A chancre is a painless open sore that appears as the first sign of syphilis, a sexually transmitted infection caused by a spiral-shaped bacterium called Treponema pallidum. It develops at the exact spot where the bacteria entered the body, typically 3 to 6 weeks after exposure. Because it doesn’t hurt, many people never notice it, which makes syphilis easy to spread without realizing you have it.
What a Chancre Looks and Feels Like
A chancre is a round or oval ulcer with a clean, “punched-out” base and firm, raised edges. The surface is usually smooth and may have a slightly moist or glistening appearance. Unlike most sores you’re familiar with, a classic syphilis chancre is not painful to touch. It feels firm or rubbery, almost like a button under the skin, which is why older medical texts sometimes call it a “hard chancre.”
Chancres are typically between a few millimeters and about an inch across, though size can vary. Most people develop a single sore, but it’s possible to have more than one. The sore is highly infectious: the bacteria are concentrated on the surface and in any fluid the ulcer produces, so direct skin-to-skin or mucous membrane contact during this stage carries a significant risk of transmission.
Where Chancres Appear
Because the sore forms at the site of contact, chancres show up wherever the bacteria first entered the body. The most common locations are the penis, vulva, cervix, anus, rectum, and perineum. They also appear on the lips or inside the mouth, and less commonly on fingers or other skin surfaces.
Chancres on the cervix, inside the rectum, or in the back of the throat are particularly easy to miss because they’re not visible or painful. This is one reason syphilis often goes undiagnosed in its earliest, most treatable stage.
Timeline: Appearance to Healing
The sore usually appears 3 to 6 weeks after infection, though some sources note the primary stage can begin around 21 days. Once it shows up, it lasts 3 to 6 weeks and then heals on its own, with or without treatment. This self-healing is deceptive. The infection hasn’t gone away. It has simply moved deeper into the body and will progress to secondary syphilis if left untreated, causing widespread rashes, fever, swollen lymph nodes, and other systemic symptoms weeks to months later.
How It Differs From Canker Sores and Herpes
People often confuse a chancre with a canker sore or a herpes outbreak because all three involve sores in or around the mouth or genitals. The differences are important.
- Chancre (syphilis): A single, painless, firm ulcer with smooth raised edges. Can appear inside or outside the mouth, on the genitals, or at other contact sites. Highly contagious through direct contact.
- Canker sore (aphthous ulcer): A round white or yellow sore with a red border that forms inside the mouth only. Canker sores are painful but not contagious, and their cause is unknown.
- Herpes sore (fever blister/cold sore): A cluster of small, fluid-filled blisters that typically appear on the outside of the mouth around the lips. Herpes sores are painful, contagious, and caused by herpes simplex virus (usually HSV-1 for oral lesions).
The key distinguishing feature of a chancre is the combination of painlessness and firmness. If you have a sore that doesn’t hurt and feels hard underneath, that pattern is characteristic of syphilis rather than the other two conditions.
Chancre vs. Chancroid
The names sound almost identical, but a chancre and a chancroid are different infections. A chancroid (sometimes called a “soft chancre”) is caused by a completely different bacterium and produces a painful ulcer with ragged, soft edges that leaks pus. Chancroid sores are tender, inflamed, and can have discolored or shiny skin around them. The classic syphilis chancre, by contrast, is painless with clean, firm edges. If a genital ulcer hurts, it’s more likely to be chancroid or herpes than syphilis, though overlap exists and testing is the only way to be sure.
How a Chancre Is Diagnosed
If a healthcare provider suspects syphilis based on the appearance of a sore, there are two main ways to confirm it. The first is examining fluid from the sore directly under a special microscope (darkfield microscopy), which can reveal the corkscrew-shaped bacteria while they’re still alive and moving. This test needs to be done within 20 minutes of collecting the sample, so it’s typically performed right in the clinic.
The second approach is a blood test. Screening blood tests detect the body’s immune response to the infection rather than the bacteria themselves. A positive screening test is followed by a second, more specific blood test to confirm the result. One important wrinkle: in the earliest days of a chancre, blood tests can come back negative because the body hasn’t yet produced enough antibodies. If a sore looks suspicious but the blood test is negative, a provider may repeat testing a few weeks later.
Treatment and What to Expect
Syphilis caught during the chancre stage is one of the most straightforward infections to cure. The standard treatment is a single injection of penicillin, which eliminates the bacteria in most cases. For people with a penicillin allergy, alternative antibiotics are available.
After treatment, the sore heals within a few weeks if it hasn’t already. Follow-up blood tests are used to confirm that the infection has cleared. The earlier syphilis is treated, the less likely it is to cause lasting damage. Left untreated beyond the primary and secondary stages, the infection can eventually affect the heart, brain, and other organs years down the line, which is why catching it at the chancre stage matters.
Why Chancres Are Easy to Miss
The single biggest reason syphilis continues to spread is that chancres don’t demand attention the way most infections do. They don’t itch, burn, or ache. They can be hidden inside the vagina, rectum, or throat. And they heal on their own, which gives the false reassurance that whatever it was has resolved. If you’re sexually active and notice any new, unexplained sore on your genitals, mouth, or anus, getting tested is the only reliable way to rule syphilis out, even if the sore doesn’t hurt.