Gut Check looks at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The claim:

Gargling with Listerine can eliminate gonorrhea throat infection, scientists reported on Tuesday in the journal Sexually Transmitted Infections.

Tell me more:

When Listerine was concocted in 1879, inventor Dr. Joseph Lawrence asserted that it could fight gonorrhea (and also clean floors). In all that time there have been no published studies of the claim. So scientists in Australia stepped up, partly because gonorrhea has become increasingly common: In Australia, rates have risen from 62 cases per 100,000 men in 2010 to 99 in 2015; in the US, from 98 cases per 100,000 in 2009 to 124 in 2015.

Scientists led by Dr. Eric Chow of the Melbourne Sexual Health Center ran two experiments. First they added Listerine Cool Mint or Total Care to lab dishes full of Neisseria gonorrhoeae bacteria (fun fact: also discovered in 1879). Undiluted, both flavors left zero bacteria alive in the dishes after a minute of exposure. Even diluted 4-to-1 they killed most of the microbes.

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Then the researchers enrolled men who have sex with men, and who tested positive for gonorrhea in their mouth or throat, in a randomized clinical trial: 104 rinsed and gargled once with Cool Mint, 92 did so with saline. The men also received antibiotics, but “antibiotics don’t do as good a job of curing gonorrhea of the throat as at other sites of infection,” said Dr. Edward Hook of the University of Alabama, Birmingham, an advisor of the American Sexual Health Association, and an expert on sexually transmitted infections who was not involved in the study.

Five minutes later, 84 percent of the saline group still tested positive for gonorrhea on a throat culture. Only 52 percent of the Listerine users did. Testing negative “means there is no viable N. gonorrhoeae present on the swab,” Chow said.

Listerine was much more effective at eliminating gonorrhea bacteria around the tonsils than further back, possibly because more of it reached the former: gargling well takes practice. “It was possible that not all men gargle[d] enough,” he said.

Listerine contains alcohol, but the researchers aren’t sure which ingredient was responsible for killing N. gonorrhoeae.

Really?

There’s a remote possibility that there were viable bacteria on the throat culture, but in amounts too small to detect. And testing people five minutes after they gargled doesn’t show whether Listerine had more than a short-lived effect.

On the other hand, in the lab bench study the microbes, once gone, stayed gone. And the finding that a single gargle eliminated gonorrhea bacteria in the throats of nearly half the men raises the possibility that doing it regularly might keep the microbes away and prevent gonorrhea transmission through oral sex.

“Their experiments suggest that the contents of Listerine do kill [gonorrhea] bacteria,” Hook said. “That’s quite interesting.”

Also important: Gonorrhea infections of the throat almost never produce symptoms, Hook said, not even a sore throat. Doctors rarely test for it, so people don’t know they have it. But, through unprotected oral sex, gonorrhea can cause genital and urethral infections, which do have symptoms, such as pain and discharge. That makes pharyngeal gonorrhea “a real public health problem,” Hook said.

The verdict:

The in vitro experiment plus the clinical trial make a strong case that Listerine can kill gonorrhea. To nail that down, a larger study will have to test people for longer to be sure infections are gone and to see what gargling technique is most effective.

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