A painful genital ulcer usually points to genital herpes or chancroid, while a painless one is the classic sign of primary syphilis. But pain is only the first clue. Herpes, syphilis, and chancroid overlap too much to settle by sight, and friction or canker-type sores can mimic all three. A swab or blood test names it.
Herpes simplex virus
Treponema pallidum
Haemophilus ducreyi
| Item | Value |
|---|---|
| Genital herpes | managed — Herpes simplex virus |
| Syphilis | curable — Treponema pallidum |
| Chancroid | curable — Haemophilus ducreyi |
Quick answer: what causes a genital sore or ulcer
When a sore shows up on or around the genitals, anus, or mouth, the short list of usual suspects is small. Three sexually transmitted infections account for most STI-related ulcers: genital herpes, syphilis, and chancroid. Two non-STI causes, friction trauma and aphthous (canker-type) ulcers, round out the picture. Pain is the single most useful feature to start sorting them, though it points the way rather than naming the cause.
- Painful — genital herpes (clustered blisters that break open) and chancroid (deep, ragged ulcers with swollen groin glands).
- Painless — the firm, round chancre of primary syphilis.
- Either, and not an STI — friction trauma from sex or shaving, and aphthous ulcers similar to mouth canker sores.
Which STIs cause a genital sore or ulcer
Genital herpes
Genital herpes is caused by two viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC, About Herpes. The tell-tale pattern is small blisters that cluster together, break open into shallow painful sores, and then crust and heal over a week or more. A first outbreak is often the worst, sometimes with flu-like symptoms such as fever, body aches, and swollen glands, and the sores can appear on or around the genitals, rectum, or mouth. Repeat outbreaks are usually shorter and milder, and many people feel a warning prodrome (tingling, itching, or burning) a day or two before sores appear.
Most people with herpes have no symptoms or very mild ones, and the majority of HSV-2 infections are never diagnosed. A clear, painful cluster of blisters is suggestive, but a quiet infection with no sore at all is far more common. If you want options for managing recurrent outbreaks, see our overview of alternative herpes treatments.
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with the right antibiotics CDC, About Syphilis. Its primary stage produces one or more painless, firm, round sores called chancres at the spot where the bacteria entered — penis, vagina, anus, rectum, lips, or mouth. The painlessness is why syphilis is so easy to miss. A chancre can sit unnoticed for 3 to 6 weeks and then heal on its own, with or without treatment, leaving you to think the problem resolved while the infection quietly advances.
The chancre typically appears about three weeks after exposure, though the incubation window runs anywhere from 10 to 90 days. Left untreated, syphilis moves into a secondary stage with a rough red or reddish-brown rash that can show up on the palms and soles, along with fever, swollen lymph nodes, sore throat, patchy hair loss, and fatigue. To understand how the stages unfold, read what is syphilis? causes, stages & risks.
Chancroid
Chancroid is a bacterial STI caused by Haemophilus ducreyi and has become rare in the United States CDC, Chancroid Tx. Its hallmark is one or more deep, painful genital ulcers paired with tender, swollen, pus-filled lymph nodes in the groin (sometimes called buboes — enlarged nodes that can become so inflamed they drain). A chancroid ulcer hurts; a syphilis sore does not. For the full picture of how it presents, see chancroid symptoms.
When it's not an STI
Not every genital sore is sexually transmitted. Friction trauma — a small raw spot or split from vigorous sex, a tight seam, or a shaving nick — can look alarming but usually maps to a known irritation and heals fast. Aphthous ulcers, the genital cousins of the canker sores people get inside the mouth, are shallow, painful, and not caused by an infection. Pain level is the most useful first clue here too, though neither of these can be confirmed by appearance alone.
How to tell them apart
A clinician reasons through a genital ulcer the same way you can start to: is it painful or painless, is it a cluster of small blisters or a single firm sore, and are the groin glands tender and swollen? Painful clustered blisters lean herpes. A painless, firm, solitary sore leans syphilis. A painful, deep ulcer with very tender, pus-filled groin nodes raises chancroid. Friction and aphthous ulcers usually have a backstory, like a recent injury or a history of mouth ulcers, and no other STI signs.
These conditions overlap far too much to name by sight, and several are frequently silent. A test settles which one, if any, it is. Because the symptoms overlap, you usually can't self-diagnose this, so treat the pattern below as a guide to what to ask for rather than a verdict.
Side-by-side comparison
| Feature | Genital herpes | Syphilis (primary) | Chancroid |
|---|---|---|---|
| Pain | Painful | Painless | Painful |
| Cause | HSV-1 or HSV-2 (virus) | Treponema pallidum (bacterium) | Haemophilus ducreyi (bacterium) |
| Look of the sore | Cluster of blisters that break into shallow sores | One or more firm, round sores (chancres) | One or more deep, ragged ulcers |
| Groin glands | May be swollen, especially in a first outbreak | Can be swollen, painless | Tender, swollen, pus-filled |
| Heals on its own? | Sores heal but the virus stays; outbreaks recur | Chancre heals in 3–6 weeks; infection persists untreated | Needs antibiotics |
| How it's confirmed | Swab of the lesion (NAAT or culture) | Two blood tests (nontreponemal + treponemal) | Special culture; often a probable diagnosis |
How it's tested
Each ulcer has its own confirmatory test. With a lesion present, herpes is confirmed by type-specific virologic testing of the sore — a swab analyzed by NAAT or culture, which works best when the sore is fresh CDC, Herpes Testing. Syphilis takes two blood tests, a nontreponemal test (RPR or VDRL) plus a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC, 2024. Chancroid is usually a probable diagnosis — painful ulcers of typical appearance plus negative herpes and syphilis testing — because the definitive culture for H. ducreyi needs special media that isn't widely available. In practice, testing is straightforward: a urine sample, a self-collected swab, or a quick exam depending on what's suspected, often free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. When you're ready, get tested, and if you're unsure about timing, check when to test after exposure.
What to do next
Don't wait for a sore to heal before getting checked, because a syphilis chancre disappears on its own while the infection keeps going. Get the sore swabbed and your blood drawn promptly, hold off on sexual contact until you have answers, and follow through on treatment for whatever turns up. Syphilis and chancroid are bacterial and cured with antibiotics; herpes is managed rather than cured, with medication to shorten and reduce outbreaks. Your clinician will match the regimen to the diagnosis.
Red flags — when to get seen urgently
- Rapidly spreading sores, severe pain, or groin glands so swollen and tender they feel like a hard lump.
- Fever, chills, or feeling generally unwell along with the ulcer.
- A sore that won't heal, keeps coming back, or is paired with a rash on the palms or soles.
- Difficulty urinating, trouble passing urine at all, or sores near the eyes.
- Pregnancy with any new genital sore — get evaluated quickly to protect the pregnancy.
- A weakened immune system (for example, from HIV or chemotherapy), where ulcers can be more severe.