New research reveals zinc can reduce the length of colds by two days – but are the side effects worth it?

A recent Cochrane review found that zinc supplementation can reduce the duration of cold symptoms by about two days.

A recent Cochrane review suggests that zinc supplementation could reduce the duration of cold symptoms by around two days. However, the results are not definitive and possible benefits must be weighed against potential side effects.

Since the 1980s, zinc products have been marketed as cold treatments and are particularly popular in the United States. Zinc is an essential mineral naturally found in many foods and plays a role in immune function. Most people in high-income countries get enough zinc through their diet, although aging and certain chronic illnesses can lead to a deficiency.

Mechanism and research

The theory behind zinc lozenges, sprays and syrups is that zinc can interfere with viral replication when it comes into contact with virus particles in the nose, mouth and throat. Zinc has been shown to interfere with viral replication in petri dishes and in mice, although that alone doesn’t tell us whether something will work in real people.

To test whether zinc is helpful in preventing or treating a cold, a team of researchers looked at 19 human trials examining zinc as a treatment and 15 as a preventative measure. They identified many variations between studies in how zinc was administered, how much was administered, how they defined a “cold,” and what they measured.

Eight studies involving 972 participants investigated zinc as a treatment to reduce the duration of colds. Combining the results of these studies provided low-certainty evidence that it could help reduce the duration by about two days, down from the average duration of one week in the groups given a placebo.

Effectiveness in preventing colds and side effects

The analysis found no strong evidence to conclude that zinc treatment had an impact on the severity of cold symptoms. Prevention studies have shown no clear evidence of benefit from taking zinc before the onset of a cold; those who took zinc as a preventative measure had similar results to those who did not.

Common side effects of zinc reported in trials included intestinal problems, nausea, and unpleasant taste. There was no clear evidence of more serious side effects directly related to zinc.

“People considering zinc to treat a cold should be aware of the limited evidence and possible side effects,” says assistant professor Daryl Nault of the University of Maryland Integrative Health, first author of the review. “Ultimately, it is up to the individual to decide whether the risk of unpleasant side effects is worth the benefit of potentially shortening their illness by a few days.” The best advice is to consult your doctor if you feel unwell and to inform them if you are using any supplements. Although many trials have been conducted on zinc, approaches vary, so it is difficult to draw conclusions with certainty.

The trials included in the review varied in many ways, including the type of zinc, the dose of zinc given, whether it was given as a lozenge or nasal spray, and how outcomes were reported and measured. Some trials measured over a fixed time window and asked participants if they still had a cold at the end. Others have measured the time from symptom onset to resolution, although this was defined slightly differently in each study. Few studies have tracked the status of individual symptoms, such as sore throat, cough, or fever. There was therefore insufficient evidence to draw reliable conclusions about specific symptoms.

“The evidence on zinc is far from established: we need more research before we can be sure of its effects,” says Assistant Professor Susan Wieland of the University of Maryland School of Medicine, lead author of the journal. “Future studies should adopt standardized methods for administering and reporting treatments, as well as defining and reporting outcomes. Additional studies investigating the most promising types and doses of zinc products and using appropriate statistical methods to assess important patient outcomes will help us understand whether zinc may have a place in the treatment of the common cold.

Reference: “Zinc for the Prevention and Treatment of Colds” by Daryl Nault, Taryn A Machingo, Andrea G Shipper, Daniel A Antiporta, Candyce Hamel, Sahar Nourouzpour, Menelaos Konstantinidis, Erica Phillips, Elizabeth A Lipski and L Susan Wieland, 9 May 2024, Cochrane Database of Systematic Reviews.
DOI: 10.1002/14651858.CD014914.pub2

The study was funded by the National Center for Complementary and Integrative Health.