The most common STDs that cause sores are genital herpes and syphilis. A few rarer infections, including chancroid and granuloma inguinale, also produce genital ulcers. Each one looks and feels different, which matters because the distinction affects how the sore is tested and treated.
Genital Herpes: Painful Clusters That Recur
Genital herpes is the most frequent cause of recurring genital sores. It’s caused by herpes simplex virus, most often type 2 (HSV-2), though type 1 (the cold sore virus) can also infect the genitals through oral sex. Sores typically appear 2 to 20 days after exposure, though some people don’t develop visible symptoms for months or years.
The sores start as small bumps or blisters around the genitals, anus, or mouth. Many people notice tingling or burning in the area before the blisters appear. The blisters then rupture into painful open ulcers that may ooze or bleed. Over the next week or two, scabs form and the sores heal on their own. The key feature of herpes is recurrence: the virus stays dormant in nerve cells and can reactivate, producing new outbreaks in the same general area. First outbreaks tend to be the most painful and widespread, while later ones are usually milder and shorter.
Herpes sores are almost always painful. They can appear inside the foreskin, on the labia, inside the vagina, or around the rectum, which sometimes makes them hard to spot. A swab test of the fluid inside a blister is the most reliable way to confirm the diagnosis, though blood tests can detect antibodies to each virus type even when no sore is present.
Syphilis: A Single Painless Sore
Syphilis produces a very different type of sore. The first sign is a small, round ulcer called a chancre that appears at the exact spot where the bacteria entered your body, usually on the genitals, rectum, tongue, or lips. It shows up anywhere from 1 to 3 months after exposure.
The defining characteristic is that the chancre is typically painless. It has a firm, raised edge and a clean base, almost like a button pressed into the skin. There may be one sore or several. Because it doesn’t hurt and sometimes appears in hidden locations like the rectum or cervix, many people never notice it. The chancre heals on its own within a few weeks even without treatment, which can create a false sense that nothing is wrong.
That self-healing is misleading. Without antibiotics, syphilis progresses through secondary and later stages that can affect the skin, brain, heart, and other organs. A blood test is the standard way to confirm syphilis, though the sore itself can also be tested directly. Early syphilis is straightforward to treat, which makes catching it at the sore stage important.
Chancroid: Deep, Painful Ulcers
Chancroid is rare in the United States and Europe but still occurs in parts of Africa, Asia, and the Caribbean. It’s caused by a specific bacterium and produces one or more deep, painful genital ulcers with ragged, soft edges. Unlike the firm, painless sore of syphilis, chancroid ulcers are tender to the touch and often accompanied by swollen, painful lymph nodes in the groin that can fill with pus.
The combination of a painful, soft-edged genital ulcer and swollen groin nodes is considered the classic sign. Testing for chancroid requires a specialized culture or molecular test that isn’t available at every clinic, so the diagnosis is sometimes made based on symptoms after ruling out herpes and syphilis.
Less Common Causes
Two other STIs occasionally cause genital sores, though both are uncommon in high-income countries. Granuloma inguinale (also called donovanosis) causes painless, slowly expanding ulcers with a distinctive “beefy red” appearance because the tissue is highly vascular and bleeds easily. These sores grow over weeks to months if untreated and typically don’t come with swollen lymph nodes.
Lymphogranuloma venereum (LGV) can produce a small sore or bump at the site of infection, but it heals so quickly that most people never notice it. By the time someone seeks care, the initial lesion has usually disappeared, and the infection has moved to the lymph nodes, causing painful swelling in the groin.
Mpox can also cause sores in the genital and anal area that may be confused with STI-related ulcers. Mpox sores tend to appear as firm, deep-seated blisters that progress through defined stages, and they’re typically painful. Unlike herpes, mpox blisters often appear on other parts of the body too, including the face, hands, and feet.
Painful vs. Painless: A Quick Guide
Whether a sore hurts is one of the fastest clues to its cause:
- Painful sores: genital herpes, chancroid, mpox
- Painless sores: syphilis, granuloma inguinale, LGV
This isn’t a perfect rule. Herpes sores in an early blister stage might not hurt yet, and syphilis chancres can occasionally be tender. But as a general pattern, pain versus no pain narrows the possibilities significantly.
How Genital Sores Are Tested
Because several infections can look similar, testing is the only reliable way to identify the cause. A standard evaluation for a genital ulcer includes a syphilis blood test, a swab of the sore for herpes (using either a molecular test or culture), and blood testing for herpes antibodies. HIV testing is also recommended for anyone with a genital ulcer who doesn’t already know their status, because open sores increase the risk of HIV transmission in both directions.
If initial tests come back negative and the sore isn’t healing, a biopsy may be needed to look for rarer causes. Sores that appear in the mouth, throat, or rectum can also be tested using molecular swab methods.
Not Every Genital Sore Is an STI
It’s worth knowing that genital sores aren’t always sexually transmitted. Friction, razor burn, allergic reactions to soaps or latex, and yeast infections can all cause breaks in the skin that look alarming. Certain autoimmune conditions, including Behçet’s disease, cause recurring oral and genital ulcers that have nothing to do with infection. If you’re dealing with a sore that doesn’t fit the patterns above, or that keeps coming back despite negative STI tests, there may be a non-infectious explanation worth exploring with a healthcare provider.