Mouth ulcers form when the protective lining inside your mouth breaks down, exposing the sensitive tissue underneath. The most common type, called a canker sore, affects roughly 20% of the population at some point. These small, painful craters on your cheeks, tongue, gums, or inner lips can be triggered by everything from accidentally biting your cheek to ongoing stress, nutritional gaps, or an underlying health condition.
Physical Injury to the Mouth Lining
The simplest and most frequent cause is mechanical damage. Biting your cheek or tongue, getting a cavity filled, or having a sharp edge on a broken tooth can all create a small wound that develops into an ulcer. Braces and retainers are especially common culprits because they rub against soft tissue repeatedly throughout the day. Even brushing too aggressively or using a hard-bristled toothbrush can injure the delicate inner lining of your mouth enough to trigger one.
These trauma-related ulcers are usually minor. They show up a day or two after the injury, hurt for about a week, and heal on their own within two to three weeks. If the source of irritation stays (a rough bracket on braces, for example), the ulcer can linger or keep coming back in the same spot until the problem is fixed.
Stress and Hormonal Changes
Stress is one of the most consistently recognized triggers for recurrent mouth ulcers. When you’re under psychological pressure, your body’s stress hormones can disrupt normal immune function and reduce the mouth’s ability to repair itself. Studies measuring cortisol levels in ulcer patients have found a clear pattern: people with moderate to severe stress scores tend to have higher cortisol levels alongside active ulcers. The relationship likely works in both directions, too. The burning pain and difficulty eating that come with ulcers add more stress, which can prolong healing or trigger new sores.
Hormonal shifts during the menstrual cycle are another commonly reported trigger, though the exact mechanism is less well understood. Many people notice ulcers appearing at predictable points in their cycle, suggesting that fluctuating hormone levels play a role in how vulnerable the mouth lining is at any given time.
Nutritional Deficiencies
If you get mouth ulcers frequently, a nutrient gap could be part of the picture. Among people with recurrent ulcers, about 21% are found to have low iron levels, roughly 5% are deficient in vitamin B12, and about 3% are low in folate. Anemia, which can result from any of these deficiencies, shows up in roughly 21% of recurrent ulcer patients.
These nutrients are essential for maintaining healthy mucosal tissue and supporting immune function. Without enough of them, the mouth lining becomes more fragile and slower to heal. If your ulcers keep returning without an obvious trigger like stress or injury, a blood test checking iron, B12, and folate levels is a reasonable step.
Food Sensitivities and Irritants
Certain foods can trigger ulcers in susceptible people, though this varies widely from person to person. Acidic fruits like oranges and tomatoes, spicy foods, and rough-textured snacks like chips or crusty bread are frequent offenders. Some people also react to chocolate, coffee, or certain nuts.
A less obvious trigger is oral allergy syndrome, which occurs in people with pollen allergies. If you’re allergic to birch pollen, for instance, eating apples, cherries, almonds, or carrots can cause a cross-reaction in your mouth. Grass pollen allergies can make kiwi, melon, and tomatoes problematic. Ragweed allergies may cause reactions to bananas, cucumbers, and watermelon. These reactions happen because proteins in these foods resemble pollen proteins closely enough to confuse the immune system.
Flavorings in toothpaste and mouthwash can also irritate the mouth lining. Ingredients like menthol, peppermint oil, and cinnamon flavoring are known to cause contact reactions in some people. Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, has long been suspected as a trigger. It can irritate the skin and mucous membranes, though research reviews have found insufficient evidence to confirm that switching to SLS-free toothpaste reliably reduces ulcer frequency. Still, some individuals find it helps.
Your Immune System’s Role
Recurrent mouth ulcers are fundamentally an immune-mediated problem. In people who are genetically predisposed, the immune system essentially overreacts and attacks the mouth’s own tissue. Certain white blood cells destroy the surface layer of the mouth lining, and the process is fueled by inflammatory signaling molecules that sustain the damage. People with active, recurring ulcers have higher levels of a specific type of immune cell that may be involved in attacking the mouth’s own tissue based on a case of mistaken identity: the immune system appears to confuse proteins from common oral bacteria with proteins found naturally in mouth cells.
This explains why ulcers tend to run in families and why some people are simply more prone to them than others, regardless of what they eat or how careful they are with their toothbrush.
Medications That Cause Mouth Ulcers
Several common medications list mouth ulcers as a side effect. These include:
- NSAIDs like ibuprofen
- Beta-blockers used for blood pressure, angina, or irregular heart rhythms
- Nicorandil, a medication sometimes prescribed for angina
- Chemotherapy and radiation therapy, which can cause widespread mouth sores known as mucositis
If you notice ulcers appearing after starting a new medication, that timing is worth mentioning to your prescriber. Medication-related ulcers often resolve once the drug is changed or stopped.
Underlying Health Conditions
Mouth ulcers that keep coming back, appear in clusters, or are unusually large or painful can sometimes signal a systemic health condition. Several diseases are known to produce oral ulcers as an early or prominent symptom.
Crohn’s disease causes oral lesions in anywhere from 0.5% to 20% of patients. These can include deep linear ulcers, diffuse swelling, and a distinctive cobblestone appearance of the inner mouth tissue. Behçet’s syndrome produces recurrent, painful ulcers that often appear in clusters on the soft palate and back of the throat. In fact, mouth sores are the most common manifestation of Behçet’s and frequently appear before any other symptoms. Celiac disease, lupus, and pemphigus vulgaris also cause oral ulcers. With pemphigus vulgaris, mouth sores are the first sign of disease in 50% to 80% of patients and can precede skin symptoms by a year or more.
When an Ulcer Needs Attention
Most mouth ulcers heal on their own within two to three weeks. One that persists beyond that window warrants a visit to your doctor or dentist. A sore that doesn’t heal is one of the key differences between a harmless canker sore and something more serious, including oral cancer. Other signs worth getting checked include ulcers that are unusually large, keep coming back in the same spot, spread in number, or come with unexplained symptoms like fever, weight loss, or difficulty swallowing.