Listerine was first invented in the late 19th century, and as early at 1879, manufacturers claimed the disinfectant was effective at both cleaning floors and curing gonorrhea.
Now, 137 years later, scientists have published the first ever randomized controlled trial testing Listerine’s gonorrhea claim in the medical journal Sexually Transmitted Infections. The verdict: the bad-breath mouthwash does indeed kill gonorrhea bacteria, both in petri dishes and in people’s throats.
Gonorrhea is a mild, sometimes asymptomatic bacterial infection that can cause infertility, sterility and even death if left untreated. And if further trials show that Listerine’s immediate effectiveness against gonorrhea translates into a long-term preventive tool, then public health officials and people at high risk of this STI have a cheap, easy way to prevent the disease, said the researchers.
Scientists led by lead study author Eric Chow, a research fellow at the Melbourne Sexual Health Centre in Australia, first tested various concentrations of Listerine to see if it was effective at reducing bacterial counts of gonorrhea in a petri dish compared to saline solution. They found that Listerine dilutions of up to one in four resulted in significant growth inhibition of gonorrhea when exposed for one minute, while saline solution prompted no change.
In the second study, a randomized controlled trial, Chow’s team recruited 58 gay or bisexual men who tested positive for gonorrhea in their throats, asking them to gargle for one minute with either 20 milliliters of normal Listerine or with saline solution.
Five minutes later, the researchers re-swabbed the 58 men and found that those who had gargled with Listerine had significantly lower proportions of viable gonorrhea on the surface of their throats than those who had just used saline solution (52 percent vs 84 percent). The researchers also calculated that the Listerine users had 80 percent lower odds of testing positive for gonorrhea than the men who gargled with salt water.
While Listerine had a moderate effect on the amount of viable gonorrhea in the throat, scientists aren’t sure how long those results last, or how long men would have to gargle and rinse with the mouthwash to prevent future gonorrhea infections in the throat. Chow’s team also points out that although mouthwash may significantly reduce bacteria levels in the throat, it’s still unclear what effect that would have on gonorrhea transmission to other parts of the body like the anus or urethra. And because the study was conducted only among men, Listerine’s effect on women still needs to be tested.
Chow and his team are currently running an even larger trial on 500 men to see if Listerine is effective against gonorrhea bacteria over a longer follow-up period. They’re also planning a series of lab experiments to test a variety of different Listerine products and other mouthwash brands to see which formulations are most effective against gonorrhea cultures in the dish. (The study was not funded or inspired by the manufacturers of Listerine.)
In the U.S., gonorrhea rates have reached unprecedented levels. A recent report showed that at 395,000 recorded cases, gonorrhea is up 13 percent from 2014, and the disease is mostly concentrated among young people ages 15 to 24 years old. Gonorrhea is also more common among men; a 2014 analysis found that 120 men out of every 100,000 had gonorrhea, while only 101 women per 100,000 tested positive for it.
Antibiotic-resistant strains of the bacteria are also on the rise. The U.S. Centers for Disease Control and Prevention notes this kind of gonorrhea has developed resistance to nearly all the medicines used to cure it,and that doctors are down to one last class of effective antibiotics. Chow is hopeful that if his initial Listerine results are confirmed, people at high risk of gonorrhea will have a non-antibiotic weapon against the infection.
“Use of mouthwash could reduce the duration of infection and hence could reduce the number of gonorrhoea cases,” said Chow. “ If the number of gonorrhoea cases [reduces], it will minimize the use of antibiotics.”
So, should you start using Listerine to prevent this common STI? Because the sample size was small, and the before-and-after bacteria samples were taken just a few minutes after gargling, we’ll have to see the results of Chow’s larger, long-term trials to assess whether or not regular use is truly an effective way to prevent gonorrhea infection. But for now, it seems that the solution’s original marketers weren’t so far off the mark when they claimed it could “cure” gonorrhea — or at least significantly reduce bacteria levels in the short term. The old claims that Listerine can cure baldness, dandruff and chapped hands remains undetermined.