Most mouth ulcers are caused by minor tissue damage, like biting your cheek or brushing too hard, combined with an overactive immune response that prevents the wound from healing quickly. These common sores, called aphthous ulcers or canker sores, affect up to 20% of the population and typically heal on their own within one to two weeks. But when ulcers keep coming back, the causes run deeper: nutritional gaps, stress, hormonal shifts, irritating ingredients in toothpaste, or underlying health conditions can all play a role.
How a Mouth Ulcer Actually Forms
A mouth ulcer isn’t just a cut that won’t close. It starts when something damages the thin lining inside your mouth, and your immune system responds disproportionately. Certain white blood cells, specifically a type of T cell, attack the damaged tissue and release inflammatory signals that break down the surface layer further instead of repairing it. This creates the shallow, painful crater you see and feel.
In people who get ulcers repeatedly, these T cells appear to be more aggressive than normal. Research suggests the immune system may be reacting to bacteria naturally present in the mouth, mistaking proteins on the lining of your cheeks and gums for something harmful. That’s why the same small injury that heals uneventfully in one person turns into a painful ulcer in another.
Physical Damage and Everyday Triggers
The most straightforward cause is mechanical injury. Biting the inside of your cheek, scraping your gum with a sharp chip, burning your palate on hot food, or irritation from braces and ill-fitting dentures can all start the process. Aggressive toothbrushing is another common culprit, especially with a hard-bristled brush.
Your toothpaste itself may be making things worse. Many toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that strips away the protective outer layers of your mouth’s lining. Research published in the British Dental Journal found that SLS can cause visible peeling of the oral tissue, and a separate study showed that people who used SLS-containing toothpaste developed mouth ulcers more frequently. Switching to an SLS-free toothpaste is one of the simplest changes you can make if you get ulcers often.
Nutritional Deficiencies
What you’re not eating can matter as much as what’s irritating your mouth. Vitamin B12 deficiency stands out as the strongest nutritional link. In one study comparing 57 people with recurrent mouth ulcers to 57 people without them, half of the ulcer group had low B12 levels. Not a single person in the control group was B12 deficient. That’s a striking difference.
Low folate levels also showed up in about 46% of ulcer patients in the same study. Iron deficiency, while commonly mentioned, had a weaker connection. The practical takeaway: if you’re getting mouth ulcers repeatedly, it’s worth checking your B12 and folate levels through a simple blood test. Vegetarians, vegans, and older adults are especially prone to B12 deficiency since the vitamin comes primarily from animal products and becomes harder to absorb with age.
Stress and Hormonal Changes
Stress doesn’t directly cause mouth ulcers, but it reliably triggers them in people who are already susceptible. In clinical studies, people with recurrent ulcers reported significantly higher stress levels during active outbreaks than during ulcer-free periods. Seventeen out of the study participants specifically linked their worst episodes to stressful life events. The working theory is that stress acts as a trigger or amplifier rather than a root cause, likely by suppressing parts of the immune system that keep inflammation in check.
Hormonal fluctuations play a similar triggering role. Some women notice ulcers appearing in the days before their period, during pregnancy, or after menopause. The pattern suggests that drops in certain hormone levels may temporarily change how the immune system responds to minor mouth injuries.
Three Types of Aphthous Ulcers
Not all mouth ulcers look or behave the same way. Minor aphthous ulcers are the most common type: less than 5 mm across (roughly the size of a pencil eraser), round or oval, and they heal within one to two weeks without scarring. These are what most people mean when they say “canker sore.”
Major aphthous ulcers are larger, often exceeding 10 mm, and significantly more painful. They can take weeks or even months to heal and frequently leave scars. These deeper ulcers sometimes appear near the back of the mouth or on the soft palate, making eating and swallowing difficult.
Herpetiform ulcers are the least common type. Despite the name, they aren’t caused by the herpes virus. They appear as clusters of tiny pinpoint sores that can merge into larger, irregular ulcers. They generally heal within a month.
Medical Conditions Linked to Mouth Ulcers
Recurring mouth ulcers can sometimes signal a systemic disease, particularly autoimmune or gastrointestinal conditions. Crohn’s disease is one of the better-known connections. People with Crohn’s may develop deep, linear ulcers inside the mouth, often along the lower gum line, sometimes with a distinctive cobblestone texture to the surrounding tissue. These oral symptoms can appear before any gut symptoms do.
Behçet’s syndrome causes painful ulcers that look similar to ordinary canker sores but tend to be more numerous and frequently involve the soft palate and back of the throat. Oral ulcers are often the very first sign of this condition and may appear years before other symptoms develop.
Lupus can produce ulcers surrounded by a distinctive pattern of white, radiating lines on a red base. Pemphigus vulgaris causes widespread, painful oral sores when the immune system attacks the proteins that hold skin cells together. Celiac disease is another condition where chronic mouth ulcers may be the most visible symptom, driven by malabsorption of nutrients like B12 and folate.
When a Mouth Ulcer Could Be Something Serious
The vast majority of mouth ulcers are harmless and temporary. But an ulcer that doesn’t heal within two to three weeks deserves medical attention, because oral cancer can initially look like a persistent sore.
There are key physical differences between a canker sore and something more concerning. Canker sores are usually flat, painful from the start, and surrounded by red, inflamed tissue. Oral cancers, by contrast, often have a small lump or bump underneath the surface that you can feel with your tongue or finger. Early-stage oral cancer is typically painless, which is the opposite of what most people expect. A canker sore hurts immediately; a cancerous lesion may not hurt at all until it’s advanced.
Other warning signs include a white patch that turns red over time, a sore that bleeds when it previously didn’t, or a small spot that keeps growing larger. Any ulcer on the side or underside of the tongue, the floor of the mouth, or the soft palate that persists beyond three weeks should be evaluated, as these are higher-risk locations for oral malignancies.
Reducing How Often Ulcers Come Back
Since recurrent mouth ulcers involve an immune overreaction to everyday triggers, the most effective strategy is removing as many triggers as possible. Switch to an SLS-free toothpaste and a soft-bristled brush. If you notice ulcers after eating certain foods, acidic items like citrus, tomatoes, and vinegar-based dressings are common offenders worth reducing.
Get your B12 and folate levels checked, especially if ulcers are a regular part of your life. Correcting a deficiency through diet or supplements can dramatically reduce recurrence in people whose levels are low. Managing stress through whatever works for you, whether that’s exercise, sleep hygiene, or structured relaxation, won’t eliminate ulcers entirely but can reduce how often they flare. For people with frequent or severe outbreaks, a dentist or doctor can prescribe topical treatments that reduce pain and speed healing by dampening the local immune response.