Quick Answer: A canker sore is a painful, shallow ulcer with a red border that typically heals in under 10 days. A syphilis sore (chancre) is usually painless, firm, and lasts 3–6 weeks without treatment. If you’ve had oral sex recently and the sore won’t heal, or it’s completely painless, get tested for syphilis.
What a Typical Canker Sore Looks and Feels Like
Canker sores, also known as aphthous ulcers, are incredibly common, affecting up to 25% of people at some point in their lives. They’re not caused by an STD, and they’re not contagious. Instead, they’re triggered by things like:
- Stress or hormonal shifts
- Minor trauma (biting your cheek, brushing too hard)
- Certain foods (like citrus or spicy dishes)
- Vitamin deficiencies (especially B12, folate, or iron)
- Underlying autoimmune conditions
Most importantly, they hurt. Especially when you eat acidic foods or touch them with your tongue. They usually appear as small, round ulcers with a white or yellow center and a bright red border. While annoying, they typically heal on their own in 7 to 10 days.
| Feature | Canker Sore |
|---|---|
| Appearance | Small, white/yellow center with red border |
| Pain | Sharp stinging or burning, especially with food |
| Location | Inside cheeks, lips, under tongue, gums |
| Healing Time | 7–10 days, no scarring |
| Contagious? | No |
Table 1. Clinical profile of a typical canker sore (aphthous ulcer).
What a Syphilis Sore (Chancre) Looks Like in the Mouth
Now let’s talk about what syphilis looks like when it shows up in your mouth. In the primary stage of syphilis, the body typically develops a chancre, a firm, round, and usually painless sore, at the site of infection. If the infection was transmitted during oral sex or kissing, the chancre will appear inside the mouth.
Unlike a canker sore, a syphilitic sore may not hurt at all. It can appear as a raised bump, a smooth red ulcer, or even a slightly crusted lesion. Some people mistake it for a cut, pimple, or pressure sore. That’s what makes it dangerous: it flies under the radar while the infection silently spreads.
The sore usually lasts 3 to 6 weeks and heals on its own, even without treatment. But if you don’t treat it? The bacteria move on to the secondary stage, affecting your skin, nervous system, and internal organs.
“It Didn’t Hurt, So I Didn’t Worry”
Vanessa, 26, noticed a small ulcer on the side of her tongue about three weeks after giving oral sex to a new partner. It didn’t hurt, so she assumed it was a burn from pizza or maybe she bit it in her sleep. She waited. It healed after a month, but two months later, she developed a rash on her chest and palms.
“I honestly didn’t think it could be an STD. It wasn’t painful. And it didn’t look gross or oozy. I had no idea syphilis could show up like that.”
Vanessa was diagnosed with secondary syphilis, a stage that occurs when the primary sore goes untreated. Fortunately, she received antibiotics and recovered fully. But she said the experience changed how she thinks about “harmless” symptoms.
People are also reading: The STD I Didn’t Feel Until It Nearly Ruined Me
Still Unsure? Ask These 3 Questions
If you have a sore in your mouth and aren’t sure whether it’s a canker or something more, ask yourself the following:
- Does it hurt? If yes, it’s more likely to be a canker sore. If no, consider syphilis.
- How long has it lasted? Canker sores heal in under 2 weeks. Syphilis sores last longer, sometimes over a month.
- Have you had recent oral sex? If so, especially without protection, syphilis should be on your radar.
You don’t have to diagnose yourself. You just have to recognize when something seems off, and take the next step.
This syphilis test kit lets you check from home. No appointments. No waiting rooms. Just discreet, fast answers.
How to Tell the Difference: Syphilis vs Canker Sore Side by Side
Let’s break this down visually. Many people glance at a sore in the mirror and assume it’s “just a mouth ulcer.” But a side-by-side comparison tells a different story. Here's what you're likely to see, if you know what to look for.
| Feature | Canker Sore | Syphilis Chancre |
|---|---|---|
| Color | White or yellow center, red border | Reddish, smooth or ulcerated surface |
| Shape | Round or oval, shallow | Firm, raised, crater-like or flat |
| Pain | Usually painful or stinging | Usually painless or mildly sensitive |
| Number of sores | One or multiple | Usually just one |
| Duration | 7–10 days | 3–6 weeks |
| Risk of transmission | None (not contagious) | Very high, active syphilis infection |
Table 2. Visual and clinical differences between a canker sore and a syphilis chancre.
Oral Syphilis Isn’t Rare, It’s Just Underdiagnosed
When people hear “syphilis,” they usually picture something scary on the genitals. But this infection is location-agnostic. If your mouth was involved during transmission, that’s where the symptoms start.
Oral syphilis is becoming more common due to rising rates of unprotected oral sex, especially among younger adults and people with multiple partners. The mouth is full of tiny vessels and breaks in the mucosa, perfect entry points for Treponema pallidum, the bacteria that causes syphilis.
And because a painless sore in the mouth is easy to miss or mislabel, oral syphilis often slips through the cracks. That’s why awareness matters.
Myth #1: “If It Was Syphilis, It Would Look Worse”
This is a dangerous assumption. Many syphilis sores look like nothing more than a flat red patch or a slightly raised bump. There’s no pus, no crust, no horror-movie symptoms. That’s exactly why people ignore them.
Here’s the real warning sign: if the sore won’t heal and doesn’t hurt, it’s not behaving like a canker sore, and you need to rule out syphilis. Especially if you've had any recent unprotected oral contact.
Myth #2: “I’d Feel Sick If I Had an STD”
Not always. Syphilis is often completely silent in its earliest stages. Some people feel a little fatigued, maybe a swollen lymph node in the neck or under the jaw, but many feel completely normal.
That’s what makes this infection so effective at spreading. People don’t feel sick. The sore heals. Life goes on, until the next stage hits.
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Early Symptoms of Syphilis in the Mouth (Stage 1)
Most people only notice one or two symptoms during the primary stage, and they’re often subtle. Here’s what to watch for:
- A painless sore on the lips, tongue, gums, or cheeks
- Slight swelling of lymph nodes in the neck
- A firm bump inside the mouth that doesn't go away
- A small open lesion that feels dry or rubbery
If you’re unsure, remember: syphilis sores usually don’t sting. That alone sets them apart from most common oral irritations.
When “It Went Away” Doesn’t Mean “It’s Gone”
One of the biggest myths about syphilis is that if the sore disappears, you’re fine. But that’s when the real trouble starts. When the initial sore (chancre) heals, the bacteria doesn’t die, it just hides. Inside your bloodstream, it begins spreading quietly to other organs and systems.
Weeks or months later, it can come back in a different form: a rash on your body, mucous patches in your mouth, patchy hair loss, fever, or flu-like symptoms. These are all part of the secondary stage of syphilis, which happens because the infection is now systemic.
This is why early testing matters. The sooner you identify syphilis, the simpler and shorter the treatment. A single penicillin injection during the early stages can cure it completely. But if you wait until later stages, the treatment becomes longer, more invasive, and recovery less certain.
“I Thought It Was a Canker. It Was Syphilis, Twice.”
Roberto, 39, was a busy restaurant manager who had gotten used to mouth ulcers popping up from stress. When he found a sore on the inside of his lower lip, he assumed it was another canker. It didn’t hurt, so he ignored it. It healed after three weeks. Six months later, he developed a faint rash on his torso and small lesions in his mouth again. Still painless. Still subtle.
“The rash didn’t itch. The mouth sore looked the same as last time. I didn’t connect the dots until I had swollen glands and night sweats. I went to a clinic, and boom, syphilis. The nurse said it was my second infection.”
Roberto had been reinfected by a partner who also didn’t know they were carrying syphilis. The first time, he’d been treated. The second time, he wasn’t so lucky, because he didn’t test again afterward. His story shows that syphilis is curable, but not one-and-done. You can catch it again if you’re re-exposed.
The Emotional Side: Shame, Silence, and Delay
When it comes to oral symptoms, shame silences a lot of people. No one wants to walk into a clinic and say, “I have a sore in my mouth, and I think it might be from sex.” But here’s what most don’t realize: clinicians have heard it all. They don’t judge. They just test, treat, and move on.
Yet stigma makes people wait, and that waiting period can be dangerous. Every week without testing gives Treponema pallidum more time to spread through the body. It also increases the chance you’ll pass it on to others without knowing.
Studies from the CDC show that over half of people with syphilis delay testing because their first sore was painless. Fear of judgment is another top reason for delay, especially for oral symptoms that are easily dismissed as “just cankers.”
People are also reading: You Don’t Have to Feel Sick to Be Infected
Why Oral Syphilis Gets Misdiagnosed
Even healthcare providers sometimes miss oral syphilis in its early form. That’s because its appearance can mimic several benign conditions:
- Canker sores or aphthous ulcers
- Oral herpes lesions
- Traumatic ulcers (from biting or braces)
- Fungal infections like thrush
- Inflammatory mouth conditions (lichen planus, lupus)
Without testing, there’s no way to know. A visual exam alone can’t reliably tell the difference. That’s why lab confirmation, through a blood test or swab, is essential. If you’ve ever had a mouth sore that seemed “off,” and it lingered longer than two weeks, don’t self-diagnose. Get checked.
Oral-to-Genital Transmission: The Overlooked Risk
Syphilis is sneaky because it can spread even without penetrative sex. Oral-to-genital contact is enough. If someone has an active chancre in their mouth and performs oral sex, the bacteria can transmit directly to their partner’s genitals, or vice versa. This means a painless sore on the lip can turn into a genital infection for someone else.
Here’s what that means in practice:
| Scenario | Transmission Risk | Prevention |
|---|---|---|
| Performing oral sex with a syphilis sore in mouth | High (transfers bacteria to partner’s genitals) | Condoms or dental dams; avoid oral contact until healed |
| Receiving oral sex from a partner with oral syphilis | Moderate (chancre may contact mucous membranes) | Regular testing; barrier protection |
| Kissing or deep contact with active sore | Low to moderate (depends on lesion presence) | Avoid kissing during outbreak, test promptly |
Table 3. Common transmission pathways for oral syphilis and preventive strategies.
Partner Conversations: How to Talk About It
It’s one thing to get tested, it’s another to talk about it. But these conversations save lives. If you’ve been diagnosed with syphilis, or even suspect it, telling your partner is part of protecting them and yourself.
You don’t have to make it heavy. Try something like this:
“Hey, I just found out I might’ve been exposed to something, so I’m getting tested. I think we both should, just to be safe.”
That line works because it’s honest without being accusatory. You’re inviting your partner into the solution, not blaming them for the problem.
FAQs
1. Can syphilis really show up as just one sore in my mouth?
Yes. One. Single. Sore. That’s all it takes. Syphilis likes to keep a low profile in the beginning, no fireworks, no drama. Just a quiet, painless bump on your tongue, inner cheek, or lip. If it sticks around for weeks and doesn’t bug you much? Get tested. Especially if your mouth had any fun recently.
2. How do I know if it’s a canker sore or something worse?
Canker sores sting. You’ll feel them every time hot sauce or toothpaste hits them. They also heal pretty fast, gone in 7–10 days. Syphilis sores? Usually painless. Stick around for 3–6 weeks. If it’s overstaying its welcome and acting chill about it, that’s your clue.
3. I had a sore, but it disappeared. So I’m good now, right?
Not so fast. A syphilis sore healing on its own is like a burglar quietly locking the door behind them. The infection just moved on to Stage Two. The bacteria’s still in your system, it’s just gotten sneakier. You’re not good until you’ve been tested and treated.
4. Can I catch syphilis from kissing?
If your partner has a syphilis sore in or around their mouth, kissing can be enough to pass it along. It’s not super common that way, but it happens. Oral sex? Much riskier. Mouths may lie, but bacteria doesn’t.
5. What if I’ve had the same partner for years?
Great! But unless both of you tested after your last outside partner, there’s still a chance something came along for the ride. STDs don’t always show up right away, and they don’t care how committed you are. Love hard, test smart.
6. Could it be something else? Like cancer or just a random ulcer?
Yep. And that’s why testing matters. Some autoimmune conditions, fungal infections, or, rarely, oral cancer can cause similar lesions. The only way to know is to get checked. Don’t spiral on WebMD at 2 a.m., just test and get real answers.
7. What if I test positive for syphilis?
You’ll get antibiotics, usually just one injection of penicillin if it’s early. No shame. No lectures. You’ll also want to tell partners so they can get checked too. And trust: the peace of mind that comes after treatment? Next-level.
8. Can I use an at-home test for syphilis?
You can, and should. At-home tests let you skip the awkward waiting room and still get medically sound results. You collect a tiny blood sample, mail it, and get your answer discreetly. It’s like DoorDash, but for sexual health.
9. Is syphilis curable?
Totally. 100%. If you catch it early, it’s one of the easiest STDs to knock out. Left untreated, though, it can mess with your nervous system, brain, and heart. So don't wait for the scary stage. Catch it while it’s still playing nice.
10. Do I need to tell my partner if I test positive?
Yes. You owe it to them, and to yourself. But you don’t have to make it dramatic. A simple “Hey, I got tested and found something, can we both check just to be safe?” goes a long way. Most people appreciate honesty way more than silence.
Ready to Take the Next Step?
Testing doesn’t have to be embarrassing or public. At-home syphilis tests are discreet, reliable, and ship in plain packaging. You collect a small sample, send it off, and get results privately, no clinic, no waiting room, no judgment.
Knowing is power. And if your sore really is just a canker? You get peace of mind, fast. If it’s not, you get treated early and move on. Either way, you win.
How We Sourced This Article: We combined current guidelines from the CDC, peer-reviewed research, and firsthand clinical experience to build this article. Our goal was to cut through fear and confusion with fact-based clarity. Around fifteen reputable sources informed this article; below, we’ve highlighted some of the most relevant and reader-friendly sources.
Sources
3. Syphilis – Symptoms & Causes | Mayo Clinic
4. Oral Syphilis – What to Know | Verywell Health
5. Syphilis in the Mouth: Causes, Symptoms, Risk Factors, and Treatment | Healthline
Don’t Guess. Check.
If you're staring at a mouth sore wondering whether it's harmless or not, you’re not alone. It’s easy to dismiss it. But if it doesn’t heal, doesn’t hurt, or feels “off,” your body may be trying to warn you.
Order a rapid at-home syphilis test today and get clarity, not fear. It’s discreet, fast, and medically sound. Because waiting for it to go away isn’t the same as knowing what it is, and taking care of it.
Remember: the sooner you test, the sooner you heal. And in most cases, one quick test can take weeks of anxiety off your plate.
About the Author
Dr. F. David, MD is a board-certified infectious disease specialist with over 15 years of experience in STD care, prevention, and digital health education. He believes in shame-free medicine, evidence-based interventions, and giving patients tools they can actually use.
Reviewed by: Arianna Lex, MSN, NP-C
Last medically reviewed: October 2025
