Mouth ulcers most commonly result from physical injury, like accidentally biting the inside of your cheek. But they can also be triggered by stress, nutritional gaps, certain foods, and underlying health conditions. Most are harmless and heal within two weeks, though recurring or persistent ulcers sometimes point to something worth investigating.
Physical Injury Is the Most Common Cause
The single most frequent trigger is mechanical trauma to the soft tissue inside your mouth. This includes biting the inside of your cheek while eating or talking, slipping with your toothbrush while brushing, or burning your mouth on hot food or drink. These injuries break the delicate lining of your mouth, and an ulcer forms at the site as the tissue tries to repair itself.
Ongoing friction is another major source. Sharp or broken teeth that constantly rub against your cheek or tongue can create ulcers that keep coming back in the same spot. The same applies to braces, dentures, orthodontic aligners, and mouth splints. If you notice ulcers repeatedly forming where a dental appliance contacts your mouth, the fit likely needs adjusting.
Stress, Sleep, and Hormones
Stress is one of the most well-documented non-physical triggers. When you’re under pressure, your body releases cortisol, which stimulates immune activity at inflammatory sites in ways that can promote ulcer formation. Paradoxically, the opposite is also true: when cortisol and related hormones are disrupted by poor sleep or chronic exhaustion, the resulting imbalance can increase inflammation and make ulcers more likely. This is why many people notice outbreaks during exam periods, work deadlines, or stretches of insomnia.
Hormonal shifts during the menstrual cycle are another commonly reported trigger, with some women experiencing ulcers at predictable points in their cycle, though the exact mechanism isn’t fully understood.
Nutritional Deficiencies
If you get mouth ulcers repeatedly, a nutrient gap could be involved. A study comparing people with recurrent ulcers to healthy controls found that about 50% of those with recurring ulcers were deficient in vitamin B12, compared to none in the control group. Low folate levels were also common, appearing in roughly 46% of ulcer patients.
Iron deficiency plays a role too, though less dramatically. The nutrients your body needs to maintain healthy mucosal tissue (the moist lining inside your mouth) include B12, folate, iron, and zinc. If your diet is low in leafy greens, legumes, eggs, meat, or fortified cereals, you may not be getting enough of these. People who follow restrictive diets or have absorption issues are at higher risk.
Food Triggers
Despite what many people assume, there’s no strong evidence that spicy or acidic foods directly cause mouth ulcers. However, they can significantly worsen existing ones. Citrus fruits, tomatoes, vinegar-based foods, and spicy dishes irritate the open tissue of an ulcer and make it more painful. Some people also report that chocolate, coffee, cheese, nuts, and wheat seem to precede outbreaks, though individual sensitivity varies widely. If you suspect a food trigger, keeping a brief log of what you ate before each outbreak can help you spot patterns.
Underlying Health Conditions
Recurring mouth ulcers sometimes signal a deeper health issue. Celiac disease, an autoimmune reaction to gluten, frequently causes oral ulcers, and for some people these are the earliest or most noticeable symptom. Crohn’s disease, a type of inflammatory bowel disease that can affect any part of the digestive tract from mouth to anus, also lists mouth sores as a recognized symptom. Behçet’s disease, a rarer condition involving blood vessel inflammation, is strongly associated with recurrent oral and genital ulcers.
Immune system suppression from conditions like HIV or from medications such as chemotherapy drugs can also lead to frequent or severe ulcers. If your ulcers are large, unusually painful, or keep returning in clusters, it’s worth mentioning to your doctor so these possibilities can be ruled out.
Types of Mouth Ulcers and Healing Times
Not all mouth ulcers are the same. The most common type, minor aphthous ulcers, are small (usually under 10mm), appear on the inner cheeks, lips, or tongue, and heal within 10 to 14 days without scarring. These account for the vast majority of cases.
Major aphthous ulcers are larger and deeper. They can take up to six weeks to heal and often leave a scar. They’re less common but considerably more painful and disruptive.
Herpetiform ulcers (despite the name, unrelated to herpes) appear as clusters of tiny sores that can merge into larger irregular shapes. They’re the least common type and tend to recur frequently.
Mouth Ulcers vs. Cold Sores
People often confuse these two, but they’re fundamentally different. Mouth ulcers (canker sores) appear inside the mouth, on the cheeks, lips, tongue, or gums. They’re typically single, round, white or yellow sores with a red border. They are not caused by a virus and are not contagious.
Cold sores (fever blisters) appear outside the mouth, usually around the border of the lips. They look like clusters of small fluid-filled blisters and are caused by the herpes simplex virus, most often HSV-1. They are highly contagious, especially when the blisters are open. If your sore is inside your mouth and isn’t a blister, it’s almost certainly a canker sore, not a cold sore.
When an Ulcer Needs Attention
Most mouth ulcers are a nuisance, not a danger. But some warrant a closer look. Oral cancer can initially look like a mouth sore that simply won’t heal. The Mayo Clinic advises seeing a doctor if any mouth sore persists for more than two weeks, especially if it’s painless (most benign ulcers hurt), grows rather than shrinks, or is accompanied by a lump, unexplained loose teeth, ear pain, or difficulty swallowing. A white or reddish patch that doesn’t go away also deserves evaluation.
Recurrent aphthous ulcers affect anywhere from 5% to 66% of the population depending on the group studied, with higher rates among younger people and those in higher socioeconomic groups. If you get ulcers frequently but they follow the typical pattern of appearing, hurting for a week or so, and resolving on their own, they’re almost certainly benign. If the pattern changes, that’s the signal to get checked.