug companies, and marketers of dietary supplements. So far nothing has stood up to scientific scrutiny, though you wouldn’t know it if you read the claims often made for most over-the-counter cold medicines and countless supplements.
Besides vitamin C and echinacea (neither found to be of value), the most widely promoted supplement for colds has been zinc in various forms. Enthusiasm for zinc has seesawed over the years as positive and negative studies have alternated in the news.
Earlier this year zinc made headlines again, thanks to a review from the Cochrane Collaboration, which evaluates scientific research. It concluded that, compared to a placebo, zinc can shorten colds by about a day and reduce their severity somewhat, particularly when started within 24 hours of the first symptoms.
This is not new research, but an analysis of data from 15 “good-quality” studies done since 1987. The studies included 1,360 healthy people who took lozenges, tablets, or syrup in the form of zinc gluconate, acetate, or sulfate, usually at the first sign of a cold. Doses ranged from 30 to 190 milligrams a day. Not all the studies found a benefit. The main adverse effects were nausea and a bad taste in the mouth.
In two of the studies in the review, children who took lower-dose zinc syrup for five to seven months developed fewer colds. This was the only evidence of a preventive effect, but these studies were of poorer quality.
It's not clear how zinc may help. Lab tests have found that zinc in the mouth and throat can deactivate cold viruses, block them from adhering to the nasal membranes, and/or stop them from replicating. The mineral may also have some anti-inflammatory and antioxidant effects.
But hardly a eureka moment
The Cochrane review left many infectious disease experts still unconvinced, however, largely because the studies varied so much. Even the Cochrane authors concluded that, due to these inconsistencies, they couldn’t make recommendations about what dose or formulation of zinc should be used or for how long.
Questions remained about whether the subjects may have known they were taking zinc, rather than a placebo, since zinc has a strong unpleasant taste—if so, that could have affected the results. Further, because of possible side effects and questions about the effectiveness of some formulations, the Cochrane authors concluded that zinc is “advised with caution.”