Men who perform oral sex on their partners have a high risk of contracting a rare form of mouth or throat cancer, according to new research.
Smokers who have had more than five sexual partners are at even greater risk of developing the cancer triggered by the human papilloma virus (HPV) - which is the main cause of cervical cancer.
Two HPV types (16 and 18) cause 70% of cervical cancers and precancerous cervical lesions, according to the World Health Organisation.
But scientists say that only 0.7 per cent of men - seven in every 1,000 - will ever develop HPV-related oropharyngeal cancer in their lifetimes.
The risk was much lower among women, those who did not smoke, and those who had less than five oral sex partners in their lifetimes.
Film star Michael Douglas was diagnosed with tongue cancer in 2013 and he famously seemed to suggest it was brought on by HPV infection contracted through cunnilingus.
An estimated eight out of 10 British adults will be infected with the HPV virus at some point in their lives.
There are hundreds of different types of HPV and while most are harmless around 12 can cause cancer.
HPV 16 or 18 triggers most cervical cancer while HPV16 most throat cancer.
It is transmitted to the mouth and throat mostly by performing oral sex and appears to cause about 70 per cent of oropharyngeal cancers.
These cancers appear at the back of the throat, base of the tongue, or tonsils.
HPV-related oropharyngeal cancers approximately tripled in British men and doubled in British women between 1995 and 2011.
But the number of cases of oropharyngeal cancer is predicted to overtake cervical cancer by 2020, US scientists warned.
Associate Professor Amber D’Souza, of Johns Hopkins Bloomberg School of Public Health in the US, said: “For these reasons, it would be useful to be able to identify healthy people who are most at risk of developing oropharyngeal cancer in order to inform potential screening strategies, if effective screening tests could be developed.
“Most people perform oral sex in their lives, and we found that oral infection with cancer-causing HPV was rare among women regardless of how many oral sex partners they had.
“Among men who did not smoke, cancer-causing oral HPV was rare among everyone who had less than five oral sex partners, although the chances of having oral HPV infection did increase with number of oral sexual partners, and with smoking.”
The study analysed behaviour and medical records of 13,089 people aged 20 to 69 taking part in the National Health and Nutrition Examination Survey (NHANES) who had been tested for oral HPV infection.
It used the numbers of oropharyngeal cancer cases and deaths to predict the risk of cancer from oral HPV infection.
Oral infections with the dozen HPV types known to cause oropharyngeal cancer especially HPV 16 were present at low prevalence in every defined group in the study,
Women ages 20 to 69, for example, had a frequency of infection of just over 1 per cent, compared to 6 per cent for men ages 20 to 69.
Men ages 50 to 59 were most likely to have an infection at 8.1 per cent of any age group.
Oral sex was clearly associated with a higher prevalence of infection, although the highest infection prevalence was seen only among men.
Women with 10 or more lifetime oral sex partners had a relatively low, 3.0 per cent prevalence of infection, whereas for men with 10 or more lifetime oral sex partners the figure was 14.4 per cent.
Prevalence of infection for those reporting zero or one lifetime oral sex partner was consistently low, between 0 and 2.4 per cent.
Smoking also was associated with higher oral HPV prevalence
Prevalence was 14.9 per cent among men who smoked and reported five or more lifetime oral sex partners, compared to less than half that - 7.3 per cent for men who reported five or more lifetime oral sexual partners but did not smoke.
Co-author associate professor Dr Carole Fakhry at the Johns Hopkins Department of Otolaryngology said: “Currently there are no tests that could be used for screening people for oropharyngeal cancer.
“It is a rare cancer and for most healthy people the harms of screening for it would outweigh the benefits because of the problem of false positive test results and consequent anxiety.
“Our research shows that identifying those who have oral HPV infection does not predict their future risk of cancer well, and so screening based on detecting cancer-causing oral HPV infection would be challenging.
“However, we are carrying out further research of oral HPV infection in young healthy men to explore this further.
“Other research is being done on different biological markers and it is possible some of them could be used for oropharyngeal cancer screening in the future in some people; for example, they might be useful in men but not in women given their lower cancer risk.
“Some studies suggest people who have antibodies against cancer-causing types of HPV have an increased risk of HPV-related cancer, but these antibodies are very rare.
“Therefore, it is not yet clear whether they will be useful for screening. Presently, these tests are not commercially available, and are still in research labs only.”
There are around 16,500 cases of oropharyngeal squamous cell carcinoma (OPSCC) every year in the USA and over 11,500 of these are HPV-related, meaning that around 70% of all OPSCC in the USA is HPV-related.
The incidence of HPV-related oropharyngeal cancer is around 6.6 per 100,000 among men and 1.4 per 100,000 women.
The study was published in the journal Annals of Oncology