STDs That Cause Sores or Ulcers: Painful vs Painless

Genital sores or ulcers most often come from one of three sexually transmitted infections: genital herpes and chancroid, which usually hurt, and syphilis, whose sore is classically painless. Friction, trauma, and aphthous ulcers can cause them too. Because these overlap, only a test reliably tells you which one — if any — it is.

Which STIs cause genital sores or ulcers?

Three STIs account for most ulcer-type lesions on the genitals, anus, or mouth. When you first notice one, the most useful clue is whether it's painful or painless. That split points you toward different infections, though it never replaces a test.

Genital herpes (usually painful)

Genital herpes is caused by two viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC, About Genital Herpes. Most people have no symptoms or very mild ones, most never know they're infected, and the majority of HSV-2 infections go undiagnosed. Never having had a sore tells you almost nothing.

When it does flare, a first outbreak tends to be the worst. Small blisters appear on or around the genitals, rectum, or mouth, then break open into painful sores that take a week or more to heal. A first episode can come with flu-like symptoms — fever, body aches, and swollen glands — because your immune system is meeting the virus for the first time. Repeat outbreaks are shorter and milder, and many people get a warning prodrome (tingling, itching, or burning in the area) a day or two before sores show up.

Syphilis (classically painless)

Syphilis is a bacterial infection caused by Treponema pallidum, and it's curable with the right antibiotics CDC, About Syphilis. Its first stage produces one or more chancres, firm, round sores at the exact spot the bacteria entered, which can be the penis, vagina, anus, rectum, lips, or mouth. The chancre is typically painless, so a sore that doesn't hurt is easy to ignore. It appears about three weeks after exposure (the incubation window runs roughly 10 to 90 days), lasts three to six weeks, and heals on its own whether or not you treat it.

That self-healing is dangerous. The infection hasn't gone; it's moved inward. The secondary stage can bring a rough red or reddish-brown rash, sometimes on the palms and soles, along with mucous-membrane lesions, fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue. For the full stage-by-stage picture, see what is syphilis? causes, stages & risks.

Chancroid (painful, now rare)

Chancroid is a bacterial STI caused by Haemophilus ducreyi, and it has become rare in the United States CDC, Chancroid Tx Guidelines. It produces one or more deep, painful genital ulcers, usually alongside tender, swollen lymph nodes in the groin that can fill with pus and break open. The chancroid ulcer hurts, whereas the syphilis chancre doesn't. Because it's uncommon here, clinicians often consider it after ruling other causes out. You can read more on the pattern under chancroid symptoms.

When it's not an STI

Not every genital sore is sexually transmitted. Friction or trauma from sex, tight clothing, shaving, or scratching can leave a raw, tender spot that mimics an early ulcer. Aphthous ulcers (the same kind of canker sore some people get in the mouth) can also appear on the genitals and aren't caused by an STI at all. These usually heal on their own, but they look enough like an infectious ulcer that you can't safely sort them out by appearance alone.

How do you tell them apart?

Pain is your first fork. Painful, blistering, recurring sores lean toward herpes; a single painful ulcer with a swollen, tender groin node leans toward chancroid; a painless, firm sore that heals itself leans toward syphilis. But these patterns overlap heavily, and several causes are frequently silent. A test settles which one it is, if any.

This is why you usually can't self-diagnose. The same sore can fit two or three of these, and people skip the painless ones. A test turns a guess into an answer.

Side-by-side comparison

How is it tested?

When a sore is present, herpes is confirmed by swabbing the lesion for type-specific virologic testing — a NAAT or culture works best while the sore is fresh CDC, Herpes Testing. Syphilis is diagnosed with two blood tests used together: a nontreponemal test (RPR or VDRL) and a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC, MMWR 2024. Chancroid is usually a probable diagnosis — painful ulcers plus negative syphilis and herpes testing — because the special culture needed to confirm H. ducreyi isn't widely available.

In real life, testing is quick: depending on what's suspected, it's a urine sample, a self-collected or clinician swab, or a brief exam, with results usually back in a few days. It's free or low-cost at health departments, Planned Parenthood, and Title X clinics. Because some of these have an incubation window, timing matters — check when to test after exposure, and when you're ready, get tested.

What to do next

Don't wait for the sore to heal to decide it was nothing. Syphilis and herpes both clear on the surface while the infection stays. Get the lesion looked at and swabbed while it's still there, since fresh sores give the most reliable results. Syphilis and chancroid are bacterial and cured with antibiotics; herpes isn't curable but is very manageable with antiviral medication, and some people also explore alternative herpes treatments alongside standard care.

Red flags — when to get seen urgently

• A sore is rapidly spreading, deeply painful, or accompanied by high fever and feeling very unwell.
• Swollen, tender groin lumps that are growing or starting to drain pus.
• A body rash appearing after a genital sore healed — especially on the palms or soles, a classic secondary-syphilis sign.
• You're pregnant and notice any new genital sore, because some of these infections can affect the baby.
• Sores that don't begin healing, keep coming back, or are paired with trouble urinating.