The risks: gonorrhea, chlamydia, herpes (oral and genital), syphilis, HIV
Fellatio is more risky than kissing, sensual touching, and cunnilingus because of the greater risk of gonorrhea. "Gonorrhea can establish an infection in the throat," says Jeanne Marrazzo, MD, an infectious disease specialist at the University of Washington in Seattle. "But it often doesn't cause a sore throat, so people can have it and not know it."
As with other kinds of oral sex, the partner performing fellatio is at risk for herpes infection in or around the mouth as well as for syphilis, while the receiving partner could experience a herpes infection in the genital area.
Even if the person receiving oral sex does not climax and ejaculate into the mouth, there is still risk of catching an STD. "There is a myth that if you don't let a man ejaculate in your mouth, you can't catch an STD. But the truth is, you can find organisms like chlamydia and gonorrhea in pre-ejaculate that will cause infections."
HIV risk from fellatio is under debate. "There is definitely a small portion of HIV-positive people who claim their only unprotected exposure was from oral sex," says Dr. Marrazzo. "So there is a small riskmostly for the person performing oral sex."
How to reduce risk
Make sure your mouth is in good health. Avoid oral sex if you have bleeding gums or open cuts or sores in or around your mouth. Don't brush your teeth right before performing oral sex because it can cause bleeding. Wearing a condom can greatly reduce risk. "We definitely recommend protectionespecially if your partner is HIV positive, even though the risk for HIV [transmission through oral sex] is probably low," says Dr. Marrazzo. You're also saferthough not by any means fully protectedif your partner does not ejaculate in your mouth.