Three sexually transmitted infections cause most genital ulcers: genital herpes (HSV-1 or HSV-2), which makes clusters of small, painful blisters and sores; syphilis, which causes a single firm, painless sore; and chancroid, a rare but deep, painful ulcer. Friction and non-STI mouth-type ulcers can mimic them, so a test settles which it is.
painful blisters that crust over; tends to recur
a single painless sore (chancre); later a body rash
painful, soft, ragged ulcer(s)
| Item | Value |
|---|---|
| Genital herpes | managed — painful blisters that crust over; tends to recur |
| Syphilis | curable — a single painless sore (chancre); later a body rash |
| Chancroid | curable — painful, soft, ragged ulcer(s) |
The short list of likely causes
When a sore or ulcer shows up on or around the genitals, the candidates fall into a small, manageable group. The STI causes are genital herpes, syphilis, and chancroid. The non-STI causes — friction, minor trauma, and aphthous (canker-type) ulcers — are common and often overlooked. Early on, ask whether the sore hurts: herpes and chancroid are painful, while the classic syphilis sore is painless.
I tell patients these conditions overlap too much to call by sight alone, and several are frequently silent. A test tells you which one you have, if any.
Which STIs cause genital ulcers or sores
Genital herpes
Genital herpes is caused by two related viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC. Most who carry it have no symptoms or only very mild ones, most don't know they're infected, and the majority of HSV-2 infections go undiagnosed. So an absence of sores never rules herpes out.
When symptoms do appear, the first outbreak tends to be the worst. Small blisters form, then break open into painful sores that take a week or more to heal. Many people also feel sick during that first episode — fever, body aches, and swollen glands, much like the flu. The sores can show up on or around the genitals, the rectum, or the mouth. Repeat outbreaks are usually shorter and milder, and some people get a warning prodrome — tingling, itching, or burning in the spot where a sore is about to surface — a day or so beforehand.
Look for a cluster of painful blisters or shallow sores, often recurring in the same area. If standard antivirals aren't a fit, it's worth reading up on alternative herpes treatments and discussing options with your clinician.
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with antibiotics CDC. Its first stage produces one or more painless, firm, round sores called chancres, right at the site where the bacteria entered — the penis, vagina, anus, rectum, lips, or mouth.
The chancre typically shows up about three weeks after exposure, though the incubation window runs anywhere from about 10 to 90 days. It then lasts roughly three to six weeks and heals on its own, with or without treatment. The sore disappears, but the infection has not. Left untreated, syphilis can move into a secondary stage with a rough red or reddish-brown rash (sometimes on the palms and soles), mucous-membrane sores, fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue.
Watch for a single, firm, painless ulcer that you might barely notice. Because syphilis can pass to a baby during pregnancy, screening matters — see syphilis in pregnancy if you're expecting or planning to be.
Chancroid
Chancroid is a bacterial STI caused by Haemophilus ducreyi, and it's a classic cause of painful genital ulcers, though it has become rare in the United States CDC. The hallmark is one or more deep, painful genital ulcers paired with tender, swollen, pus-filled lymph nodes in the groin.
That groin gland involvement, plus the depth and tenderness of the ulcer, is the giveaway. Unlike the syphilis chancre, the chancroid ulcer hurts. You can read more about how it presents in our overview of chancroid symptoms.
When it's not an STI
Not every genital sore is an infection. Friction from sex or tight clothing, minor trauma (a nick from shaving or a zipper), and aphthous ulcers — the same kind of canker sore people get in the mouth — can all produce sores in the genital area. These usually heal on their own and aren't contagious.
The most useful split when you're trying to reason it through is painful versus painless: herpes and chancroid are painful, while the syphilis chancre is painless. But that's a starting point, not a diagnosis. Friction sores hurt too, and a herpes outbreak can be mild enough to mistake for irritation.
How to tell them apart
The discriminating features clinicians lean on are pain, number, depth, and the company the sore keeps. Herpes tends to be multiple small blisters that break into sores, often recurring. Syphilis is typically a single firm painless sore. Chancroid is a deep painful ulcer with swollen, pus-filled groin nodes.
These patterns overlap, and several of these infections are frequently silent. You can't reliably self-diagnose a genital ulcer by looking at it, so a test turns a guess into an answer.
Side-by-side comparison
| Feature | Genital herpes | Syphilis (primary) | Chancroid |
|---|---|---|---|
| Cause | HSV-1 / HSV-2 (virus) | Treponema pallidum (bacteria) | Haemophilus ducreyi (bacteria) |
| Pain | Painful | Painless | Painful |
| Appearance | Clusters of blisters that break into sores | Firm, round sore (chancre) | Deep ulcer |
| Number | Often multiple | One or more | One or more |
| Groin glands | Swollen glands possible | Swollen lymph nodes possible | Tender, pus-filled groin nodes |
| Timing / course | First sores heal in a week or more; recurs | Chancre ~3 weeks after exposure; lasts 3–6 weeks, then heals | Persists until treated |
| Often silent? | Yes — many undiagnosed | Yes — sore can be missed | Less so; ulcer is painful |
| Curable? | No, but treatable | Yes, with antibiotics | Yes, with antibiotics |
How it's tested
When a sore is present, herpes is confirmed by type-specific testing of the lesion itself — a swab analyzed by NAAT or culture works best CDC. Syphilis is diagnosed with blood work: it takes two serologic tests together, a nontreponemal test (RPR or VDRL) plus a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC, 2024. Chancroid is a probable diagnosis when you have a typical painful ulcer with negative syphilis and herpes results; a definitive diagnosis needs a special culture for H. ducreyi that isn't widely available.
In practice, testing is simpler than the list looks. Depending on what's suspected, it's a urine sample, a self-collected or clinician-collected swab, or a quick exam, often free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. See your options to get tested, and if you're counting days since a possible exposure, check when to test after exposure.
What to do next
If you have a new genital sore, don't wait it out or try to match it to a photo online. Get a swab and blood draw so you know what you're treating. Syphilis and chancroid are curable with antibiotics, and herpes is very manageable. The treatment depends entirely on the diagnosis, so testing comes first.
Red flags — when to get seen urgently
- You have a genital sore along with fever, severe pain, or swollen, tender glands in the groin.
- A painless sore that healed on its own — get checked anyway, because syphilis doesn't leave when the chancre does.
- A rough rash on your palms, soles, or body, which can signal secondary syphilis.
- You're pregnant or could be, since untreated syphilis can harm the baby.
- A first herpes outbreak with intense pain, trouble urinating, or feeling very unwell.
- Any ulcer that isn't healing, keeps coming back, or is spreading.