Zinc lozenges may triple the rate of recovery from the common cold, according to a new meta-analysis of three studies. However, the promising results come with a caveat: the studies looked at doses much higher than are commonly recommended by doctors, and the authors say that not all zinc lozenges on the market are effective.
For the new review, published in Open Forum Infectious Diseases, Finnish researchers combined data from three randomized clinical trials, in which about 200 people who had cold symptoms took either zinc acetate lozenges or a placebo. Daily doses of zinc ranged from 80 to 92 mg a day—substantially higher than the recommended daily intake in the United States, which is 11 mg for men and 8 mg for women.
When the studies were analyzed together, results showed that 70% of the people taking zinc lozenges recovered from their colds after five days, compared to just 27% of people who took the placebo. After adjusting for several factors, the authors concluded that zinc provided a 3.1-fold increase in improvement. These effects were seen regardless of age, gender, race, smoking status or presence of allergies, the researchers note, suggesting that zinc’s healing powers could apply to a wide variety of people.
In the United States, the National Academies of Sciences, Engineering and Medicine recommends a tolerable upper intake level (UL) of 40 mg a day for people not taking zinc under the care of a doctor. Too much zinc can cause nausea, vomiting, loss of appetite, stomach cramps, diarrhea and headaches, and large doses over a long period of time can lead to low copper levels, low immunity and low levels of HDL “good” cholesterol.
However, no one in the studies reported serious side effects, and the authors point out that zinc has been shown to be safe and well tolerated in quantities up to 150 mg a day for months at a time in clinical trials unrelated to the common cold. It’s also prescribed long-term at 150 mg a day for patients with Wilson’s disease, a rare genetic condition in which the body can’t process copper properly.
For these reasons, short-term doses of up to 100 mg a day are unlikely to be harmful for most people, says lead author Dr. Harri Hemilä, a researcher in public health at the University of Helsinki in Finland.
Merrill Christensen, professor of nutritional science at Brigham Young University, is more cautious. (He was not involved in the new review.) In a larger meta-analysis of zinc for the common cold from 2012, side effects including nausea were common.
Christensen says that people should aim to get the recommended daily amount of nutrients, including zinc, for overall health. But he also recommends staying below the upper limit set by the government and getting as many nutrients as possible from whole foods. "Problems occur when people start popping supplements," he says.
There’s another catch, too: Many zinc lozenges on the market contain ineffective amounts of the active ingredient, says Hemilä. Some also contain ingredients such as citric acid, which interact with zinc molecules to make them less available in the body.
The optimal formulation and dosage schedule for zinc lozenges should be studied further, Hemilä says. In the meantime, he recommends that common-cold sufferers try zinc acetate lozenges themselves to see if they notice a faster-than-usual improvement. (Lozenges containing zinc gluconate don't seem to work as well, he adds.)
“The most positive effects have been found when common cold patients have started to take zinc lozenges within 24 hours after the onset of symptoms,” Hemilä says. “The benefit is possibly less if treatment is started later.” People should avoid formulas that contain citric acid, he adds, and aim to take a total of 80 to 100 mg of elemental zinc per day.
Hemilä says his advice does not apply to pregnant women and children, who “might want to wait for further research to guarantee the safety of zinc lozenges." He also says that high doses of zinc might cause stomach irritation for some people.
And while zinc lozenges are often known for having a bad taste, Hemilä says that not all formulas taste the same—and that those used in his meta-analysis were not considered unpleasant. People who find the taste unacceptable may want to try a different brand, he says.