Mouth ulcers in adults are most often triggered by minor physical injury, stress, or nutritional gaps, though they can also signal an underlying health condition. These small, painful sores on the inner cheeks, lips, tongue, or gums affect anywhere from 5% to 66% of the population depending on the study, and they tend to first appear in adolescence before recurring less frequently with age. Most heal on their own within two weeks, but understanding what’s behind them can help you reduce how often they come back.
Physical Injury and Irritation
The most straightforward cause is mechanical damage to the soft tissue inside your mouth. Biting the inside of your cheek, jabbing yourself with a toothbrush, or burning your mouth on hot food can all produce an ulcer at the injury site within a day or two. These one-off sores usually heal without any intervention.
Ongoing irritation is a bigger problem. Braces, dentures, or a tooth with a sharp or broken edge can rub against the same spot repeatedly, creating ulcers that won’t fully heal until the source of friction is corrected. If you notice sores consistently forming in one location, a rough dental surface or poorly fitting appliance is a likely culprit.
Stress and the Cortisol Connection
Stress is one of the most consistently reported triggers for recurrent mouth ulcers. The link appears to be hormonal: when you’re under chronic stress, your body overproduces cortisol and other stress hormones, which disrupts immune function in the delicate lining of the mouth. Research published in the Oral and Maxillofacial Pathology Journal found a meaningful association between anxiety levels, elevated salivary cortisol, and recurrent ulcer flare-ups.
This doesn’t mean stress “causes” ulcers in the way a burn does. Rather, stress lowers the threshold at which your immune system attacks oral tissue. Many people notice a pattern: ulcers appear during exam periods, work deadlines, or after emotionally difficult stretches, then disappear once the pressure lifts.
Vitamin and Mineral Deficiencies
Three nutrients are most closely tied to recurrent mouth ulcers: iron, vitamin B12, and folate. Deficiencies in any of these can leave the oral lining thinner and more vulnerable to breakdown. The NHS lists mouth ulcers as a recognized symptom of B12 or folate deficiency anaemia, alongside a sore, reddened tongue.
You don’t need to be severely anaemic for this to matter. Even borderline-low levels of these nutrients can contribute to more frequent sores. If your ulcers keep returning without an obvious trigger like injury or stress, a blood test checking iron, B12, and folate is a reasonable step. Vegetarians and vegans, people with heavy menstrual periods, and those with digestive conditions that reduce nutrient absorption are at higher risk for these deficiencies.
Toothpaste Ingredients
A foaming agent called sodium lauryl sulfate (SLS) is found in most commercial toothpastes. It creates the lather you feel while brushing, but research has shown it can strip away the protective layers of oral tissue in some people. A study published in the British Dental Journal found that SLS can cause visible peeling of the mouth’s inner lining, and earlier research demonstrated that SLS-containing toothpaste increases the frequency of recurrent ulcers.
Not everyone is sensitive to SLS, but if you get frequent ulcers with no clear cause, switching to an SLS-free toothpaste for a few months is an easy experiment. Several brands market themselves as SLS-free, and they clean just as effectively.
Hormonal Fluctuations
Some women notice mouth ulcers appearing on a monthly cycle, flaring just before or during menstruation. Shifts in estrogen and progesterone affect blood flow and immune activity in the oral lining, making it more susceptible to breakdown at certain points in the cycle. The Illinois Department of Public Health notes that women prone to mouth sores may develop a pattern of recurrence tied to every menstrual cycle. Pregnancy and menopause can also shift the frequency of ulcers, though some women find their sores actually improve during pregnancy due to sustained high progesterone levels.
Food Sensitivities
Certain foods seem to provoke ulcers in susceptible people, though the mechanism isn’t fully understood. Chocolate, peanuts, eggs, and highly acidic foods like citrus and tomatoes are among the most commonly reported triggers. No studies have directly proven that food allergies cause recurrent ulcers, but the clinical pattern is consistent enough that tracking your diet against flare-ups can be revealing. If you notice sores appearing within a day or two of eating specific foods, an elimination trial, cutting that food out for several weeks, can confirm the connection.
Medications That Trigger Ulcers
Several common drug classes list mouth ulcers as a side effect:
- NSAIDs like ibuprofen, particularly with frequent use
- Beta-blockers prescribed for high blood pressure, angina, or irregular heart rhythms
- Nicorandil, a medication used for angina
- Chemotherapy and radiation therapy, which can cause widespread mouth sores known as mucositis
If your ulcers started or worsened after beginning a new medication, that timing is worth mentioning to your prescriber. In many cases, an alternative drug can be substituted.
Underlying Health Conditions
Recurrent mouth ulcers can be an early or overlooked sign of a systemic disease. The conditions most strongly associated with oral ulcers include:
- Celiac disease, where the immune reaction to gluten damages the gut lining and often produces mouth sores, sometimes as the only visible symptom
- Crohn’s disease, an inflammatory bowel condition that can cause ulceration anywhere in the digestive tract, mouth included
- Behçet’s syndrome, a rare condition involving blood vessel inflammation that typically causes ulcers in the mouth and genitals
- Iron deficiency or other immune conditions, including HIV and other immunodeficiencies that reduce the body’s ability to maintain healthy oral tissue
The underlying biology of common recurrent ulcers also points to the immune system. The damage is primarily driven by a type of white blood cell (T-cells) that attacks the mouth lining, with inflammatory signaling molecules fueling the process. This immune-mediated mechanism explains why conditions that disrupt immune regulation, from autoimmune diseases to simple sleep deprivation, can tip the balance toward more frequent sores.
Genetics and Family History
Recurrent mouth ulcers tend to run in families. If one or both of your parents dealt with frequent canker sores, your risk is significantly higher. The genetic component likely involves inherited variations in immune response, particularly in how aggressively your T-cells react to minor triggers in the oral lining. This also explains why some people get ulcers constantly while others, exposed to the same stress or dietary triggers, never develop them.
When an Ulcer Needs Attention
Most mouth ulcers resolve within one to two weeks. The key threshold to remember is three weeks: any mouth ulcer that hasn’t healed after three weeks warrants a visit to your doctor or dentist. Persistent ulcers can occasionally indicate oral cancer or another condition that requires treatment. Ulcers that are unusually large, painless (paradoxically, cancerous lesions often don’t hurt), or accompanied by unexplained weight loss, fever, or new ulcers appearing in other parts of the body also deserve prompt evaluation.