A new Cochrane review has found that taking zinc may help to reduce the duration of common cold symptoms by about two days, but the evidence is not conclusive and potential benefits must be balanced against side-effects.
Since the 1980s, zinc products have been marketed as treatments for the common cold and are particularly popular in the USA. 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 diets, although aging and some chronic diseases may lead to deficiency.
The theory behind zinc-based lozenges, sprays and syrups is that the zinc may interfere with viral replication when it comes into contact with viral particles in the nose, mouth and throat. Zinc has been shown to interfere with viral replication in petri dishes and mice, although this alone doesn't tell us whether something will work in real people.
To test if zinc is useful 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 a lot of variation between the studies in how zinc was administered, how much was given, how they defined a 'cold' and what they measured.
Eight studies with 972 participants investigated zinc as a treatment to reduce cold duration. Combining the results of these studies yielded low-certainty evidence that it may help to reduce duration by around two days, down from an average week-long duration in the groups who received placebo.
The review found no strong evidence to conclude that zinc treatment impacts the severity of cold symptoms. The prevention studies showed no clear evidence of benefit from taking zinc before the onset of a cold; those taking zinc preventatively had similar outcomes to those who didn't.
Common side-effects of zinc reported in the trials included bowel problems, nausea and unpleasant taste. There was no clear evidence of more serious side-effects directly resultant from zinc.
"People considering zinc to treat a cold should be aware of the limited evidence base and possible side-effects," says Assistant Professor Daryl Nault of Maryland University of Integrative Health, first author of the review. "Ultimately, it's up to the individual to decide whether the risk of potential unpleasant side-effects is worth the benefit of potentially shortening their illness by a few days. The best advice remains to consult your physician if you're feeling unwell and inform them if you use any supplements. While there have been many trials investigating zinc, the 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 the outcomes were reported and measured. Some trials measured for a fixed time window and asked participants if they still had a cold at the end. Others measured the time between symptoms starting and resolving, although this was defined slightly differently by each study. Few studies monitored the status of individual symptoms, such as sore throat, cough or fever, so there was insufficient evidence to draw any reliable conclusions about specific symptoms.
"The evidence on zinc is far from settled: we need more research before we can be confident in its effects," says Assistant Professor Susan Wieland of the University of Maryland School of Medicine, senior author of the review. "Future studies should adopt standardized methods for administering and reporting treatments and defining and reporting outcomes. Additional studies focusing on the most promising types and doses of zinc products and using appropriate statistical methods to assess outcomes that are important to patients will enable us to understand whether zinc may have a place in treatment of the common cold."
