Canker sores form when your immune system mistakenly attacks the soft tissue lining inside your mouth. About 10 to 20 percent of people get them repeatedly, and while they feel like simple injuries, the process behind them is driven by the same type of immune cells that fight infections.
What Happens Inside Your Mouth
The formation of a canker sore starts with an immune signaling error. In people prone to these ulcers, a specific pathway that helps the immune system recognize threats becomes overactive. This triggers a type of immune response normally reserved for fighting bacteria and viruses, but instead it targets healthy tissue on the inside of your cheeks, lips, tongue, or soft palate.
One leading explanation is that certain immune cells react to a common, harmless mouth bacterium called Streptococcus sanguinis. Proteins on this bacterium closely resemble proteins found in your own oral tissue. Your immune system confuses the two, and the attack begins. Specialized immune cells, including ones designed to kill infected cells, flood the area and start destroying the surface layer of tissue. Local chemical signals called cytokines keep the destruction going and sustain the inflammation, which is why canker sores stay painful for days even after the initial damage is done.
The result is a shallow, round or oval crater in the soft lining of your mouth, usually with a white or yellowish center surrounded by a red border. That white center is exposed tissue where the top layer has been stripped away.
The Stages From First Tingle to Healing
Canker sores follow a predictable timeline. One to two days before an ulcer appears, you’ll feel a burning or tingling sensation at the spot where it’s forming. No blister or bump shows up during this phase, just discomfort that seems to come from nowhere.
Then the ulcer opens. Pain peaks during the first four to seven days and is often surprisingly intense relative to the sore’s small size. A tiny ulcer the width of a pencil eraser can make eating, drinking, and talking genuinely difficult. After that initial window, pain gradually fades as the tissue begins to repair itself. Most standard canker sores heal completely within about 10 days and leave no scar behind.
What Triggers the Process
The immune overreaction doesn’t happen randomly. Specific triggers can set it off, and most people who get canker sores repeatedly can identify one or more of their personal triggers with some attention.
Physical irritation is one of the most common. Biting the inside of your cheek, scraping your gums with a chip, brushing too aggressively, or getting poked by orthodontic hardware can all kick-start the process in susceptible people. The minor tissue damage seems to activate the immune pathway in a way it wouldn’t in someone who isn’t prone to canker sores.
Nutritional deficiencies play a measurable role. Low levels of vitamin B12, folate, and iron are all linked to more frequent outbreaks. These nutrients support healthy cell turnover in the mouth lining, and when they’re depleted, the tissue becomes more vulnerable. Stress and poor sleep are also well-established triggers, likely because they shift immune function toward the kind of inflammatory response that drives ulcer formation. Hormonal changes, particularly around menstruation, and certain acidic or spicy foods round out the most common culprits.
Some people also react to sodium lauryl sulfate, a foaming agent found in many toothpastes. Switching to an SLS-free toothpaste reduces outbreak frequency for a subset of people who get canker sores often.
Three Types and How They Differ
Not all canker sores behave the same way. The vast majority, around 80 percent of cases, are minor aphthous ulcers. These are small, heal within 10 days, and don’t scar. They’re the ones most people picture when they think of a canker sore.
Major aphthous ulcers are larger, deeper, and far more disruptive. They can take weeks or even months to heal and often do leave scars. These tend to appear on the soft palate, the back of the throat, or the inner lips, and the pain can interfere significantly with eating.
The third type, herpetiform ulcers, looks different from the other two. Instead of one or two distinct sores, dozens of tiny pinpoint ulcers appear at once and then merge into larger, irregular patches. Despite the name, they have nothing to do with the herpes virus. These clusters typically heal within about two weeks.
Why Some People Get Them and Others Don’t
Genetics plays a significant role. If both your parents get canker sores, your chances of getting them are substantially higher. The underlying difference appears to be in how the immune system’s threat-detection pathways are calibrated. In people who get recurrent canker sores, those pathways are set to a lower threshold, making them more likely to misfire against harmless triggers.
Certain systemic conditions also increase susceptibility. Celiac disease, inflammatory bowel disease, and immune deficiencies are all associated with more frequent oral ulcers. In these cases, the canker sores are a symptom of a broader pattern of immune dysregulation rather than a standalone problem. If you suddenly start getting canker sores frequently as an adult after rarely having them, or if they come alongside digestive symptoms, fatigue, or joint pain, that pattern is worth mentioning to a doctor.
When a Sore Isn’t a Canker Sore
Canker sores are sometimes confused with cold sores, but the two are fundamentally different. Cold sores are caused by the herpes simplex virus, appear on or around the lips (outside the mouth), and start as fluid-filled blisters. Canker sores are not caused by a virus, appear only inside the mouth, and never form blisters.
More importantly, a sore that doesn’t heal within a few weeks deserves attention. Oral cancers can initially look like a stubborn ulcer. Red flags that distinguish a potentially serious sore from a canker sore include: bleeding that won’t stop, a lump under the skin beneath the sore, rough or crusty texture changes in the surrounding tissue, white or mottled patches, visible swelling in the neck or jaw, and any sore that persists beyond two weeks without improvement. A standard canker sore, even a painful one, follows a clear arc of worsening, plateauing, and healing. A sore that just stays or grows is a different situation entirely.