Performing oral sex on a penis can transmit at least six well-documented STIs: gonorrhea, chlamydia, syphilis, herpes, HPV, and HIV. The risk is lower than vaginal or anal sex for most of these infections, but “lower risk” is not “no risk,” and some of these are easier to catch orally than people realize.
Gonorrhea: The Most Common Oral STI
Gonorrhea is the infection most efficiently spread through fellatio. The bacteria thrive in the throat, and you can pick them up from contact with an infected partner’s genitals. Throat gonorrhea often causes no symptoms at all, which means you can carry and spread it without knowing. When symptoms do appear, they typically look like a mild sore throat, making it easy to dismiss.
The tricky part is that a standard urine test won’t detect a throat infection. You need a pharyngeal (throat) swab. If you’re having oral sex and only getting urine-based screening, a gonorrhea infection in your throat will go undetected. It’s worth specifically asking for a throat swab when you get tested.
Chlamydia in the Throat
Chlamydia can also infect the throat through oral sex, though it happens less frequently than gonorrhea. Like throat gonorrhea, throat chlamydia rarely produces noticeable symptoms. It’s detected the same way: a throat swab using a nucleic acid amplification test (NAAT). Standard genital screening alone won’t catch it.
Syphilis and Oral Sores
Syphilis spreads through direct contact with a syphilis sore, called a chancre. These sores can appear on the genitals, rectum, tongue, or lips. If your partner has a chancre on their penis and you perform oral sex, the bacteria can enter through your mouth and establish an infection.
The first sign of syphilis is a small, painless sore that shows up about three weeks after exposure, right at the spot where the bacteria entered your body. For someone who contracted it through oral sex, that means the sore could appear on the lips, tongue, or inside the mouth. It heals on its own within three to six weeks, which sounds reassuring but isn’t. Without treatment, syphilis progresses to more serious stages affecting the skin, heart, and nervous system. A simple blood test can detect it, and it’s curable with antibiotics when caught early.
Herpes (HSV-1 and HSV-2)
Both types of herpes simplex virus can be transmitted during fellatio. HSV-1, the type most people associate with cold sores, is extremely common and spreads readily between mouths and genitals during oral sex. HSV-2, traditionally linked to genital herpes, can also infect the mouth, though this happens less often.
Herpes doesn’t require visible sores to spread. The virus can shed from skin that looks completely normal, a process called asymptomatic shedding. This is why many people contract herpes from partners who had no idea they were infectious. If you already carry HSV-1 orally (as roughly half of adults do), you have some protection against getting it again in a new location, but it’s not absolute.
HPV and Throat Cancer Risk
Human papillomavirus is the STI with the most serious long-term consequences from oral transmission. HPV spreads to the mouth and throat during oral sex and usually clears on its own without causing problems. But in some cases, the virus persists and, over years or decades, can cause oropharyngeal cancer, which affects the back of the throat, base of the tongue, and tonsils.
HPV is now thought to cause 60% to 70% of oropharyngeal cancers in the United States. The risk increases with the number of oral sex partners over a lifetime. The HPV vaccine protects against the strains responsible for these cancers, and vaccination before exposure is the most effective prevention available. The vaccine is approved for people up to age 45, so even if you didn’t get it as a teenager, it may still be an option.
HIV: Low but Not Zero Risk
HIV transmission through fellatio is possible but considerably less likely than through anal or vaginal sex. The risk is highest for the person performing oral sex, particularly if they have cuts, sores, or bleeding gums that create an entry point for the virus. Risk also increases with exposure to more fluid, especially if blood is present (from genital sores, for example).
For context, the per-act risk is low enough that documented cases of HIV transmission through oral sex alone are relatively rare. But “rare” still means it happens. If your partner’s HIV status is unknown and you want to minimize risk, avoiding ejaculation in the mouth reduces exposure significantly.
Hepatitis B
Hepatitis B virus is present in semen and blood, and it can transmit during oral sex when those fluids contact mucous membranes or broken skin in the mouth. Most adults who contract hepatitis B clear the infection on their own, but a small percentage develop chronic infection that can damage the liver over time. A highly effective vaccine exists and is part of routine immunization schedules, so many people are already protected.
What Makes Oral Sex Riskier or Safer
Several factors shift your risk level in either direction. Having open sores, cuts, or bleeding gums in your mouth increases vulnerability to virtually every infection on this list. Recent dental work or brushing your teeth right before oral sex can create micro-abrasions that serve as entry points. A partner with an active, untreated infection is far more contagious than someone whose viral load is suppressed or whose bacterial infection has been treated.
Condoms used during oral sex reduce skin-to-skin and fluid contact, lowering the risk of transmission for most of these infections. They’re most effective against fluid-borne infections like gonorrhea, chlamydia, and HIV. They’re less protective against herpes and syphilis if the sore is on an area the condom doesn’t cover.
Getting Tested After Oral Sex
If you’re sexually active and oral sex is part of your routine, standard genital STI panels may not be enough. Gonorrhea and chlamydia in the throat require a throat swab to detect. Many clinics don’t automatically test the throat unless you ask, so be specific about what kind of sex you’re having when you request screening.
Syphilis and HIV are detected through blood tests regardless of where the infection entered the body. Herpes blood tests can identify whether you carry HSV-1 or HSV-2 but can’t tell you where on your body the infection is active. HPV has no routine screening test for the mouth or throat in people without symptoms.
Timing matters for accurate results. Gonorrhea and chlamydia can be detected as early as one to two weeks after exposure. Syphilis blood tests typically become positive within three to six weeks. HIV tests vary by type, with most becoming reliable at four to six weeks after exposure.