Fifty or so years ago, the owners of Listerine — the mouthwash with “the taste people hate twice a day” — claimed their magic but foul-tasting elixir could cure numerous maladies, including sore throats, the common cold, tooth decay and gum disease.

The Federal Trade Commission began putting an end to those false, unsubstantiated claims in 1978, when it ordered the company to spend $10 million on an ad campaign informing the public that, “Contrary to prior advertising, Listerine will not help prevent colds or sore throats or lessen their severity.”

So, the results of two recent peer-reviewed studies that found Listerine and an array of cosmetic and therapeutic mouthwashes kill the novel coronavirus in the laboratory should be approached cautiously.

The studies are in vitro, in glass, in the test tube, in the petri dish, and we won’t know if these compounds work on actual living organisms, in this case human beings, until in vivo studies are done.

Nevertheless, for dentists, dental hygienists and other dental healthcare providers, there’s plenty to celebrate in the studies, since they both validate some practices already put in place by dental offices when the pandemic took off in the United States last March and point the way forward for future research.

In July, The Journal of Infectious Diseases published a study by multiple authors, “Virucidal Efficacy of Different Oral Rinses Against Severe Acute Respiratory Syndrome Coronavirus 2.”

In September, The Journal of Medical Virology published “Lowering the transmission and spread of human coronavirus.” Both studies put cosmetic and therapeutic mouthwashes available over the counter and by prescription to the test.

The former study tested the effects of various mouthwashes on SARS-CoV-2. The latter study tested mouthwashes as well as several nasal rinses against human coronavirus, HCoV, which costs less to replicate than SARS CoV-2 and has a similar fatty, protective covering.

Cracking that fatty covering is what virucides do, and both studies suggest that the common household chemical hydrogen peroxide, H2O2, may be an effective virucide for eradicating novel coronavirus in oral cavities.

“The three products with H2O2 as their active ingredient all demonstrated similar abilities to inactivate HCoV,” the Journal of Medical Virology study found. “Crest Pro‐Health decreased infectious virus by 99.9 percent to more than 99.99 percent; again, the contact times used made little difference.”

The contact times in the study were 30 seconds, 1 minute and 2 minutes. Theoretically, gargling with a diluted hydrogen peroxide solution for just 30 seconds may reduce novel coronavirus in the patient’s mouth and throat by 90% to 99.9%.

The American Dental Association concurs. Since the onset of the pandemic it has advised dentists to require patients “rinse with 1.5 percent hydrogen peroxide or commercially available rinses that contain 1.5 percent hydrogen peroxide just prior to beginning treatment.”

Lori Gordon Hendrick, MS, RDH, CDA, CDT and a dental office infection control consultant offered more details about hydrogen peroxide on the Registered Dental Hygienist website last March.

“The concentration of any rinse used should be at least 1.5 percent,” she wrote. “Patients should rinse for at least 1 minute. Over-the-counter peroxide is typically 3 percent; therefore, one should dilute 1-part hydrogen peroxide to 2-parts water. In the dental industry, we have several products [that contain H2O2]. Two examples are Colgate Peroxyl and Listerine Whitening Mouthrinse, both of which contain 1.5 percent peroxide. Also available is EverSmile which contains a 3.8 percent concentration of peroxide.”

Hendrick also endorsed oral rinses containing the active ingredient povidine-iodine, or PVP-I. Similarly, the study published by the Journal of Infectious Diseases found that a commercially available mouthwash containing PVP-1, Iso Betadine 1 Percent, was one of three products tested that “significantly reduced viral infectivity to undetectable levels.”

Somewhat unbelievably, given the dubious medical and dental claims made in its past, Listerine, the old-school nasty tasting stuff (not the brand’s numerous spinoff products), may be one of the most potent SARS-CoV-2 virucides on the market according to both studies.

“Listerine Antiseptic is an alcohol‐based eucalyptol, menthol, methyl salicylate, and thymol formulation that historically has claimed numerous antimicrobial properties,” notes the Journal of Medical Virology study. “It currently lists only a claim to kill germs that cause bad breath. Our tests show that it is highly effective at inactivating HCoV in solution. Even at the lowest contact time of 30 seconds it inactivated greater than 99.99 percent of HCoV.”

Critics of this study have pointed out it used HCoV, human coronavirus, which isn’t a perfect stand-in for SARS-CoV-2. But the study published by the Journal of Infectious Diseases did use SARS-CoV-2 and expressed similar findings about Listerine.

“In agreement with our observation, different studies using Listerine observed antiviral activities specifically against enveloped viruses, implying an impact on the viral lipid envelope,” the study found. “The in vivo effects of the oral solutions require further analysis during clinical studies.”

The study concludes by noting that said in vivo experiments are already in the works.

Oral and naval cavity rinses — a 1% solution of Johnson and Johnson’s Baby Shampoo up the nose has proved effective at treating certain rhinoviruses — by definition are only a partial solution. Novel coronavirus thrives throughout the respiratory system and can hide in places medicine can’t yet reach.

In a story responding to the studies, The New York Times put it this way:

“Relying on mouthwash or a nasal rinse to rid the body of infectious virus would be about as futile as trimming the top of a cluster of weeds, paying the roots little mind, and expecting the garden pests to disappear.”

Still, both studies end on an upbeat note.

“Nasal rinses and mouthwashes, which directly treat the major sites of reception and transmission of HCoV, may provide an additional level of protection against the virus,” states the study published in The Journal of Medical Virology.

“While clinical trials will be necessary to confirm the virucidal potential of these products and assess their ability to limit transmission of HCoV within the general population, in the current manuscript we have demonstrated here that several commonly available healthcare products have significant virucidal properties with respect to HCoV.”

The study published in The Journal of Infectious Diseases, which did use SARS-CoV-2 in its experiments, ends with a forward-looking statement.

“Our findings clearly advocate the evaluation of selected formulations in clinical context to systematically evaluate the decontamination and tissue health of the oral cavity in patients and healthcare workers to potentially prevent virus transmission,” it states.

Let the experiments begin. In the dental setting, it seems likely that the SARS-CoV-2 viral load—the risk to workers in the dental workplace — has been lowered by requiring patients to rinse with hydrogen peroxide and povidine-iodine solution before dental procedures.

Like wearing proper personal protective gear and maintaining social distance, rinsing your oral and nasal cavities out with a virucidal solution may become yet another tool for stopping the spread of the pandemic.

Not that Listerine, now owned by Johnson and Johnson, dares make such a bold claim. On its COVID-19 FAQ page, it makes it plainly clear that the only thing Listerine Antiseptic can do is kill “99.9% of germs that cause bad breath, plaque and gingivitis.”

“Listerine mouthwash has not been tested against the coronavirus and is not intended to prevent or treat COVID-19,” the company continues. “Consumers should follow the preventive measures issued by the World Health Organization including washing hands frequently, maintaining social distance and avoid touching your eyes, nose and mouth.”

Sound advice.