A mouth ulcer is a small, open sore that forms on the soft tissue inside your mouth, such as the inner cheeks, gums, tongue, or the floor of your mouth. Most mouth ulcers are round or oval with a white or yellow center and a red border. They’re usually harmless and heal on their own within 10 to 14 days, but they can make eating, drinking, and talking surprisingly painful while they last.
Mouth Ulcers vs. Cold Sores
People often confuse mouth ulcers with cold sores, but they’re different in almost every way. Mouth ulcers (also called canker sores or aphthous ulcers) form inside the mouth. Cold sores, or fever blisters, appear outside the mouth, typically around the border of the lips. Cold sores look like clusters of small fluid-filled blisters, while mouth ulcers are single round sores with that distinctive white or yellow center.
The biggest practical difference: cold sores are caused by herpes simplex virus (usually type 1) and are very contagious. Mouth ulcers have no known single cause and are not contagious at all. You can’t pass them to someone by sharing a drink or kissing.
What Causes Them
There’s no single trigger for mouth ulcers, and for many people the cause is never pinpointed. That said, several well-established factors make them more likely:
- Minor mouth injuries. Biting your cheek, brushing too hard, dental work, or sharp-edged food like chips can break the delicate lining of your mouth and start an ulcer.
- Nutritional gaps. Deficiencies in vitamin B12, folate, iron, and zinc are linked to recurrent ulcers. Low B12 or folate can cause a sore, red tongue along with mouth ulcers as part of a broader pattern of deficiency.
- Stress and fatigue. Emotional or physical stress is one of the most commonly reported triggers, though the exact mechanism isn’t fully understood.
- Hormonal changes. Some women notice ulcers flaring at certain points in their menstrual cycle.
- Certain foods. Acidic or spicy foods, particularly citrus fruits, tomatoes, and chocolate, seem to provoke ulcers in some people.
What’s Happening Under the Surface
Although the exact biology is still being worked out, the immune system plays a central role. In people prone to mouth ulcers, certain white blood cells (T cells) appear to attack the mouth’s own lining. These immune cells release inflammatory signals that break down the thin tissue inside the mouth, creating the open sore you see and feel. People with active ulcers have higher levels of several inflammatory markers in their blood compared to people without them.
One theory is that the immune system mistakes proteins in the mouth’s lining for bacterial proteins it wants to destroy. This cross-reactivity between normal tissue and bacteria may explain why some people’s bodies essentially turn on their own mouth tissue. There also appears to be a genetic component. If your parents dealt with frequent mouth ulcers, your odds of getting them are higher.
How Long They Take to Heal
Most mouth ulcers fall into the “minor” category, measuring less than about a centimeter across. These heal on their own in roughly one to two weeks without leaving a scar. They’re painful for the first few days and then gradually become less bothersome as the tissue repairs itself.
Major ulcers are larger, deeper, and far less common. They can take weeks or even months to fully heal and sometimes leave scarring. A third type, called herpetiform ulcers (despite having nothing to do with the herpes virus), appear as clusters of many tiny sores that can merge together. These also tend to heal within a couple of weeks.
If any mouth ulcer hasn’t healed after three weeks, that’s worth a visit to your doctor or dentist. Persistent sores that don’t resolve are one of the things screened for during oral cancer checks.
Easing the Pain at Home
You can’t force a mouth ulcer to heal faster, but you can make it hurt less and avoid irritating it further. A simple saltwater rinse is one of the most effective home remedies. Mix one teaspoon of salt into eight ounces (about one cup) of warm water and swish it gently around your mouth. If that stings too much, cut the salt to half a teaspoon for the first day or two.
Over-the-counter products come in pastes, gels, and liquids that coat the sore or numb it. Numbing agents containing lidocaine or benzocaine can take the edge off pain before meals. Protective pastes create a barrier over the ulcer so food and drink don’t make direct contact. These work best when applied early, as soon as you notice the ulcer forming.
While the ulcer is healing, avoid crunchy, sharp, spicy, or highly acidic foods. Soft, cool foods are generally the most comfortable. Drinking through a straw can help keep liquids away from the sore.
When Ulcers Keep Coming Back
Some people get mouth ulcers once or twice a year and barely think about them. Others deal with near-constant outbreaks, a pattern called recurrent aphthous stomatitis. If that sounds familiar, it’s worth looking at a few angles.
First, consider your diet. Research has found a meaningful link between antioxidant-rich diets and lower rates of recurrent ulcers. Zinc intake, in particular, showed a significant protective association in one study, with higher dietary zinc reducing the odds of recurrence by roughly 36%. Foods rich in zinc include meat, shellfish, legumes, seeds, and nuts. More broadly, eating a diet high in fruits, vegetables, and whole grains provides the range of antioxidants that may help keep your mouth’s lining healthier.
You may have heard that switching to a toothpaste free of sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, can help. The evidence on this is mixed. A 2019 review found there wasn’t enough data to confirm that SLS-free toothpastes reduced ulcer frequency, duration, or pain. That said, some individuals do seem to notice improvement, so it’s a low-risk experiment if nothing else has helped.
If you’re getting frequent ulcers, it’s also worth asking your doctor to check your B12, folate, and iron levels. Correcting a deficiency, if one exists, can sometimes resolve the problem entirely.
Mouth Ulcers as a Sign of Something Else
In most cases, mouth ulcers are a standalone nuisance. But recurrent or unusually severe ulcers can occasionally signal an underlying condition. Behcet’s disease, a rare inflammatory disorder, causes painful mouth sores that look identical to ordinary canker sores as its most common symptom. These tend to heal in one to three weeks but keep returning. Behcet’s also causes sores in other areas of the body, eye inflammation, and joint pain.
Inflammatory bowel diseases like Crohn’s disease can also produce mouth ulcers, sometimes even before gut symptoms appear. Celiac disease is another condition where mouth ulcers are a recognized symptom, triggered by the body’s reaction to gluten.
Oral cancer can appear as a sore or ulcer in the mouth that simply won’t heal. The key difference is persistence: cancerous lesions don’t resolve on their own. They may appear as red or white patches and are often painless in the early stages, which is the opposite of a typical canker sore. During an oral cancer screening, your dentist checks for exactly these kinds of non-healing sores. If something looks suspicious, the only way to rule out cancer is a biopsy, where a small sample of tissue is removed and tested.
A single mouth ulcer that heals within two weeks is almost certainly nothing to worry about. But ulcers that are unusually large, that come in frequent waves, that spread with other symptoms like fatigue or joint pain, or that stick around past three weeks all warrant a closer look.