By Michelle Roberts
BBC News health reporter
Homosexual men are requesting a controversial "sex disease" vaccine designed to prevent a female cancer.
Condoms are not guaranteed to stop HPV transmission
Gardasil protects against the most common of sexually transmitted infections, human papillomavirus (HPV), which can cause cervical cancer.
But HPV also causes genital warts and anal and penile cancer, and men argue the jab would guard against these.
Many private clinics are offering it to men. One in London says it has immunised dozens in the last six weeks.
Gardasil has been causing controversy since it was launched in the UK late last year, mainly because it is designed to be given to children before they become sexually active and can catch HPV.
The government is considering whether all girls, and possibly boys, aged 11 or 12 should get it routinely in schools, ultimately to cut cervical cancer rates.
Gardasil is licensed for boys and girls aged nine to 15 and women aged 16 to 26. But doctors can opt to give it to other people "off licence" if they wish.
Dr Sean Cummings at the Freedom Health clinic in Harley Street, where dozens of men have had the jab, said he was happy to recommend Gardasil to his adult men, at £450 for a three-dose course.
"We've had a strong demand for it. I had a man come in for the vaccine this morning. He was 24. Then I have one this afternoon who is 67 years old.
"The motivation is to protect themselves and to prevent spreading HPV to their partners."
Opponents say there is no point in immunising people who are already sexually active.
But Dr Paul Fox, a genito-urinary medicine expert at the Chelsea and Westminster and Ealing hospitals, believes it can be worthwhile.
He argues that it is unlikely a person will have encountered all of the four strains of HPV found in Gardasil, including the two linked to cancers, even if they are leading a very promiscuous sex life.
"We should not just be looking at vaccinating people in their pre-teen years. Other people would benefit as well."
Dr Jo Longstaff, of the Independent General Practice private clinic in Cardiff, which also offers the Gardasil vaccine, agrees.
"Our first enquiry about Gardasil was from a male patient. I think they should be considering it."
Dr Anne Szarewski, clinical consultant for Cancer Research UK who has been involved in evaluating both Merck's Gardasil and GSK's rival jab Cervarix, says there may be a case for immunising men.
"Men who have sex with men are at a much higher risk than average of anal cancer and genital warts, particularly if they are HIV-positive.
"Clearly it would be very important if the vaccine could protect. The problem is we do need proof."
Trials in men
Merck is currently testing the vaccine's efficacy in 4,000 men, including 500 men who have sex with men.
And the US National Institute of Health is also carrying out trials to see what benefits it could have for people with HIV.
Merck said its priority was to tackle cervical cancer, but has not ruled out giving the vaccine to other groups - including men who have sex with men.
Roger Peabody of the Terrence Higgins Trust said if the trials were successful, there would be a good case for vaccinating young boys, not only to stop the spread of HPV to women, but to protect men against HPV-related disease.
Dr Szarewski agreed, saying: "It is bad enough suggesting to people that their 12-year-old daughter might need a vaccine against a sexually transmitted infection.
"I would be interested to see the response of suggesting to parents that they should vaccinate their boys at 12 in case they become gay."
She said heterosexual men and women also risked anal cancer.
About 400 people are diagnosed with anal cancer each year in the UK. The disease is slightly more common in women than men.