Mouth ulcers happen when your immune system attacks the thin lining of tissue inside your mouth, creating small, painful craters. Between 5% and 25% of people get them repeatedly, with some studies estimating rates as high as 60% in certain populations. They peak in frequency during your teens and twenties, then often become less common with age. The reasons you get them range from a minor bite on your cheek to nutritional gaps to underlying health conditions.
What Actually Happens Inside Your Mouth
The tissue lining your cheeks, lips, and tongue is soft and unprotected compared to the tougher, keratinized skin on the outside of your body. When something irritates or damages this lining, your immune system launches an inflammatory response that can spiral beyond what’s needed.
Research using immune cell markers shows a clear pattern. In the earliest stage, before a visible ulcer forms, helper immune cells flood the area and begin coordinating an attack. Then, as the ulcer opens up, a different type of immune cell takes over: cytotoxic cells that destroy tissue. The ratio shifts dramatically, from roughly two helper cells for every one cytotoxic cell to one helper cell for every ten cytotoxic cells. This wave of tissue destruction is what creates the open sore. Once the damage peaks, the balance flips again, helper cells return to dominate the area, and healing begins. Natural killer cells also appear in the earliest stages, adding to the initial assault.
This pattern suggests that mouth ulcers aren’t simply wounds. They’re a localized immune overreaction where your body essentially attacks its own tissue.
Physical Triggers That Start the Process
The most straightforward cause is mechanical injury. Biting your cheek, scraping your gums with a sharp chip, or burning the roof of your mouth with hot food can all break the mucosal layer and set off the inflammatory chain reaction described above. For most people, these heal without much trouble.
Dental braces and orthodontic appliances are a particularly common culprit. Brackets, wires, and bands create constant friction against the soft tissue inside your mouth. This repeated rubbing disrupts the protective lining and produces ongoing inflammation. Orthodontic hardware also creates small spaces that trap food debris and bacterial plaque. The resulting bacterial buildup triggers additional inflammation in the gums and surrounding tissue, and research shows a strong link between increased plaque accumulation and more frequent ulcer formation in orthodontic patients.
Poorly fitting dentures, rough tooth edges, and aggressive toothbrushing can all produce similar effects.
Nutritional Deficiencies
Your body needs specific nutrients to maintain and repair the mucosal lining inside your mouth. When those nutrients are missing, the tissue becomes more vulnerable to breakdown. Three deficiencies are most consistently linked to recurrent mouth ulcers: iron, vitamin B12, and folate. All three play roles in producing healthy red blood cells and maintaining tissue integrity. When levels drop low enough to cause anemia, mouth ulcers and a sore, reddened tongue are recognized symptoms.
If you get mouth ulcers frequently and also feel unusually tired, short of breath, or lightheaded, a nutritional deficiency is worth investigating with a simple blood test. Correcting the deficiency often reduces or eliminates the ulcers.
The Role of Stress
Many people notice mouth ulcers appearing during stressful periods, and stress is widely cited as a trigger. The biological mechanism, however, remains surprisingly unclear. One study measuring both perceived stress levels and blood cortisol (the body’s primary stress hormone) in 50 patients with recurrent mouth ulcers found no significant relationship between either measure and ulcer duration. Cortisol levels showed no meaningful connection at all.
This doesn’t mean stress plays no role. Stress changes behavior in ways that matter: people sleep less, eat differently, skip meals, grind their teeth, and neglect oral hygiene. These indirect effects may explain the connection better than a direct hormonal pathway. If you notice a pattern between stressful periods and ulcer outbreaks, the practical response is the same regardless of the mechanism: prioritize sleep, nutrition, and oral care during high-pressure stretches.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, has long been suspected of triggering mouth ulcers. SLS strips away the protective mucus layer inside your mouth, potentially making the tissue more vulnerable to irritation. Some patch-testing studies have identified SLS as one of the most common allergens found in toothpaste, alongside titanium dioxide and menthol.
The clinical evidence, though, is mixed. A 2019 review concluded there wasn’t enough data to confirm that SLS-free toothpastes reduced ulcer frequency, duration, pain, or number. A double-blind study found no significant change in ulcer patterns when patients switched to SLS-free products. Still, if you get frequent ulcers and want to test the theory, switching toothpaste is a low-cost experiment with no downside.
Connections to Digestive Conditions
Mouth ulcers can be an early or recurring sign of inflammatory conditions in the gut. In an Italian survey of patients with celiac disease and inflammatory bowel disease (including Crohn’s disease), about 66% reported oral symptoms during active periods of their intestinal disease. Among those with oral symptoms, canker sores were the most common complaint, reported by 63% of affected patients.
The connection makes biological sense. Celiac disease involves an immune reaction to gluten that damages the lining of the small intestine, and that same immune dysfunction can target the oral mucosa. Crohn’s disease causes inflammation anywhere along the digestive tract, and the mouth is technically where that tract begins. If you get persistent mouth ulcers alongside digestive symptoms like bloating, diarrhea, abdominal pain, or unexplained weight loss, the ulcers may be pointing toward something systemic rather than local.
How Long They Take to Heal
Most mouth ulcers are the minor type: small, round or oval, with a white or yellowish center and a red border. These typically heal within 10 to 14 days without leaving a scar. You’ll usually feel a tingling or burning sensation for a day or two before the ulcer becomes visible, followed by several days of peak pain, then a gradual tapering as the tissue rebuilds.
Over-the-counter treatments can shorten that timeline and reduce pain. Topical corticosteroid pastes applied directly to the ulcer reduce both pain and duration, though they don’t prevent future episodes. Antiseptic mouth rinses containing chlorhexidine can reduce the severity and pain of an active ulcer but similarly don’t change how often ulcers come back. Anti-inflammatory adhesive patches or tablets that stick to the ulcer site also show benefits for pain relief and faster healing.
When a Mouth Ulcer Needs Attention
A typical canker sore, while painful, resolves on its own within two weeks. Ulcers that persist beyond three weeks, grow larger, spread, or keep returning in clusters warrant a closer look. A non-healing ulcer can sometimes indicate oral cancer, particularly if it’s painless, has irregular borders, or appears on the side of the tongue or floor of the mouth.
Clinicians will often try to identify and remove a suspected cause first. If the ulcer persists after the irritant is gone, a biopsy is the standard next step to rule out malignancy. Any ulcer that changes in size or appearance after an initial evaluation also calls for re-examination. This is especially relevant for people who smoke or drink heavily, as both significantly raise the risk of oral cancers.