Listerine (Johnson & Johnson Consumer Inc) mouthwash inhibits the growth of oral gonorrhea bacteria in the mouth, a new study has found.

In a randomized trial, 52% (95% confidence interval [CI], 34% - 69%), of the pharyngeal surfaces of men who rinsed and gargled with the mouthwash for a minute tested positive for Neisseria gonorrhoeae compared with 84% (95% CI, 64% - 95%) of the pharyngeal surfaces of men who rinsed and gargled with a saline solution.

"With daily use it may increase gonococcal clearance and have important implications for prevention strategies," Eric P. F. Chow, PhD, from Melbourne Sexual Health Centre in Australia, and colleagues write in an article published online December 20 in Sexually Transmitted Infections.

Gonorrhea infections among men are increasing in many countries as condom use declines, mostly among men who have sex with men (MSM), the researchers explain.

In Australia, new diagnoses doubled from 6892 to 11,508 in the last 5 years, with 70% of these infections in MSM, the researchers write.

As the incidence of the infections rises, so do concerns about the risk for antibiotic resistance, highlighting the need for new preventive measures, they say.

As early as 1879, well before the advent of antibiotics, the manufacturer of Listerine claimed it could be used to cure gonorrhea. Until now, however, no published research has tested this claim.

To fill that gap, Dr Chow and colleagues assessed whether the mouthwash could curb the growth of N gonorrhoeae in laboratory tests and in a clinical trial of MSM.

In the laboratory tests, the researchers applied various dilutions up to 1:32 of Listerine Cool Mint and Total Care to cultures of N gonorrhoeae to see which, if any, of them might curb growth of the bacteria. Both formulations contain 21.6% alcohol. The authors also applied a saline solution to an identical set of cultures.

Listerine at dilutions up to 1 in 4, applied for 1 minute, significantly reduced the number of N gonorrhoeae on the culture plates. The saline solution had no effect.

Encouraged, the researchers undertook a clinical trial, starting with 196 MSM who had tested positive for gonorrhea in their mouths or throats and were returning for treatment at Melbourne Sexual Health Centre between May 2015 and February 2016.

At this return visit, 58 of the men once again tested positive for the bacteria on their pharynxes. The median age of these men was 27 years. They had had a median of six sexual partners in the previous 3 months.

The researchers assigned 33 men to rinse and gargle for 1 minute with the mouthwash, and 25 to rinse and gargle with a saline solution for 1 minute.

The researchers then retested the men and found that those in the mouthwash group were 80% less likely to test positive for gonorrhea on their pharyngeal surface compared with the other men (odds ratio, 0.20; 95% CI, 0.07 - 0.72).

Similarly, of the men who gargled and rinsed with the mouthwash, 57% (95% CI, 34% - 77%) tested positive for gonorrhea at the tonsillar fossae compared with 90% (95% CI, 68% - 99%) of those in the saline group (P = .016).

Men in the mouthwash group were 86% less likely to test positive on this surface than men in the saline group (odds ratio, 0.14; 95% CI, 0.03 - 0.77).

Of the men in the mouthwash group, 57% (95% CI, 34% - 77%) were infected on their posterior oropharynx compared with 70% (95% CI, 46% - 88%) of those in the saline group.

Although the difference between groups in pharyngeal surface infections was statistically significant (P = 0.013), as was the difference in tonsillar fossae infections (P = .016), the difference in posterior oropharynx infections was not (P = .277).

The researchers acknowledge that the follow-up time in the study was short, so it is possible that the effects of the mouthwash might be short-lived. However, the laboratory test results suggest a longer-term effect, they reason.

Whether or not an oral mouthwash can reduce gonorrhea infections of the anus and urethra remains to be seen. Some studies have suggested that pharyngeal gonorrhea spreads to the anus and urethra, the researchers note.

As people are less likely to have symptoms from oral gonorrhea infections, they are less likely to receive treatment rapidly than with urethral infections, the researchers point out.

A larger trial is currently underway to confirm the results of this study and see whether the use of mouthwash could curb the spread of gonorrhea, according to a news release from the journal.

The study was funded by the Australian National Health and Medical Research Council. The researchers have disclosed no relevant financial relationships.

Sex Transm Infect. Published online December 20, 2016. Full text