Kissing with tongues can spread the sexually transmitted infection gonorrhoea, researchers have warned.
Scientists say the STI can be caught through saliva – despite the NHS dismissing kissing as a way of passing it on.
And spreading the infection this way may be more common than expected, the Australian researchers claimed.
They found gay men were more likely to have gonorrhoea in their throat than their penis – and the risk of spreading it was greater for kissing than for oral sex.
Antiseptic mouthwash could, the scientists suggested, be created to try and kill the bacteria, which can cause a sore throat and swollen lymph nodes.
The research into oropharyngeal gonorrhoea comes amid growing concerns about 'super' strains of the STI that are becoming resistant to medicine.
Scientists at Monash University and the Melbourne Sexual Health Centre in Australia surveyed around 3,000 gay and bisexual men in the city.
'A number of pieces of evidence suggest transmission from the oropharynx [back of the throat] may be more common than previously thought,' Professor Eric Chow and his colleagues wrote in the paper.
'[The bacteria] can be cultured from saliva, suggesting that the exchange of saliva between individuals may potentially transmit gonorrhoea.
'Several case reports in the 1970s suggested kissing as a mode of transmission for oropharyngeal gonorrhoea... but kissing has always been neglected as a risk factor'.
Professor Chow said a rise in global reports of gonorrhoea strains which are evolving to stop antibiotics working against them is cause for concern.
Doctors and scientists must find new ways of stopping the infection, he warned, and efforts are currently focused on encouraging condom use during sex.
Discovering a way the bacteria are spreading under the radar could lead to new ways of spotting it sooner and preventing infections.
Men in the study were more likely to have gonorrhoea in their throat than in their penis or anus, the researchers found.
More than six per cent had the infection in their mouth while just under six per cent had it in their anus, and only three per cent in the penis.
And the proportion of men with the STI was higher in a small group (52 men) who reported having only kissed men in the past three months without having sex.
Those who did the opposite – sex without kissing – were the least likely to have the STI, compared to only-kissers or those who did both.
Kissing more partners also led to a significantly higher risk of the infection – those who French-kissed four or more men were 46 per cent more likely to have it.
This risk rose to 81 per cent if they also had sex with the people they kissed, showing the risk increase for sex (35 per cent) was smaller than that for kissing.
'Previously, it has generally been assumed that oropharyngeal gonorrhoea can only be acquired from an infected penis or anus, not from another man’s [throat],' the researchers said.
'Hence, the importance of the oropharynx has been neglected in gonorrhoea transmission.
'Our results suggest kissing with or without sex may be a risk factor for oropharyngeal gonorrhoea.'
In contrast to the team's findings, the NHS website says: 'Gonorrhoea is not spread by kissing, hugging, swimming pools, toilet seats, or sharing baths, towels, cups, plates or cutlery.'
The scientists added that using spit as lubricant during sex or oral contact with the anus could also spread the infection from the throat to the anus or genitals.
They acknowledged the study, based on homosexual men at only one sexual health centre, may be difficult to apply to the wider population, but women can also catch oral gonorrhoea.
Super gonorrhoea – strains of the infection which have evolved to resist treatment with typical antibiotics – is becoming a growing concern around the world.
Experts have warned it will only be a matter of time until the infection becomes untreatable with current methods.
Two women in the UK were this year among the first to be diagnosed with super gonorrhoea, with both their infections being traced back to holidayers in Ibiza.
The research is published in the journal Sexually Transmitted Infections, part of the British Medical Journal.