Several sexually transmitted infections can produce ulcers in or around the mouth, with syphilis, herpes simplex virus, and lymphogranuloma venereum being the most recognized causes. A few others, including HIV-related opportunistic infections and even gonorrhea, can damage oral tissue in ways that sometimes mimic ulcers. The tricky part is that many of these oral lesions look similar to one another and to harmless conditions like canker sores, which means the underlying STI often goes undiagnosed until it has progressed.
Syphilis and the Oral Chancre
Syphilis is the STI most strongly associated with mouth ulcers, and it can produce them in both its primary and secondary stages. In primary syphilis, the bacterium Treponema pallidum enters through a break in the mucous membrane and forms a chancre, a firm, usually painless ulcer, at the site of contact. When that contact point is the lips, tongue, or palate, the chancre appears in the mouth rather than on the genitals. Because a single painless sore inside the mouth doesn’t alarm most people, oral chancres are frequently mistaken for a canker sore or a minor injury and left alone. The chancre heals on its own within a few weeks, but the infection silently advances.
Secondary syphilis, which develops weeks to months later, brings a wider range of oral findings. Mucosal plaques, shallow ulcerations on the hard palate, and sores at the corners of the lips are typical features of this stage.1PubMed Central. Oral findings in secondary syphilis These lesions are highly infectious. Syphilis has earned the nickname “the great imitator” because secondary-stage oral lesions can mimic a wide variety of other conditions, from lichen planus to squamous cell carcinoma, making clinical diagnosis a genuine challenge.2PubMed. Oral Manifestations of Secondary Syphilis That mimicry is exactly why clinicians are urged to include syphilis in the differential diagnosis of any unusual oral ulceration or white patch.3Head and Neck Pathology. Syphilis: the renaissance of an old disease with oral implications
The practical lesson here is that a persistent, painless mouth sore, especially one that doesn’t respond to the usual remedies, warrants testing for syphilis. A simple blood test can confirm or rule it out, and treatment with penicillin is highly effective when caught early.
Herpes Simplex Virus
Herpes simplex virus type 1 (HSV-1) is probably the most common sexually transmitted cause of oral ulcers, though many people contract it through non-sexual contact in childhood. HSV-1 classically causes cold sores on or around the lips, but during the initial infection or in people with weakened immune systems, it can produce painful ulcers on the tongue, inner cheeks, gums, and palate.4PubMed. Herpes simplex virus in oral mucosal ulcers in patients with hematological malignancy HSV-2, typically thought of as genital herpes, can also infect the mouth through oral sex, producing similar-looking sores.
What makes herpes distinctive from syphilis is the pain. Herpes ulcers tend to start as small clusters of fluid-filled blisters that rupture and leave shallow, intensely painful sores. They recur, sometimes triggered by stress, illness, or sun exposure. Between outbreaks, the virus hides in nerve tissue, meaning a person can be infectious even when no sore is visible. Studies using sensitive DNA detection methods have found that HSV-1 can be present in the saliva of people with no active lesions roughly a third of the time.5PubMed. Asymptomatic shedding of herpes simplex virus (HSV) in the oral cavity This asymptomatic shedding is a major reason herpes spreads so efficiently.
There is no cure for herpes, but antiviral medications can shorten outbreaks, reduce their frequency, and lower the risk of passing the virus to a partner. People who get frequent or severe oral herpes outbreaks often take a daily suppressive dose of an antiviral rather than waiting for each flare.
HIV and Oral Ulceration
HIV itself doesn’t typically produce mouth ulcers in the way syphilis or herpes does, but the immune suppression it causes opens the door to a range of oral problems. Oral lesions of some kind affect roughly 30 to 80 percent of people living with HIV, and the appearance of certain mouth conditions can signal that the immune system is deteriorating.6PubMed Central. Oral lesions: A true clinical indicator in human immunodeficiency virus
The most relevant ulcer-causing conditions in people with HIV include severe herpes outbreaks that don’t heal on their own schedule, aphthous-like ulcers that are larger and more persistent than typical canker sores, and opportunistic fungal or viral infections that erode the oral lining. Oral candidiasis (thrush) is extremely common and, while not an ulcer in the classic sense, can produce raw, painful patches that look and feel similar. Kaposi sarcoma, an HIV-associated cancer, can also appear in the mouth as dark red or purple lesions that may ulcerate.
For someone already diagnosed with HIV, a new or worsening mouth ulcer is worth reporting to a clinician because it may indicate a shift in immune status. For someone not yet diagnosed, persistent oral lesions that don’t respond to standard treatment sometimes end up being the clue that leads to an HIV test.
Gonorrhea in the Mouth
Oral gonorrhea doesn’t usually produce the kind of well-defined ulcers that syphilis or herpes does, but it belongs in this conversation because it can cause oral tissue damage that overlaps visually. The most common presentation of pharyngeal gonorrhea is a sore throat, often with fever, and sometimes tonsillitis or inflamed gums.7Urogenital Tract Infection. Why Should You Care About Oral Gonorrhea and Oral Human Papillomavirus Infection? When the infection is more aggressive, the back of the throat can become red and develop a mucous discharge along with swollen lymph nodes in the neck.
The bigger concern with oral gonorrhea is how often it goes unnoticed. Fewer than one in five people with a positive throat culture for gonorrhea show any symptoms at all.7Urogenital Tract Infection. Why Should You Care About Oral Gonorrhea and Oral Human Papillomavirus Infection? An asymptomatic person can unknowingly transmit the bacterium to a sexual partner through kissing or oral sex. Standard STI panels often skip a throat swab unless specifically requested, so if you’ve had unprotected oral sex and have persistent throat or mouth symptoms, asking for a pharyngeal gonorrhea test by name is a good idea.
Lymphogranuloma Venereum
Lymphogranuloma venereum (LGV) is caused by specific strains of Chlamydia trachomatis and is far more common in genital and rectal tissue than in the mouth. But oral cases are being reported with increasing frequency, particularly among men who have sex with men. In one documented case, a patient developed large, purulent ulcers on the lower lip, including a deep ulcer on the central lip and another near the corner of the mouth.8PubMed Central. Orolabial Lymphogranuloma Venereum, Michigan, USA Another case involved an ulcer on the tongue in a patient who also had an asymptomatic rectal infection.9PubMed. Lymphogranuloma venereum presenting as an ulcer on the tongue
These cases are still uncommon enough to be published as individual reports, which tells you how rare oral LGV remains. But the fact that LGV cases overall have been climbing in recent years means oral presentations may become less exotic over time.9PubMed. Lymphogranuloma venereum presenting as an ulcer on the tongue LGV responds well to a course of antibiotics, but it has to be diagnosed first, and that requires a clinician who thinks to test for it.
Granuloma Inguinale (Donovanosis)
Donovanosis is a chronic bacterial infection caused by Klebsiella granulomatis that primarily affects the genitals and groin. Oral involvement is rare but documented. In one reported case, a patient with genital ulcers also developed chronic ulcerative lesions in the mouth that initially looked like a completely different disease.10PubMed. Granuloma inguinale (donovanosis) of the oral cavity. A case report Donovanosis is largely confined to tropical and subtropical regions and is extremely uncommon in high-income countries. Still, for someone with a persistent, beefy-red oral ulcer that won’t heal, particularly if genital lesions are also present, donovanosis enters the list of possibilities that should be considered.
HPV and the Mouth
Human papillomavirus is sexually transmitted and extremely common in the oral cavity, but it typically doesn’t cause ulcers. Instead, HPV tends to produce wart-like growths: papillomas, condylomas, or focal thickened patches on the oral lining.11PubMed Central. HPV and oral lesions: preventive possibilities, vaccines and early diagnosis of malignant lesions These are benign in most cases, though certain high-risk HPV types, especially HPV-16 and HPV-18, are strongly linked to cancers of the tonsils and oropharynx. As an oropharyngeal cancer grows, it can ulcerate, so HPV is an indirect contributor to oral ulceration at a late stage rather than a direct one.
The main reason HPV is worth mentioning in this context is that people who notice a persistent lump or rough patch in their mouth sometimes worry it’s an ulcer caused by an STI. An HPV-related papilloma feels and looks different from the ulcers caused by syphilis or herpes: it’s usually a raised, painless, fleshy bump rather than a flat sore. If it doesn’t go away on its own within a few weeks, a biopsy can clarify what it is.
Telling STI Ulcers Apart from Canker Sores
The question most people actually have when they spot a mouth sore is whether it’s something harmless or something they need to worry about. Canker sores, known clinically as recurrent aphthous stomatitis, are the most common oral ulcers and are not caused by an infection at all. They tend to appear on movable tissue like the inside of the cheeks, the floor of the mouth, or the soft palate, and they’re painful from the start. Herpes sores, by contrast, tend to appear on tissue that’s firmly attached to bone, like the hard palate and gums, and they usually begin as tiny clustered blisters before breaking open.12The Journal of Contemporary Dental Practice. Differential Diagnosis: Is It Herpes or Aphthous?
A few practical clues can help you decide when to seek testing:
- Pain level: A painless ulcer is more suspicious for syphilis. Herpes is painful. Canker sores are painful but don’t begin as blisters.
- Location: Herpes favors the hard palate, gums, and lips. Canker sores favor the inner cheeks and soft tissue. Syphilis chancres can appear anywhere contact occurred.
- Duration: A canker sore usually heals within one to two weeks. A syphilitic chancre also self-resolves but is followed by secondary symptoms weeks later. A herpes sore that persists beyond two weeks, especially in someone with a weakened immune system, may need treatment.
- Recurrence: Herpes recurs in roughly the same spot. Canker sores can recur but move around. A single non-recurring ulcer that healed on its own could have been a primary syphilis chancre and still warrants testing if you had a new sexual contact beforehand.
None of these features are foolproof on their own. A clinician can usually narrow the diagnosis by combining what the sore looks like with your sexual history and targeted lab tests.
Why Oral STIs Are Frequently Missed
Several factors work together to keep oral STIs underdiagnosed. First, many people and even some clinicians still think of STIs as genital diseases and don’t consider the mouth as a site of infection. Second, as noted with gonorrhea, a large proportion of oral infections cause no symptoms whatsoever, so there’s nothing to prompt a visit. Third, when symptoms do appear, they often mimic everyday conditions: a sore throat mistaken for a cold, a mouth ulcer chalked up to stress, a bump assumed to be a bitten cheek. Fourth, routine STI screening typically involves urine or genital swabs, not throat swabs, so even someone who gets tested regularly may never be tested at the site that matters.
The stigma around STIs adds another layer. Research into care-seeking behavior has found that low knowledge about how STIs present, combined with high perceived stigma, significantly delays the point at which people seek testing or treatment.13PubMed Central. Knowledge, perceived stigma, and care-seeking experiences for sexually transmitted infections: a qualitative study from the perspective of public clinic attendees in Rio de Janeiro, Brazil A person who develops a mouth sore after oral sex may avoid bringing it up with a clinician because of embarrassment, or may not realize the connection exists at all.
If you have oral sex as part of your sexual activity, letting your healthcare provider know is the simplest step toward getting appropriate screening. Asking specifically for a throat swab for gonorrhea and chlamydia, and for a blood test for syphilis and herpes antibodies, fills most of the gaps that a standard genital-only panel leaves open.
Asymptomatic Shedding and Transmission Through Oral Sex
One of the most underappreciated aspects of oral STIs is that many of them spread most efficiently when the infected person has no visible symptoms. The herpes data is striking: in studies that sampled saliva daily using sensitive detection methods, HSV-1 DNA turned up on about a third of the testing days even when people had no active sores.5PubMed. Asymptomatic shedding of herpes simplex virus (HSV) in the oral cavity That means a person who has never had a noticeable cold sore can still pass herpes to a partner through kissing or oral sex on any given day.
Gonorrhea behaves similarly in the throat. Most pharyngeal gonorrhea cases are completely silent, and the bacterium can persist for weeks or months without producing symptoms, all while remaining transmissible. Syphilis, during its secondary stage, produces oral lesions teeming with bacteria, and these can be so subtle or painless that the person has no idea they’re there.
Barrier methods like condoms and dental dams reduce the risk of transmission during oral sex, though they don’t eliminate it entirely since they may not cover all areas where shedding occurs. For syphilis specifically, the single most effective prevention strategy after potential exposure is early testing. The window between exposure and a detectable blood test is roughly three to six weeks for standard syphilis screening, so a test taken too early may miss a new infection.
When Multiple STIs Overlap
It’s common for more than one STI to be present at the same time, and having one infection can make it easier to acquire another. An open ulcer in the mouth from syphilis or herpes provides a direct entry point for HIV, and HIV-related immune suppression in turn makes herpes outbreaks more frequent and severe. Similarly, oral gonorrhea and chlamydia can coexist, and LGV has been documented alongside asymptomatic rectal infections in the same patient.9PubMed. Lymphogranuloma venereum presenting as an ulcer on the tongue
This overlap means that when one oral STI is found, testing for others is warranted even if you feel fine otherwise. A clinician who diagnoses oral herpes should at least discuss syphilis and HIV testing, and someone diagnosed with syphilis should be offered a full STI panel. The interconnected nature of these infections is one of the strongest arguments for comprehensive screening rather than testing for a single pathogen in isolation. If you’re dealing with a persistent or unusual mouth sore and you’re sexually active, the most efficient approach is to have a frank conversation with your provider about your oral sexual history so the right tests get ordered the first time.