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If four out of five TV doctors tell you something is true, chances are, there’s not any evidence to actually back up that claim. But now, for at least one of those claims, there is evidence—even if it took 137 years to get it. The manufacturers of Listerine have been claiming that the mouthwash can cure gonorrhea, a common sexually transmitted infection that has been increasing in the U.S. the past two years, since 1879.

For reference, Rutherford B. Hayes was president then, the Civil War had only been over for 14 years, the first female lawyer argued before the Supreme Court, the first ice rink had just been established at Madison Square Garden, and Thomas Edison first demonstrated the incandescent lamp after perfecting the light bulb. (Or, for real history buffs, that was the year Henry Morton Stanley returned to the Congo to continue raping the country, having found “Dr. Livingstone, I presume?” eight years earlier.)

That’s quite a claim for an era before antibiotics were available. But no one actually tried to prove it—until now. The small study, published Dec. 20, did not find evidence that Listerine actually cures gonorrhea, but it did find that the over-the-counter mouthwash kills enough of the bacteria to render the disease absent from the mouths of about half of those infected.

That means that a good gargle before engaging in oral sex may reduce the risk of transmitting the infection to the recipient, though additional research would need to confirm those findings before any medical professionals actually recommended that strategy. The study was not funded by Listerine manufacturers but rather by the Australian National Health and Medical Research Council.

Gonorrhea is the second most common “notifiable disease”—those that are required to be reported to the CDC—in the U.S. and increased to 124 cases per 100,000 last year. It’s a leading cause of pelvic inflammatory disease, which can lead to tubal infertility, ectopic pregnancy and chronic pelvic pain in women, according to the CDC. The bacterial infection also increases the risk of contracting HIV, the virus that causes AIDS.

To test the claims about Listerine, scientists first used Listerine Cool Mint and Listerine Total Care, both 21.6% alcohol, in petri dishes with a colony of gonorrhea bacteria. They diluted the Listerine at different levels, from a 1:2 dilution to a 1:32 dilution, and compared its ability to kill the bacteria to saline. At dilutions of 1:2 and 1:4, the Listerine successfully wiped out the gonorrhea, unlike the saline.

Then, the researchers conducted a randomized controlled trial with 196 men who have sex with men. All the men received a standard treatment of antibiotics ceftriaxone and azithromycin, but 104 of them also gargled for one minute as far back in their throats as they could with 20 mL of Listerine Cool Mint. The other 92 gargled with saline.

Five minutes later, the scientists swabbed the men’s tonsils and the backs of their throats. Ultimately, only 58 of the men tested positive for gonorrhea in throat and tonsil swabs before gargling, so only these men’s results were analyzed: 33 who gargled with Listerine and 25 who gargled with saline.

Of these post-gargling swabs, 84% of the men who gargled with saline had a positive result for gonorrhea, but just 52% of the men who gargled with Listerine did. So half the men who gargled with Listerine for one minute no longer had detectable quantities of gonorrhea bacteria in their mouths.

This is good news, but several major caveats should be immediately noted: First, the study is very small. Second, the men were tested only 5 minutes after one minute of gargling, and the researchers did not test longer-term effects of Listerine on mouth bacteria. For all we know, the bacteria could have returned a day later, or an hour later or 10 minutes later.

Third, half the men who gargled with Listerine still tested positive for having detectable levels of gonorrhea bacteria in their mouths, so the short-term effect is only half effective based on this small group. Fourth, the study in no way suggests that Listerine can “cure” gonorrhea. Only prescribed antibiotics have been shown to treat the infection. Fifth, the intervention only addresses oral gonorrhea—no one is testing or suggesting using mouthwash in the genital area. That may not work out too well.

And yet, the findings, especially if confirmed in larger studies, do suggest that gargling with Listerine can reduce the risk of oral transmission of the STI, and that’s no small thing.

“If daily use of mouthwash was shown to reduce the duration of untreated infection and/or reduce the probability of acquisition of N. gonorrhoeae then this readily available, condomless and low-cost intervention may have very significant public health implications in the control of gonorrhoea in men who have sex with men,” the researchers wrote. “Interventions such as this are urgently needed in the context of rising rates of gonorrhoea in men who have sex with men and the likelihood that condom use may fall further as condom-free HIV preventions are more widely adopted.”

The authors recommend that future studies look at long-term effects and long-term use of Listerine, such as daily use over months. This study, then, was more of a proof-of-concept study that provides the evidence needed to justify additional larger, longer, more expensive and more involved studies. The longer lasting effects of Listerine in the petri dish experiment suggest the same may be possible in people.

“In addition, if the load of N. gonorrhoeae was reduced by the mouthwash after a single dose then one might expect that daily mouthwash over weeks would potentially reduce the viable number of N. gonorrhoeae, and hence reduce further transmission or reduce the mean duration of infection,” the authors wrote.

The researchers don’t know if it’s the alcohol content or another ingredient or combination of ingredients in Listerine that killed the bacteria. Therefore, gargling with, say, a shot of vodka before fellatio may not necessarily have any protective effect.

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