People falsely believe that unprotected oral sex is safe, according to a new study.

The Centers for Disease Control and Prevention report shows that a substantial proportion of syphilis (search) cases occurred through unprotected oral sex. The CDC recommends that people should use condoms during oral sex to reduce the risk of sexually transmitted diseases.

Syphilis is a sexually transmitted disease, and can be transmitted through oral sex. Infection with bacteria can cause serious problems. Called 'the great imitator,' syphilis can cause problems ranging from a painless sore, occurring on the genitals, lips, or mouth, or a rash in its later stages of infection.

If untreated, syphilis can eventually cause problems in blood vessels or the heart, as well as mental disorders, blindness, nerve system problems, and even death.

Syphilis can also be a gateway for HIV transmission (search). Syphilis lesions around the mouth increase the risk for HIV transmission, and syphilis "might also increase progression of HIV disease," says the CDC in the Oct. 22 issue of its publication, Morbidity and Mortality Weekly Report.

A recent study by Chicago's Department of Public Health showed that syphilis is more common than people may think.

From 1998-2002, the city of Chicago had a total of 1,528 cases of primary and secondary syphilis. That's more than any other American city.

During much of the 1990s, most of those cases were reported among heterosexuals. But since 2001, men who have sex with men have accounted for nearly 60 percent of the city's primary and secondary syphilis cases, according to the CDC.

To find out what caused the change, officials from the Chicago Public Health Department interviewed people with syphilis.

Overall, almost 14 percent of the participants surveyed said that they only had oral sex during the time frame when they were likely infected with syphilis.

Among male syphilis patients with male sex partners, 20 percent said oral sex was their only sexual exposure during the transmission period.

In contrast, 6 percent of heterosexual male syphilis patients and 7 percent of heterosexual women with syphilis said they only had oral sex around the time they got syphilis.

Many people think oral sex is "safe" sex that doesn't need a condom. However, the CDC says that's a misguided belief.

"Persons who are not in a long-term monogamous relationship and who engage in oral sex should use barrier protection (e.g., male condoms or other barrier methods) to reduce the risk for sexually transmitted diseases and HIV transmission," reports the CDC.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Centers for Disease Control, Morbidity and Mortality Weekly Report, Oct. 22, 2004; vol 53: pp 966-968. News release, Centers for Disease Control and Prevention. WebMD Medical Reference from Healthwise: "Syphilis: Topic Overview." WebMD Medical Reference from Healthwise: "Syphilis: What Happens."