A mouth ulcer is an open sore on the soft tissue inside your mouth, most commonly appearing on the inner cheeks, lips, tongue, or floor of the mouth. Most are small, shallow, and heal on their own within 10 to 14 days. They’re one of the most common oral complaints, and while they can be intensely painful, they’re usually harmless.
Types of Mouth Ulcers
Not all mouth ulcers are the same. The most common variety, called minor aphthous ulcers (better known as canker sores), are small, round or oval sores that resolve in about 10 to 14 days without treatment. These account for the vast majority of cases.
Major aphthous ulcers are deeper, larger, and far more stubborn. They can last up to a month, sometimes leave scarring, and tend to be significantly more painful. A third type, herpetiform ulcers, appear as clusters of tiny sores that can merge into larger irregular ulcers. These clusters make eating and speaking extremely difficult and can persist anywhere from 10 to 100 days. Despite the name, herpetiform ulcers aren’t caused by the herpes virus.
Traumatic ulcers are a separate category entirely. These result from a physical injury: biting your cheek, a sharp edge on a broken tooth, or irritation from braces. They typically heal once the source of trauma is removed.
What Causes Them
For most people, a single mouth ulcer traces back to something straightforward: accidentally biting the inside of your cheek, burning your mouth on hot food, or irritation from dental work. These heal quickly and don’t come back unless the injury repeats.
Recurrent ulcers are more complex. The triggers include local trauma, hormonal changes (many women notice ulcers around their menstrual cycle), stress, and nutritional gaps. Research has specifically linked reduced dietary intake of vitamin B12 and folate to recurrent outbreaks. Iron deficiency is another well-established contributor. The immune system plays a central role: in people prone to canker sores, the body’s own immune cells appear to attack the oral lining, breaking down the surface tissue and creating the characteristic crater-shaped wound.
Stress is a particularly common trigger. A study among dental students found a clear association between high stress levels and recurrent oral ulcers, a pattern most people recognize from their own experience during exams, work deadlines, or periods of poor sleep.
When Ulcers Signal Something Deeper
Occasional mouth ulcers are normal. Frequent or unusually severe ones can be a sign of an underlying condition. Celiac disease, Crohn’s disease, and other inflammatory bowel disorders are all associated with recurrent mouth ulcers because they involve chronic immune dysfunction that affects mucosal tissue throughout the body.
Behçet’s disease is a rarer but important example. Painful mouth sores are typically the first sign of this condition, appearing on the lips, tongue, inner cheeks, roof of the mouth, throat, and tonsils. About 75 percent of people with Behçet’s also develop similar ulcers on the genitals. HIV and other conditions that compromise the immune system can also cause persistent or severe oral ulceration.
How They Heal
A typical minor ulcer follows a predictable pattern. The first two to three days tend to be the worst, with sharp, burning pain that flares when you eat acidic or salty foods. After that peak, the pain gradually decreases. By day seven or eight, most ulcers are noticeably smaller. Full healing takes about 10 to 14 days for minor ulcers, and no scar is left behind.
Major ulcers take a different course. Because they penetrate deeper into the tissue, they can take up to a month to heal and may leave a visible scar. If you’re dealing with a cluster of herpetiform ulcers, the timeline is even less predictable, potentially stretching past three months in severe cases.
Relief and Treatment
Most mouth ulcers don’t need medical treatment, but the pain can be significant enough that you’ll want to manage it. Over-the-counter gels and pastes containing numbing agents like benzocaine can be applied directly to the sore. These provide temporary relief, especially before meals. Products containing hydrogen peroxide work as antiseptic rinses that help keep the area clean.
A simple saltwater rinse is one of the most accessible home remedies, and there’s actual science behind it. Lab research published in PLOS One found that rinsing with saline solution promoted wound-healing cell activity, specifically boosting cell migration and increasing production of collagen and other proteins involved in tissue repair. The practical recipe: dissolve about one teaspoon of salt in a cup of warm water and swish gently. It won’t numb the pain, but it supports the healing environment inside your mouth.
For severe or persistent ulcers, a dentist or doctor may prescribe a topical steroid gel to reduce inflammation and speed healing.
Reducing Recurrence
If you get mouth ulcers regularly, a few practical changes can lower your frequency. Avoiding foods that irritate your mouth is a good starting point: citrus fruits, tomatoes, spicy dishes, and rough-textured foods like chips and crusty bread are common culprits.
Switching to an SLS-free toothpaste is often recommended. Sodium lauryl sulfate is the foaming agent in most standard toothpastes, and some studies have reported up to a 70% reduction in ulcers after switching to SLS-free formulas. The evidence is mixed, though. A well-designed double-blind trial of 90 people with recurrent ulcers found that SLS-free toothpaste didn’t reduce the number or frequency of outbreaks. It did, however, result in participants reporting less pain and slightly faster healing. That alone may be worth the switch if you’re dealing with frequent sores.
Addressing nutritional gaps matters too. If your ulcers keep coming back, it’s worth checking whether you’re getting enough B12, folate, and iron through your diet, or asking for a blood test to rule out deficiency. Managing stress, getting adequate sleep, and protecting your mouth from trauma (using orthodontic wax on braces, for example) all help reduce the likelihood of recurrence.
When a Mouth Ulcer Needs Attention
The key warning sign is an ulcer that doesn’t heal. A standard canker sore should be well on its way to healing within two weeks. An ulcer that persists beyond three weeks, keeps growing, or feels unusually hard warrants a professional evaluation. Mouth cancer can present as a painless ulcer that simply won’t resolve, which is easy to dismiss precisely because it doesn’t hurt.
Other reasons to seek evaluation include ulcers that are unusually large, keep recurring in rapid succession, come with a high fever, or make it too painful to drink fluids. Ulcers appearing alongside sores on the genitals or persistent eye inflammation could point to Behçet’s disease or another systemic condition that requires diagnosis.