Most mouth ulcers are caused by minor physical damage to the lining of your mouth, like biting your cheek, brushing too hard, or irritation from a sharp tooth edge. These common sores (called aphthous ulcers or canker sores) are round, white or yellow with a red border, and heal on their own within one to two weeks. But when ulcers keep coming back or won’t go away, the causes run deeper, from nutritional gaps and stress to underlying health conditions.
Physical Damage and Irritation
The most straightforward cause is mechanical trauma. Accidentally biting the inside of your cheek, scraping your gums with a hard toothbrush, or rubbing against braces, dentures, or a rough filling can all break the delicate tissue lining your mouth. Once that tissue is damaged, a painful ulcer forms at the site. These ulcers are not contagious and typically heal within 7 to 14 days once the source of irritation is removed.
Hot food and drinks can also burn the lining of your mouth enough to trigger an ulcer. So can acidic foods like citrus fruits, tomatoes, and vinegar-based dressings, which irritate already-vulnerable tissue and may make existing ulcers worse.
Toothpaste as a Surprising Trigger
A foaming agent found in most toothpastes, sodium lauryl sulfate (SLS), is linked to more frequent mouth ulcers in people who are prone to them. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared with a standard one containing SLS. If you get ulcers regularly, switching to an SLS-free toothpaste is a simple first step worth trying. People with naturally delicate mouth tissue seem especially sensitive to this ingredient.
Nutritional Deficiencies
Recurrent mouth ulcers are one of the recognized symptoms of being low in iron, folate, or vitamin B12. These deficiencies reduce the body’s ability to maintain healthy tissue in the mouth, making the lining more fragile and slower to repair itself. The NHS lists mouth ulcers alongside a sore, red tongue as symptoms of B12 or folate deficiency anaemia.
You don’t need to be severely anaemic for this to matter. Even borderline-low levels of these nutrients can contribute to ulcers that keep returning. A blood test from your doctor can check all three. Dietary sources of these nutrients include red meat, leafy greens, eggs, fortified cereals, and legumes, though supplements may be needed if levels are genuinely low.
Stress and Your Immune Response
Stress is one of the most well-documented triggers for recurrent mouth ulcers, and the connection is biological, not just anecdotal. When you’re stressed, your body releases cortisol. Research published in the Journal of Oral Medicine and Oral Surgery found that people with recurrent mouth ulcers had significantly elevated cortisol levels not only during active ulcers but even after healing, suggesting that chronic stress and anxiety keep the body in a state that favors ulcer formation.
Stress disrupts the immune system on multiple levels. It alters the balance of immune cells, shifts hormone levels, and changes nervous system signaling. All of this can make your mouth lining more vulnerable. Stressful events are particularly likely to trigger new ulcers in people who already have a history of them. This helps explain why ulcers often cluster around exams, deadlines, or difficult life events.
Immune System Involvement
Mouth ulcers are fundamentally an immune-mediated process. Researchers believe that in people prone to canker sores, the immune system overreacts to a trigger and attacks the cells lining the mouth. This creates the characteristic painful crater. The trigger might be physical damage, a food sensitivity, stress, or something else entirely, but the immune response is what turns a minor irritation into a full ulcer.
Some people also develop ulcers from allergic reactions to certain foods or dental materials. These are delayed hypersensitivity reactions, meaning symptoms appear hours or days after exposure rather than immediately. Common culprits include certain preservatives, flavorings, and metals used in dental work.
Underlying Health Conditions
When mouth ulcers are frequent, severe, or slow to heal, they can signal a systemic health condition. Several diseases are closely associated with recurrent oral ulcers:
- Celiac disease: An immune reaction to gluten that damages the gut lining. Mouth ulcers are sometimes the first noticeable symptom, even before digestive problems appear.
- Crohn’s disease: An inflammatory bowel condition that can cause ulcers anywhere in the digestive tract, including the mouth.
- Behçet’s syndrome: A rare condition causing inflammation in blood vessels throughout the body. Painful, recurrent oral ulcers are its hallmark symptom.
- HIV and other immunodeficiencies: A weakened immune system makes the mouth lining more susceptible to ulceration.
- PFAPA syndrome: A condition primarily affecting children, causing periodic fevers alongside mouth ulcers, sore throat, and swollen neck glands.
If you’re getting ulcers several times a year with no obvious trigger, it’s worth mentioning to your doctor. A few blood tests can rule out the most common systemic causes.
Mouth Ulcers vs. Cold Sores
Mouth ulcers and cold sores are often confused, but they’re different conditions with different causes. Canker sores (aphthous ulcers) appear inside the mouth, on the inner cheeks, lips, or tongue. They look like single round sores with a white or yellow center and red border. They are not caused by a virus and are not contagious.
Cold sores (fever blisters), on the other hand, are caused by the herpes simplex virus. They appear on the outside of the mouth, typically around the border of the lips, as clusters of small fluid-filled blisters. Cold sores are contagious, especially when the blisters are open. If your sore is inside your mouth and looks like a single round crater, it’s almost certainly a canker sore, not a cold sore.
When an Ulcer Needs Medical Attention
Most mouth ulcers are harmless and heal without treatment. But Cancer Research UK advises that any mouth ulcer lasting longer than three weeks without a clear explanation should be checked by a doctor or dentist. A persistent ulcer that won’t heal is one of the possible signs of mouth cancer, particularly if it feels hard or thickened, or if you notice an unexplained lump in your neck at the same time.
Red or mixed red-and-white patches inside the mouth that a dentist considers abnormal also warrant an urgent referral. This doesn’t mean every slow-healing ulcer is cancer. The vast majority are not. But the three-week mark is the point where getting it looked at becomes important, because early detection makes a significant difference in outcomes for the small number of cases that do turn out to be serious.