The FDA today approved Merck’s Gardasil HPV vaccine for prevention of anal cancer in both males and females ages 9 through 26 years.

HPV — human papillomavirus — is a sexually transmitted infection. It most commonly causes genital warts, but it also causes several cancers and precancerous lesions.

Gardasil already is approved for preventing cervical, vulvar, and vaginal cancer in females, and for preventing genital warts in both males and females.

However, the vaccine is routinely recommended only for girls. It remains optional for boys. Another HPV vaccine, Cervarix, is approved only for cancer prevention in girls.

Anal cancer is relatively rare in the U.S. — about 5,300 cases a year. However, cases have been steadily increasing.

While men who have sex with men are at highest risk, anal cancer is more common in women than in men.

“Treatment for anal cancer is challenging: The use of Gardasil as a method of prevention is important as it may result in fewer diagnoses and the subsequent surgery, radiation, or chemotherapy that individuals need to endure,” Karen Midthun, MD, director of the FDA center for biologics evaluation and research, says in a news release.

In November 2010, an FDA expert advisory panel recommended that the agency approve Gardasil for anal cancer.

HPV vaccines cannot prevent cancer in women or men already infected with the strains of HPV included in the vaccines. That’s why the vaccine is most commonly given to girls and boys before they become sexually active.

The CDC’s Advisory Committee on Immunization Practices, which advises the U.S. Department of Health and Human Services on vaccination policy, likely will discuss extending routine Gardasil vaccination at its February 2011 meeting.