Safe sex: it’s easier to talk about than to practice consistently. All methods of birth control have drawbacks, and if to the goal of preventing pregnancy you add the imperative to protect against sexually transmitted diseases—an imperative given new urgency in the age of AIDS—the concept of safe, really safe sex becomes still more complicated, since some widely used measures prevent pregnancy but not STDs.

One such measure is oral sex—a practice many people, especially young people, see as “safe” because it doesn’t cause pregnancy. But STDs are a different matter, and a new health warning about oral sex has been sounded by a recently published study of oral HPV (human papilloma virus) infection.

HPV causes mouth and throat cancer; the number of cases of those cancers has been rising for 25 years. In particular, since the millennium, the disease has become much more common among women, who used to get oral cancer very rarely.

A study entitled Prevalence of Oral HPV Infection in the United States, 2009-2010, published in January by the Journal of the American Medical Association, offers data showing a strong connection between HPV infection and oral sex. The study involved 5,579 people aged 14 to 69; the highest prevalence of oral HPV occurred among people in their early 60s.

The study showed that HPV was present in the mouths of 7 percent of those tested. The rate for men (10.1 percent) was much higher than for women (3.6 percent); the reason for that difference, the researchers said, is simply unknown, though study leader Dr. Maura Gillison of the Ohio State University Comprehensive Cancer Center allowed discreetly that it might have something to do with specific ways of engaging in oral sex.

(From a very different source, Cosmo—ever ready to offer advice on the sex front—comes a hint about practices that may be protecting women. To a reader who wrote, “I love giving my guy oral sex, but I don’t like when he comes in my mouth. Can you give me sexy ways to end the session without swallowing?” the magazine replied, “One way to give your man earth-shattering oral pleasure without having him come in your mouth is to finish him off manually.”)

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Though surveys have shown that some 90 percent of adults in the U.S. have had it, the reputation of oral sex as a “safe” sex practice is exaggerated. Oral sex helps spread herpes and HIV (to say nothing of contributing to bladder infections). But the new information is startling because HPV, most notorious as the cause of cervical cancer, is now emerging as a more potent cause of oral cancer than tobacco. Last October, research published in the Journal of Clinical Oncology named HPV as the cause of more than 70 percent of newly diagnosed cases of oral cancer.

And a study published in the New England Journal of Medicine in 2007 concluded that the more oral sex partners a person had had, the more likely they were to develop throat cancer. People who had had one to five oral sex partners are twice as likely as those who have never had oral sex to develop throat cancer, the study found; those who have had six or more partners increase their chances by 250 percent.

HPV is extremely common and often innocuous; at any given time it’s present in the mouths of many people, and healthy immune systems usually dispose of it with no bad effects. But a persistent strain of it can linger in a person’s system for years and eventually cause cancer.

Vaccines to prevent the worst outcomes of HPV infection may be available in the near future. Two vaccines, Gardasil and Cervarix, have been approved by the government for young women because they protect against the kinds of HPV that cause cancer of the cervix; Gardasil is also recommended for boys because it protects against HPV strains that cause genital warts. The federal Centers for Disease Control recommends that it be administered to children of both sexes at ages 11 or 12. On the other hand, the effectiveness of Gardasil at preventing oral and throat cancers has not yet been established.

Meanwhile, oral sex should be practiced in a protected way, experts say. That means that some sort of barrier—cut-open condoms, dental dams, Sheer Glyde dams or plastic wrap—should be in place between mouths and bodily fluids.

Regular visits to alert dentists who can spy out sores, lesions or discolorations in the mouth have been known to help in the detection of incipient cancers.

And there is good news here: cancers of the type caused by HPV are generally easy to cure if detected in the early stages, especially in young people.