Most mouth ulcers are caused by minor physical damage to the lining of your mouth, such as accidentally biting your cheek, irritation from braces, or scraping from a sharp piece of food. These small, round sores (also called canker sores or aphthous ulcers) typically heal on their own within one to two weeks. But when ulcers keep coming back or won’t go away, the cause is often something less obvious: a nutritional gap, an ingredient in your toothpaste, or an underlying health condition.
Physical Injury to the Mouth Lining
The tissue inside your mouth is delicate, and even minor trauma can break it open enough to form an ulcer. The most common physical triggers include biting the inside of your cheek or lip, sharp edges on a broken tooth or filling, poorly fitting dentures, and orthodontic braces rubbing against soft tissue. Hard or sharp foods like chips, crusty bread, or toast can also scratch the lining and start an ulcer.
Aggressive tooth brushing is another frequent culprit, especially with a hard-bristled brush. Even vigorous flossing can nick the gums enough to trigger a sore. These injury-related ulcers are usually isolated events. If you can identify what caused the damage (a new retainer wire, a rough dental crown), fixing the source typically prevents recurrence.
Your Toothpaste May Be a Factor
Many commercial toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that can irritate the mouth’s protective lining. Research published in the British Dental Journal found that SLS strips away the delicate outer layers of the oral mucosa, the soft tissue that lines the inside of your mouth. For some people, this makes the tissue more vulnerable to ulcers.
An earlier clinical study found that SLS-containing toothpaste can increase the frequency of repeated mouth ulcers. If you notice ulcers appearing regularly, switching to an SLS-free toothpaste is a simple first step. Several brands now market SLS-free formulas specifically for sensitive mouths, and they clean just as effectively.
Nutritional Deficiencies
Recurring mouth ulcers are sometimes a sign that your body is low on specific nutrients. The deficiencies most closely linked to ulcers are vitamin B12, iron, folate, and zinc. In one study of 273 patients with recurrent aphthous ulcers, about 20% had iron deficiency, roughly 21% had anemia, nearly 5% were low in vitamin B12, and about 3% were deficient in folic acid.
Vitamin B12 deficiency deserves particular attention. Clinical case reports have documented a clear relationship between recurring mouth ulcers and low B12 levels, and researchers have recommended that anyone with persistent aphthous ulcers be checked for this deficiency. B12 is found primarily in animal products like meat, fish, eggs, and dairy, so vegetarians and vegans are at higher risk. Iron and folate gaps are also common in people with restricted diets or heavy menstrual periods. A simple blood test can identify whether a nutritional shortfall is driving your ulcers.
Stress and Immune Response
Many people notice mouth ulcers appearing during stressful periods, and this connection is well recognized in clinical practice. Stress suppresses parts of the immune system while ramping up inflammatory responses, creating conditions where the mouth lining is more easily damaged and slower to heal. Hormonal shifts, particularly around menstruation, can trigger ulcers through a similar immune mechanism. Sleep deprivation compounds both of these factors.
This immune component helps explain why mouth ulcers cluster during difficult stretches of life: exam periods, work deadlines, illness, or emotional upheaval. The ulcers themselves aren’t caused by stress alone, but stress lowers the threshold at which other triggers (a minor bite, a scratchy food) turn into a full sore.
Food Sensitivities
Certain foods seem to provoke mouth ulcers in some people, though the mechanism is irritation rather than a true allergic reaction. The American Academy of Allergy, Asthma and Immunology notes there is no evidence that immunologic food reactions are responsible for aphthous ulcers. However, many people consistently report that specific foods trigger new sores or worsen existing ones.
Common reported triggers include citrus fruits, tomatoes, chocolate, coffee, strawberries, cheese, nuts, and spicy foods. Acidic foods are the most obvious offenders because they directly irritate the mouth lining. If you suspect a food connection, keeping a brief food diary alongside your ulcer episodes can help you identify your personal triggers.
Underlying Health Conditions
When mouth ulcers are unusually frequent, severe, or accompanied by other symptoms, they can signal a systemic condition. Several diseases are known to produce oral ulcers as one of their features:
- Celiac disease causes nutrient malabsorption that leads to the same B12, iron, and folate deficiencies described above. Mouth ulcers are sometimes the first noticeable symptom.
- Crohn’s disease can produce deep linear ulcers in the mouth, along with diffuse mucosal swelling and a distinctive “cobblestone” texture on the inner cheeks.
- Behçet’s syndrome causes recurrent, painful ulcers that are often numerous and tend to appear on the soft palate and back of the throat.
- Lupus can present with irregularly shaped mouth ulcers, along with redness, purplish spots, and raised plaques inside the mouth.
- Pemphigus vulgaris produces widespread, painful oral ulceration that often appears before skin symptoms develop.
Immunosuppressive medications, certain blood pressure drugs, and some anti-inflammatory painkillers can also cause mouth ulcers as a side effect. If your ulcers started after beginning a new medication, that timing is worth noting.
Canker Sores vs. Cold Sores
People often confuse these two conditions, but they are fundamentally different. Canker sores (aphthous ulcers) appear inside the mouth, on the cheeks, lips, tongue, or gums. They are single, round sores with a white or yellow center and a red border. They are not contagious, and their exact cause is unknown, though the triggers listed above all play a role.
Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. They look like clusters of small, fluid-filled blisters rather than open sores. Cold sores are caused by herpes simplex virus (usually type 1) and are highly contagious. If your sore is inside your mouth and isn’t a cluster of blisters, it’s almost certainly a canker sore, not a cold sore.
When Mouth Ulcers Need Attention
Most mouth ulcers are harmless and resolve within one to two weeks without treatment. But the NHS recommends seeing a dentist or doctor if an ulcer lasts longer than three weeks, is larger than your typical sores, appears near the back of your throat, or becomes increasingly painful, red, or starts bleeding (which may indicate infection). A long-lasting mouth ulcer is sometimes a sign of oral cancer, so persistent sores that don’t follow the normal healing pattern deserve a professional evaluation.
Ulcers that come in clusters, recur monthly, or are accompanied by symptoms elsewhere in the body (joint pain, skin rashes, digestive problems, fatigue) point toward a systemic cause worth investigating through blood work or further testing.