A chancre is a firm, round, painless ulcer that forms at the spot where the syphilis-causing bacterium enters the body. It is the hallmark sign of primary syphilis, the earliest stage of infection, and it typically appears on the genitals, anus, or mouth. Because it doesn’t hurt and often shows up in hard-to-see locations, many people never notice it.
How a Chancre Forms
Syphilis is caused by a corkscrew-shaped bacterium called Treponema pallidum. When this organism enters the body through a break in the skin or mucous membrane during sexual contact, it begins multiplying at the entry site. The immune response at that site produces a small, raised bump (a papule) that eventually opens into the characteristic ulcer known as a chancre.
The sore is usually solitary, meaning just one appears. It starts small, has clean, raised edges, and feels firm or rubbery to the touch. Unlike most ulcers people are familiar with, a chancre is typically painless, which is one of the main reasons it goes unnoticed or gets dismissed.
Where Chancres Appear
Most chancres show up on the external genitalia or around the anus, since these are the most common points of contact during sex. However, about 2% of patients develop chancres in extragenital locations. Of those, the mouth accounts for 40 to 70% of cases, with the lip being the most frequent oral site, followed by the tongue. Rarely, chancres can form on the tonsils or the back of the throat.
Because a chancre develops precisely where the bacterium entered, it can technically appear anywhere skin-to-skin or skin-to-mucous-membrane contact occurred. A chancre inside the rectum or on the cervix is particularly easy to miss entirely.
Timeline: Appearance to Healing
A chancre doesn’t show up immediately after exposure. There’s an incubation period, typically around 21 days, though it can range from 10 to 90 days depending on the person and the bacterial load. Once the sore appears, it usually lasts 3 to 6 weeks and then heals on its own, whether or not the person receives treatment.
This self-healing is one of the most dangerous features of primary syphilis. The disappearance of the sore often convinces people that whatever was wrong has resolved. In reality, the bacterium has simply moved deeper into the body. Without treatment, syphilis progresses through secondary, latent, and eventually tertiary stages, potentially affecting the heart, brain, and other organs years or even decades later.
How a Chancre Differs From Other Genital Sores
Several infections cause sores in the genital or anal area, and telling them apart matters because each requires different treatment. In the United States, most genital ulcers in sexually active people are caused by either herpes or syphilis, and occasionally both can be present in the same sore at the same time.
- Syphilis chancre: Begins as a firm papule, opens into a painless ulcer with clean edges, usually solitary.
- Genital herpes: Begins as small fluid-filled blisters (vesicles) that burst into shallow, painful sores, often appearing in clusters.
- Chancroid: A less common infection that causes soft, ragged-edged ulcers that are distinctly painful, often accompanied by swollen lymph nodes that may drain pus.
The key distinguishing feature of a syphilis chancre is the combination of firmness and painlessness. If you have a genital sore that doesn’t hurt and feels hard at the base, syphilis should be high on the list of possibilities.
How It’s Diagnosed
Diagnosing a chancre can be tricky because standard blood tests for syphilis sometimes come back negative in the earliest days of primary infection. The body hasn’t yet produced enough antibodies to trigger a positive result. When a suspicious sore is present, a more direct approach may be used: a sample of fluid from the ulcer can be examined under a specialized microscope (darkfield microscopy) to look for the spiral-shaped bacteria. This method can provide a definitive diagnosis even when blood tests are still negative.
Blood-based screening tests detect antibodies the immune system produces in response to the infection. These become more reliable as the infection progresses, but they may miss the very earliest cases. If your blood test is negative but you have a sore that looks and feels like a chancre, your provider may recommend retesting in a few weeks or using a direct detection method on the sore itself.
Treatment and Transmission
Syphilis at the primary stage is highly curable. The standard treatment is a single injection of penicillin, which has remained the preferred drug for all stages of syphilis for decades. For people with a penicillin allergy, alternative antibiotics exist, though penicillin remains first-line.
While the chancre is present, the risk of transmitting syphilis to a sexual partner is significant. The sore is teeming with bacteria, and direct contact with it during vaginal, anal, or oral sex is the primary route of spread. Even after the chancre heals, the infection remains transmissible during the secondary stage, when other symptoms like rashes and mucous membrane lesions appear. Treatment eliminates the bacterium and stops the infection from progressing or spreading, but it cannot undo any damage the later stages may have already caused, which is why catching it at the chancre stage is ideal.