Gay and bisexual men in San Francisco are signing up to take Gardasil, the vaccine intended to prevent the human papilloma virus, or HPV, in women, reports one AIDS advocacy organization.

According to Jason Riggs, spokesman for San Francisco's STOP AIDS Project, some gay and bisexual men are asking their doctors for the vaccine in the hope that it will ward off anal and penile cancer which are also caused by HPV.

"The prevalence of anal cancer among gay and bisexual men is very high. So that's why some people are looking at this as a possible preventive cure for anal cancer and HPV that causes anal cancer," Riggs said.

A clinic run by the San Francisco Department of Public Health is offering Gardasil and some men have been paying the $350 for the standard three doses.

No one is sure about the effectiveness of the vaccine in men, however.

"It would be nice to see some published data that the vaccine works in men," said Dr. Jeff Klausner, who is also in charge of prevention and control of sexually transmitted diseases in San Francisco.

According to Klausner the difference between the male and female anatomy makes it difficult to assume that if Gardasil works in women it will also work in men.

HPV is considered the most common STD in the U.S. among both men and women.

One study estimated that approximately 95 percent of HIV-positive gay men have been infected with HPV. That same study estimated that 65 percent of HIV-negative gay men have been infected with HPV. Those infections were usually in the anal canal or surrounding skin.

As HPV is determined via clinical diagnosis without a confirmatory lab test, it is almost impossible to determine the exact number of people who have the virus.

The incidence of anal cancer is more prevalent in the gay and bisexual population, at rates 35 times higher than that of the general population, reports VaccineRX.com. It's even higher among those who are infected with HIV.  This is probably due in part to the fact that men are now surviving longer with recent effective HIV treatments, and are therefore experiencing rising rates of other, previously uncommon cancers.

Still, concerns have been raised over the validity of men taking Gardasil, since it is not yet clear whether the drug is as effective for men as it is for women. 

Anal cancer is caused by "high risk" strains of HPV that can only be detected with an anal Pap test. In this procedure, a health care provider passes a swab into the rectum and collects cells. These cells are then put on a slide that is examined by an experienced pathologist in a lab under a microscope.

It takes many years for pre-cancerous anal lesions to develop into cancer. If the cells show changes of the kind that could lead to cancer, the patient should undergo a thorough rectal exam by an experienced proctologist. If the problem is caught early, before a cancer develops, it can easily be treated in a doctor's office.

Treatment for these anal lesions varies from simple in-office procedures, such as freezing the cells or applying a topical cream, to surgery or biopsy, if the lesions are widespread. Treatment is very effective in removing lesions, but they may come back in the same or other areas, so it is important to continue to have regular screenings. Persons with advanced tumors will often need other treatment.