Chancroid and syphilis both cause genital ulcers, but the classic fork is pain: a chancroid ulcer is a soft, painful sore with tender, pus-filled groin nodes, while the primary syphilis chancre is a painless, firm, clean-based sore with rubbery, non-tender nodes. Looks overlap enough that only testing confirms which one you have.
Haemophilus ducreyi
Treponema pallidum
| Item | Value |
|---|---|
| Chancroid | curable — Haemophilus ducreyi |
| Syphilis | curable — Treponema pallidum |
What each infection is
Chancroid
Chancroid is a bacterial STI caused by Haemophilus ducreyi, a fastidious organism that invades broken skin and mucosa to produce painful genital ulcers CDC, 2021. It has become rare in the United States, so many clinicians may never see a case, which makes a careful history about travel and partners useful. Look for one or more deep, ragged, tender sores with swollen, pus-filled lymph nodes in the groin. For how the sores and nodes evolve, see the dedicated rundown of chancroid symptoms.
Syphilis
Syphilis is caused by the spirochete bacterium Treponema pallidum and is curable with the right antibiotics CDC. Its defining feature in the earliest stage is a painless sore, the opposite of chancroid. Left untreated, syphilis moves through stages over months to years and can eventually damage the heart, brain, and nerves, so catching it early matters. The stage-by-stage course is laid out in what is syphilis? causes, stages & risks.
Symptoms compared
Chancroid typically shows up as one or more deep, painful genital ulcers together with tender, swollen, pus-filled lymph nodes in the groin, which can soften and drain. Patients notice the pain first, and at the bedside that's often what flags it.
Primary syphilis produces one or more painless, firm, round sores — chancres — at the site of infection, which can be the penis, vagina, anus, rectum, lips, or mouth. The sore lasts roughly three to six weeks and heals on its own with or without treatment, which dangerously reassures some people that the infection is gone when it is still present.
If primary syphilis goes untreated, secondary syphilis can follow weeks later with a rough red or reddish-brown rash that can appear on the palms and soles as well as the trunk, plus mucous-membrane lesions, fever, swollen lymph nodes, sore throat, patchy hair loss, headache, weight loss, muscle aches, and fatigue. Chancroid has no comparable second stage of body-wide symptoms.
How to tell them apart
Clinicians weigh a handful of discriminating features when an ulcer walks in the door:
- Pain: the chancroid ulcer hurts; the syphilis chancre is classically painless.
- Ulcer texture: chancroid produces a soft, ragged, undermined ulcer (the "soft chancre"), while the syphilis chancre is firm and indurated (the "hard chancre") with a clean base.
- Lymph nodes: chancroid causes tender, pus-filled groin nodes that can rupture; syphilis causes firm, rubbery, non-tender enlargement.
- Course: a chancroid ulcer stays and worsens, whereas a syphilis chancre heals on its own and may be followed by a body rash.
These textbook signs blur in real life. An infected or atypical syphilis chancre can be tender, and herpes can mimic either one. The symptoms overlap enough that you usually can't tell these apart by feel, so a test is what settles it.
Side-by-side comparison
| Feature | Chancroid | Syphilis (primary) |
|---|---|---|
| Cause | Haemophilus ducreyi | Treponema pallidum |
| Ulcer pain | Painful | Painless |
| Ulcer texture | Soft, ragged (soft chancre) | Firm, indurated (hard chancre) |
| Groin lymph nodes | Tender, pus-filled, may drain | Firm, rubbery, non-tender |
| Natural course | Persists and worsens without treatment | Heals on its own in 3–6 weeks; may progress to secondary stage |
| Body-wide rash | No | Yes, in the secondary stage (can involve palms and soles) |
| U.S. frequency | Rare | Common and rising |
Testing
The two are diagnosed in very different ways. A probable chancroid diagnosis rests on painful genital ulcers with a typical appearance plus negative syphilis and herpes testing. A definitive diagnosis requires identifying H. ducreyi on special culture media that isn't widely available, part of why probable diagnosis is the practical norm.
Syphilis diagnosis requires two serologic blood tests used together — a nontreponemal test (RPR or VDRL) and a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC lab recs, 2024. One test alone isn't enough; the combination confirms the infection and tracks the response to treatment.
In practice, evaluation may involve a urine sample, a self-collected swab, or a quick exam depending on which infection is suspected, and it's available free or low-cost at health departments, Planned Parenthood, and Title X clinics. You can get tested for syphilis and the common STIs at the same visit. Antibodies take time to appear, so read up on when to test after exposure before assuming a single early negative clears you.
Treatment compared
Chancroid is cured with a single dose: azithromycin 1 g orally, or ceftriaxone 250 mg IM. Alternatives are ciprofloxacin 500 mg twice daily for three days, or erythromycin base 500 mg three times daily for seven days. Your clinician should re-examine the ulcer three to seven days after you start therapy to confirm it's improving.
Syphilis is treated with penicillin, the only reliable cure, and the dose follows the stage CDC, STI Tx Guidelines. Primary, secondary, and early latent syphilis are treated with benzathine penicillin G 2.4 million units IM in a single dose, while late or unknown-duration disease takes three weekly injections CDC. There is no oral drug that dependably substitutes.
The correct product is benzathine penicillin G (Bicillin L-A). The combination product Bicillin C-R (benzathine plus procaine) is NOT an acceptable substitute and has caused treatment-failure errors. Confirm the label before you accept the shot.
Don't be alarmed if you feel feverish after the first syphilis injection. The fever, chills, and headache some people get within a couple of hours is the Jarisch-Herxheimer reaction — dying bacteria releasing inflammatory signals, not a penicillin allergy. It affects roughly 95% of people treated for secondary syphilis and settles within about 24 hours StatPearls.
Can you have more than one at once?
Yes. Genital ulcers from one infection break the skin barrier and make co-infection with another STI easier to acquire, and chancroid, syphilis, and herpes can coexist. The probable chancroid criteria call for excluding syphilis and herpes first, and a full STI panel — not just one targeted test — is the right move when an ulcer appears. A single negative test for one organism never rules out the others.
When to see a clinician
See a clinician for any new genital sore, whether or not it hurts. The painless ones are the easiest to ignore and the most dangerous to miss. Also seek care for tender or draining groin swelling, a rash on the palms or soles, or any sore in a recent partner. Syphilis keeps climbing in the U.S., with about 53,000 primary-and-secondary cases reported in 2023 and the late or unknown-duration stage jumping from 14 to nearly 30 per 100,000 between 2020 and 2023 CDC AtlasPlus, 2023, so early evaluation isn't being overcautious.