Mouth ulcers are small, painful sores that form on the soft tissue inside your mouth, most commonly on the inner cheeks, lips, tongue, or gums. They have a wide range of triggers, from something as simple as biting your cheek to underlying health conditions that need medical attention. Most are minor and heal on their own within a few weeks, but understanding what’s behind them can help you avoid repeat outbreaks.
Physical Injury to the Mouth Lining
The most straightforward cause is mechanical damage. Accidentally biting your cheek or tongue, eating hard or crunchy food, brushing too aggressively, or catching the inside of your mouth on a sharp tooth edge can all break the delicate tissue and create an ulcer. Dental braces, retainers, and ill-fitting dentures are especially common culprits because they create repeated friction against the same spot day after day.
Burns from hot food or drinks can also trigger ulcers. These injuries typically heal within one to two weeks as long as the source of irritation is removed. If a sharp tooth or dental appliance keeps rubbing the same area, the ulcer may persist or keep coming back in the same location.
Stress and Immune Function
Stress is one of the most well-documented triggers for recurrent mouth ulcers. When you’re under sustained emotional or mental pressure, your body releases stress hormones that suppress immune function. This makes the lining of your mouth more vulnerable to breakdown and slower to repair itself.
Stress also contributes indirectly. People under stress are more likely to clench their jaw, grind their teeth, or bite the inside of their cheeks, all of which damage oral tissue. Stress can also reduce saliva production, leaving the mouth drier and more prone to irritation. If you notice ulcers tend to appear during high-pressure periods at work or school, the connection is likely real.
Foods That Irritate the Mouth
Certain foods are known to provoke ulcers in people who are susceptible. The main offenders include:
- Acidic fruits: Pineapples, oranges, lemons, limes, and strawberries. The acid stresses mouth tissue and can aggravate the gums.
- Nuts: Walnuts, peanuts, cashews, and almonds contain an amino acid that contributes to canker sores. Salted varieties are worse because the sodium dries out the mouth and inflames the lining.
- Chocolate: Contains a compound called theobromine that some people’s mouths are particularly sensitive to, producing a reaction similar to an allergy.
- Coffee and alcohol: Both are highly acidic and can erode the protective layers of the oral lining over time.
Not everyone reacts to these foods. If you get frequent ulcers, keeping a food diary for a few weeks can help you identify your personal triggers.
Toothpaste Ingredients
A foaming agent called sodium lauryl sulfate (SLS), found in most mainstream toothpastes, can strip away the delicate outer layers of the mouth’s lining. Research published in the British Dental Journal found that SLS acts as a direct irritant to oral tissue, and a separate study showed that people who switched to SLS-free toothpaste experienced four times fewer soft tissue lesions after just 30 minutes of use compared to those using a standard formula. If you’re prone to recurrent ulcers, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Medication Side Effects
Several common medications can cause mouth sores. Pain relievers like aspirin and naproxen sometimes produce chemical burns on the oral lining, appearing as white or yellow lesions with tissue sloughing. Blood pressure medications, including certain ACE inhibitors, can trigger swelling or irritation inside the mouth. Some anti-seizure medications carry a risk of more serious oral reactions. If mouth ulcers started shortly after beginning a new medication, that timing is worth mentioning to your prescriber.
Underlying Health Conditions
Occasional mouth ulcers are almost always harmless. But ulcers that keep returning, are unusually large, or refuse to heal on their own can signal a deeper issue. Conditions linked to chronic or severe mouth ulcers include Behçet’s syndrome (an inflammatory disorder that also causes genital ulcers and eye problems), Crohn’s disease and other gastrointestinal conditions, celiac disease, and immune deficiencies including HIV/AIDS. In these cases, the ulcers look similar to ordinary canker sores but tend not to resolve on their own with age the way typical canker sores do.
Canker Sores vs. Cold Sores
These two conditions are often confused, but they have different causes and appear in different places. Canker sores form inside the mouth, on the inner cheeks, lips, or tongue, and look like a single round white or yellow sore with a red border. They are not contagious and are not caused by a virus.
Cold sores (fever blisters) appear on the outside of the mouth, usually around the lips. They start as clusters of small fluid-filled blisters and are caused by the herpes simplex virus. If your sores are inside your mouth and are not blistered, they are almost certainly canker sores, not cold sores.
Types and Healing Times
Not all canker sores are the same. Minor canker sores, which account for the vast majority of cases, are smaller than a centimeter across (roughly the size of a pea or smaller). These heal within a few weeks without scarring. Major canker sores are larger than one centimeter and can take months to heal, sometimes leaving scars. A third type, called herpetiform ulcers, appears as clusters of tiny pinpoint sores. Despite the name, these are not related to herpes. They typically heal within about two weeks without scarring.
If a mouth ulcer hasn’t improved after three weeks, is unusually painful, keeps coming back in the same spot, or is accompanied by fever, that pattern warrants a closer look from a dentist or doctor to rule out something beyond the common triggers listed above.