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.
In 2011, zinc made headlines again, thanks to a review from the Cochrane Collaboration, which evaluates scientific research. The review, which was updated in 2013, 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. (Another 2012 research review on zinc, published in the Canadian Medical Association Journal, came to similar conclusions, though it noted that “uncertainty remained about its clinical benefit.”)
The Cochrane research is not new research, but is 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.
Zinc: 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.”
Zinc as a cold treatment “is controversial because researchers have used all kinds of preparations and doses, and subjects often did not take lozenges within 24 hours of the first symptoms,” according to Ananda Prasad, M.D., Ph.D., a leading zinc researcher at Wayne State University School of Medicine in Detroit. “If zinc is used properly, and you take lozenges that release it effectively—zinc acetate, though gluconate may also be okay—you can cut the duration of a cold by 50 percent and the severity very significantly as well.” But, he warns, ingredients that are often added to the products to mask the zinc taste—including ascorbic acid (that is, vitamin C), citric acid and sorbitol—may inactivate the zinc. And there is little or no evidence that zinc will prevent colds.
Think about zinc
Like many nutrients, zinc has its good-guy/bad-guy aspects. It is needed for cell division, growth, wound healing and the proper functioning of the immune system, and also plays a role in acuity of taste and smell.
Zinc is plentiful in some foods, such as meat and seafood. The recommended dietary allowance, or RDA, for zinc is eight to 11 milligrams a day for people 14 or older, and three to eight milligrams for children, depending on their age. Prolonged use of high doses of zinc can interfere with the body’s absorption of copper, as well as depress the immune system, impair blood cell formation, and have other potential adverse effects. In adults, even 50 to 75 milligrams of zinc a day for long periods can be a problem, and it would take even less to adversely affect children.
In short-term studies the common complaints have been nausea and bad taste, along with diarrhea, vomiting, cramps, and mouth sores. One of the worst side effects of zinc as a cold treatment is loss of smell, called anosmia, but this is caused only by forms applied directly to the nose. Thus, in 2009 the Food and Drug Administration (FDA) warned against the use of Zicam gels and nasal swabs. Don’t buy nasal zinc products if you see them around.
Bottom line: If you want to try zinc lozenges when you have a cold, start as soon as you have symptoms and follow the dosing advice on the label (usually every two hours), though admittedly this is a shot in the dark. Don’t take the lozenges for more than a week, and don’t take them long-term to prevent colds.
Originally published June 2011. Updated December 2013.