Mouth ulcers are most commonly caused by minor physical trauma, like biting your cheek or irritation from braces, but they can also result from nutritional deficiencies, certain foods, stress, and underlying health conditions. Most are harmless and heal on their own within two to three weeks. Understanding the trigger behind yours can help you prevent the next one.
Physical Injury and Irritation
The most straightforward cause is mechanical damage to the soft tissue inside your mouth. Accidentally biting the inside of your cheek, slipping with a toothbrush, or burning your mouth on hot food can all break the surface of the delicate lining and leave behind a painful sore. These ulcers typically appear right at the site of injury and heal without treatment once the tissue repairs itself.
Ongoing friction is a bigger problem than a one-time bite. Braces, dentures, orthodontic aligners, and mouth splints can rub against the same spot repeatedly, preventing the tissue from healing and sometimes creating new ulcers. Sharp or broken teeth do the same thing. If you notice ulcers forming in the same location again and again, the cause is likely a dental appliance or tooth edge that needs adjustment.
Foods and Drinks That Trigger Ulcers
Certain foods are well-known triggers, especially for people who are already prone to ulcers. Acidic fruits top the list: pineapples, oranges, lemons, limes, and strawberries can irritate the mouth lining enough to cause a breakout. Spicy foods like curries, hot sauce, and jalapeños have a similar effect, disrupting the tissue with their high acidity.
Some triggers are less obvious. Nuts, particularly walnuts, peanuts, cashews, and almonds, contain an amino acid called L-arginine that contributes to canker sores in sensitive people. Chocolate contains a compound called theobromine that can provoke something like a mild allergic reaction in the mouth, leading to sores on the tongue or inner cheeks. Dairy products, coffee, and alcohol are also common culprits. If you get ulcers frequently, keeping a food diary for a few weeks can help you identify your personal triggers.
Your Toothpaste May Play a Role
Many toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that can irritate the mouth’s lining. For some people, SLS worsens existing ulcers or triggers new ones. If you deal with recurring sores, switching to an SLS-free toothpaste is a simple change worth trying. Several brands market specifically as SLS-free, and they’re widely available at pharmacies.
Vitamin and Mineral Deficiencies
Recurring mouth ulcers can be a signal that your body is low on certain nutrients. In a study of 273 patients with recurrent ulcers, about 20% had low iron levels, roughly 21% were anemic, nearly 5% were deficient in vitamin B12, and about 3% were low in folic acid. A separate study found B12 deficiency in 7% of 208 patients with recurring sores.
These numbers mean that while nutritional deficiency isn’t behind every mouth ulcer, it’s a real factor for a significant minority of people who get them repeatedly. Iron, B12, and folate all play roles in maintaining healthy tissue lining. If your ulcers keep coming back without an obvious physical trigger, a blood test checking these levels is a reasonable step. Correcting the deficiency often reduces or eliminates the ulcers.
Stress and Immune System Changes
Many people notice ulcers appearing during stressful periods, and there’s a biological reason for that. Recurrent mouth ulcers involve an immune response where certain white blood cells attack the mouth’s own tissue. In people prone to ulcers, immune cells called gamma-delta T cells are present in higher proportions during active flare-ups. These cells, along with other immune signals, drive inflammation and break down the surface layer of the mouth lining.
Stress, poor sleep, and illness can all shift immune function in ways that make this process more likely to kick in. Hormonal changes, particularly around menstruation, are another common trigger. The ulcers themselves aren’t an infection. They’re your immune system overreacting to a stimulus that wouldn’t normally cause damage.
Underlying Health Conditions
In some cases, recurring mouth ulcers are a symptom of a broader medical condition. Crohn’s disease causes oral lesions in anywhere from 0.5% to 20% of patients, and these can include deep, linear ulcers along with swelling and a distinctive cobblestone texture to the mouth’s lining. Lupus produces oral sores in 8% to 45% of patients with the systemic form, often appearing as well-defined areas of redness or ulceration surrounded by white, radiating lines.
Celiac disease is another condition closely linked to mouth ulcers. People with undiagnosed celiac may get recurrent sores that improve once they start a gluten-free diet. Other conditions associated with oral ulcers include pemphigus vulgaris, an autoimmune blistering disease, and Behçet’s syndrome, which causes ulcers in the mouth and other parts of the body. If your ulcers are unusually frequent, severe, or accompanied by other symptoms like digestive problems, joint pain, or skin changes, these conditions are worth investigating.
Three Types of Canker Sores
Not all mouth ulcers look or behave the same. Minor canker sores, the most common type, are smaller than a centimeter across and heal within a few weeks without leaving a scar. Major canker sores are larger than a centimeter, extremely painful, and can take months to heal, often leaving scars behind. Herpetiform canker sores are clusters of tiny pinpoint sores that group together. Despite the name, they’re not caused by the herpes virus. They typically heal within about two weeks.
When a Mouth Ulcer Could Be Something Else
Most mouth ulcers are painful but harmless. A small number, however, can be early signs of oral cancer, and knowing the differences matters. Canker sores are typically flat, painful from the start, and surrounded by red, inflamed edges. Oral cancers in their earliest stages are usually not painful. They often have a small lump or bump beneath the surface that you can feel with your tongue or finger, something a standard canker sore won’t have.
The clearest warning sign is time. If a sore hasn’t healed within two to three weeks, it needs professional evaluation. Other red flags include a small spot that keeps growing larger, a white patch that turns red, or a lesion that starts bleeding when it didn’t before. These changes don’t necessarily mean cancer, but they do mean the sore isn’t behaving like a typical ulcer and deserves a closer look.