Laboratory studies have also suggested potential benefits from gargling, though a major limitation of these test-tube studies is that what works in the lab may not translate to health benefits for patients. A recent German lab study sponsored by a manufacturer of povidone-iodine sore throat gargle solution, for example, reported that the solution was shown to eliminate over 99 percent of the coronaviruses that cause SARS and MERS (very close cousins to the current Covid-19). An earlier Japanese lab study revealed that povidone-iodine products outperformed other common antiseptics such as chlorhexidine gluconate and benzalkonium chloride in inactivating many other common problematic viruses, such as coxsackie, rhinovirus, adenovirus, rotavirus, influenza, to name a few. Clinical trials in people would be needed to determine the clinical relevance for patients.
In the United States, povidone-iodine solutions are sold as skin disinfectants, which contain ingredients that can cause serious harm if ingested; preparations that are suitable for gargling are not generally available in this country. It’s critical that people not gargle with skin disinfectant solutions, including those that contain povidone-iodine. In Canada, a povidone-iodine gargle solution is sold under the brand name Betadine. Some people are allergic to iodine, however, and iodine can cause problems in patients with thyroid problems.
There is less evidence for the potential antimicrobial benefits of other gargle solutions. Listerine antiseptic, for example, has been shown to have antiviral activity in laboratory test-tube studies against some viruses that had been exposed for at least 30 seconds, though studies have not looked at coronavirus.
A recent intriguing clinical study from England involving 66 patients suggested that using a homemade hypertonic saline solution for nasal irrigation and gargling significantly reduced the incidence of flu and colds. (Some common colds are caused by coronaviruses, though they are far less dangerous than the strain of coronavirus that is currently circulating.) While nasal irrigation requires a sterile preparation, gargling does not. The gargling solution in the study involved a heaping teaspoon of salt in a cup of water to make an approximate 3 percent saline solution, gargled up to six times a day. A follow-up lab study by the same group of investigators discovered a potential mechanism of action of the saline solution, whereby throat cells took up extra chlorine from the saline solution to produce a compound that has known antiviral properties.
Other small studies have suggested potential beneficial anti-viral activity of gargling with green tea or solutions containing catechins, active ingredients of green tea, or with apple cider vinegar. These studies, however, were done in the laboratory, so have unknown clinical relevance for patients, and none looked at coronavirus specifically. One study even suggested that gargling with tap water alone may even be helpful in reducing the incidence of upper respiratory infections in a healthy population, although a later study did not confirm this finding.