A new review of existing evidence conducted by researchers in Australia points to a modest but real benefit from using zinc to combat the common cold. The study found that zinc supplementation can likely prevent and shorten the duration of typical respiratory infections. At the same time, there’s much less known about the best way to take zinc for a cold, and the jury is still out on whether zinc can help covid-19.
The history of zinc being touted as a common cold treatment dates back to at least the 1980s. Zinc is an essential mineral and trace element, important for a variety of biological functions, and it’s known that zinc deficiency can increase our susceptibility to infection. But the primary theory behind zinc as a cold remedy is that it can broadly interfere with the replication of viruses that invade the upper respiratory tract when taken early, preventing or at least weakening the infection. It can be taken as a lozenge, nasal spray, or liquid.
The evidence for zinc’s benefits as a cold treatment has been mixed, though, and occasionally muddled with controversy. In 2009, the Food and Drug Administration infamously warned the public to stay away from brands of a zinc nasal spray and swab sold by the company Zicam, following reports of users losing their sense of smell afterward (Zicam denied culpability but did pull the products off the market and settled a class action lawsuit over the claims for $US12 ($16) million).
The current paper, according to the authors, wasn’t originally intended to study whether zinc was any good for colds in general. Their primary goal had been to look at the usefulness of zinc for covid-19, since the mineral has emerged as a popular home remedy for the viral illness. But none of the registered clinical trials testing zinc for covid-19 that they found were finished yet. Instead, they decided to treat this new review, published Tuesday in BMJ Open, as an update to existing reviews of the literature on zinc and colds.
The group looked at data from 28 randomised and controlled trials of zinc, collectively involving nearly 5,500 participants. Some studies tracked people taking zinc to prevent a cold; others looked at people who took zinc soon after they reported symptoms; and some even deliberately exposed healthy people to the human rhinovirus, a mild but ubiquitous source of the cold. Most were conducted in the U.S., though some also occurred in China, Western Europe, and Australia.
Overall, they found modest benefits throughout for zinc compared to placebo. They estimated, for instance, that zinc lozenges or nasal sprays would prevent five cold cases for every 100 people who took them for a month. On average, they also found that lozenges and nasal sprays were associated with two fewer days of symptoms, compared to placebo. And they estimated that about 19 fewer people out of every 100 would have cold symptoms by day seven if they had been taking zinc.
At the same time, not all of the findings were positive. The team found data indicating that zinc users would experience less severe symptoms by day three, but overall, they didn’t find evidence that zinc reduced the average daily severity of a cold. In the trials that deliberately exposed volunteers to a rhinovirus cold, they also found no evidence of benefits in prevention or treatment from taking a zinc lozenge. And people taking zinc did experience more adverse events than the placebo group, including nausea, mouth irritation, and altered taste, though none were severe and no cases of anosmia (the loss of smell) were reported by nasal spray users.
Perhaps most importantly, the group found no consistent pattern in the data for how to take zinc optimally — both in terms of the safest dose to take, the best route of administration, and whether any other treatments may work in conjunction with zinc to boost its effects.
Despite the shortcomings of the data, though, the researchers do say that doctors and patients should consider zinc a viable option for preventing and treating the common cold, particularly in comparison to over-the-counter medications that have worse side effects or medicines that will definitely have no effect but are sometimes prescribed to reassure patients, like antibiotics for a suspected viral infection.
“The marginal benefits, strain specificity, drug resistance, and potential risks of other over-the-counter and prescription medications makes zinc a viable ‘natural’ alternative for the self-management of non-specific [respiratory tract infections],” the authors wrote.
However, they add, “clinicians and consumers need to be aware that considerable uncertainty remains regarding the clinical efficacy of different zinc formulations, doses, and administration routes. They also note the possibility that zinc may not work for all viruses that cause respiratory infections. Where covid-19 fits into that picture is still an open question, the authors note, though some studies haven’t found a real benefit. And there needs to be more and better research into zinc in general before it’s likely to be widely recommended by health authorities as a cold remedy.
“Clarification of the efficacy and mechanism of zinc in viral respiratory infections, including SARSCoV-2 infections, warrants further research,” they wrote.