Mouth ulcers form when the protective lining of your oral tissue breaks down, exposing the sensitive nerve-rich layers underneath. The most common type, called canker sores (aphthous ulcers), affects anywhere from 5% to 66% of the population depending on the group studied. They develop when your immune system triggers an inflammatory response against your own mucosal tissue, though the exact reason this happens varies from person to person.
What Happens Inside the Tissue
The inside of your mouth is lined with a thin, moist layer of tissue called mucosa. Unlike the tough outer skin on your body, this layer is delicate and turns over rapidly. When something disrupts it, whether physical damage, an immune reaction, or a chemical irritant, the surface cells die off faster than they can be replaced. This creates a shallow crater in the tissue.
Your immune system plays a central role. White blood cells flood the damaged area and release inflammatory signals that, instead of helping repair the tissue, actually accelerate the destruction. This is why a tiny nick from a chip or a toothbrush bristle can balloon into a painful ulcer over the course of a day or two. The inflammation also explains the hallmark appearance: a white or yellowish center (dead tissue and immune cells) surrounded by a bright red border of swollen, irritated tissue.
Common Triggers
Most mouth ulcers don’t have a single cause. They result from a combination of factors hitting at the same time. The most frequent triggers include:
- Physical trauma: Biting your cheek, aggressive brushing, sharp food edges, or poorly fitting dental work can all breach the mucosal surface and start the process.
- Stress and fatigue: Periods of high stress or sleep deprivation suppress normal immune regulation, making overreactive inflammatory responses more likely.
- Hormonal shifts: Many people notice ulcers appearing at specific points in their menstrual cycle, suggesting hormonal changes affect mucosal resilience.
- Certain foods: Acidic fruits, spicy dishes, and foods you have a sensitivity to (chocolate, coffee, nuts, and wheat are common culprits) can irritate the lining or provoke an immune response.
The Toothpaste Connection
A foaming agent called sodium lauryl sulfate (SLS), found in most commercial toothpastes, can contribute to ulcer formation. SLS strips away the protective mucin layer that coats and shields the inside of your mouth. Once that barrier is compromised, irritants can penetrate deeper into the tissue, triggering inflammation and making the lining more vulnerable to breakdown. SLS also denatures proteins in the surface cells and dissolves structural fats that hold those cells together.
There is limited but consistent evidence that people prone to recurrent ulcers experience fewer outbreaks, shorter healing times, and less pain when they switch to an SLS-free toothpaste. It’s one of the simplest changes you can try if you get ulcers frequently.
Nutritional Deficiencies
Your mouth lining replaces itself every one to two weeks, which means it has a high demand for the nutrients involved in cell growth. When those nutrients run low, the tissue becomes fragile and more prone to breaking down. Three deficiencies in particular are closely linked to recurrent mouth ulcers: vitamin B12, iron, and folate.
Low iron stores (measured as ferritin) are one of the more common findings in people with chronic ulcers. Low B12 and low folate both impair your body’s ability to produce healthy red blood cells and regenerate mucosal tissue. If you get mouth ulcers repeatedly and can’t identify an obvious trigger, a simple blood test can check these levels. Correcting a deficiency often reduces or eliminates the ulcers entirely.
Genetics and Age Patterns
Family history is one of the strongest predictors. Children whose parents both get canker sores have up to a 90% chance of developing them, compared with roughly 20% in children whose parents don’t. Most people experience their first outbreak between ages 10 and 19, and the frequency tends to decrease with age. This pattern suggests the immune system’s tendency to overreact to oral tissue damage is partly inherited and partly something you grow out of.
Three Types and How They Differ
Not all mouth ulcers look or behave the same. They fall into three categories based on size and number.
Minor ulcers are by far the most common. They’re small, typically 2 to 3 millimeters across and always under 8 millimeters. They heal within about 10 days and leave no scar. Most people who get canker sores only ever experience this type.
Major ulcers are larger than 1 centimeter and significantly more painful. They can take weeks to months to fully heal and sometimes leave scar tissue behind. These often form on the soft palate, the back of the throat, or the inner lips.
Herpetiform ulcers (named for their visual resemblance to herpes sores, though they are not caused by a virus) appear as clusters of many tiny ulcers, sometimes up to 100 at once. Each one is under a centimeter, and they typically heal within two weeks. The clusters can merge into larger irregular sores, which makes them particularly uncomfortable.
How an Ulcer Heals
Mouth ulcers follow a predictable healing pattern. First, if there’s any bleeding, your body forms a clot to seal the wound. Most canker sores don’t bleed unless you irritate them, so this stage is often skipped. Next, inflammatory cells clean out damaged tissue, which is the period when the ulcer feels most painful and looks most inflamed.
The turning point is the growth phase, when new skin starts forming from the outer edges of the ulcer and works inward toward the center. You’ll notice the red ring fading and the white center taking on a grayish tone. Pain and burning decrease noticeably during this stage. Finally, the new tissue strengthens and matures. For a typical minor ulcer, the entire process wraps up in about 10 days.
When Ulcers Signal Something Else
Recurrent mouth ulcers can occasionally be an early sign of a systemic condition. In Behçet’s disease, a rare inflammatory disorder, painful ulcers on the soft palate and throat are often the very first symptom, appearing before any other signs of the disease. In Crohn’s disease, oral lesions can actually show up before any digestive symptoms develop. These tend to look different from standard canker sores: deep, linear ulcers along the inner cheek, sometimes with surrounding tissue that has a bumpy, cobblestone texture.
Celiac disease is another condition associated with recurrent oral ulcers. The chronic inflammation and nutrient malabsorption it causes, particularly of iron and B vitamins, can make the oral lining especially vulnerable.
Canker Sore vs. Oral Cancer
The critical distinction is healing time. A canker sore should resolve within two to three weeks. An ulcer that lingers beyond that window needs professional evaluation. Canker sores are typically flat with inflamed red edges. Oral cancers, by contrast, often have a small lump or bump underneath the lesion that you can feel with your tongue or finger. Cancerous lesions also tend to lack the angry red border that characterizes a canker sore. Any painless, firm ulcer that doesn’t heal is worth getting checked promptly.