Most mouth ulcers are canker sores, small shallow wounds on the soft tissue inside your lips, cheeks, or tongue. They’re extremely common, usually harmless, and heal on their own within 10 to 14 days. But they can be surprisingly painful, and understanding what triggered yours can help you avoid the next one.
The Most Common Triggers
Mouth ulcers rarely have a single, clear-cut cause. Instead, several factors tend to overlap. The most frequent trigger is simple physical trauma: biting the inside of your cheek, scraping your gums with a chip, brushing too aggressively, or irritation from braces or a rough tooth edge. Even a minor injury to the lining of your mouth can develop into a full ulcer within a day or two.
Stress is another major driver. College students, for example, are noticeably more prone to outbreaks during exam periods. Hormonal shifts, poor sleep, and illness can have similar effects, likely because they suppress your immune system’s ability to manage inflammation in the mouth’s delicate tissue.
Certain foods make outbreaks worse. Chocolate, peanuts, and eggs are commonly reported culprits, along with acidic fruits like oranges and tomatoes, and spicy foods. These don’t cause ulcers through an allergic reaction. Instead, they seem to irritate the tissue or trigger an inflammatory response in people who are already susceptible.
Your Toothpaste Might Be Making It Worse
A surprisingly common and overlooked factor is sodium lauryl sulfate (SLS), the foaming agent in most toothpastes. Research has shown that SLS strips away a protective layer on the inside of your mouth, making the tissue more vulnerable to damage. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared to a standard one. If you get ulcers frequently, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Nutritional Gaps That Show Up in Your Mouth
Recurring mouth ulcers can be a sign that your body is low on specific nutrients. Deficiencies in vitamin B12, folate (vitamin B9), iron, and zinc are all linked to frequent outbreaks. Your mouth lining replaces itself rapidly, roughly every one to two weeks, so it’s one of the first places to show signs when the raw materials for cell repair run short. If your ulcers keep coming back and you can’t pin them on an obvious trigger like stress or injury, it’s worth having your levels checked with a simple blood test.
What Most Mouth Ulcers Look and Feel Like
About 85% of canker sores are the minor type: small (typically 2 to 3 millimeters across), round or oval, with a white or yellowish center and a red, inflamed border. They hurt, especially when you eat, drink, or talk, but they heal within two weeks without scarring.
Major canker sores are less common, accounting for about 10% of cases. These are larger than a centimeter, deeper, and can take weeks or even months to fully heal. They often leave scars. The remaining 5% are called herpetiform ulcers, which appear as clusters of dozens of tiny sores that can merge together. Despite the name, these are not caused by the herpes virus.
When Ulcers Signal Something Deeper
For most people, mouth ulcers are a standalone nuisance. But when they recur frequently or appear alongside other symptoms, they can point to an underlying condition. Celiac disease, Crohn’s disease, and other inflammatory or autoimmune conditions can all cause recurring mouth ulcers as one of their symptoms.
Behcet’s disease is a less well-known but important example. It causes inflammation in blood vessels throughout the body, and painful mouth sores that look identical to ordinary canker sores are often the first sign. What distinguishes Behcet’s is the pattern of symptoms that follow: genital sores, eye inflammation with redness and blurred vision, skin rashes or tender raised nodules on the lower legs, and joint pain, particularly in the knees. If you’re experiencing mouth ulcers alongside any combination of these symptoms, that’s worth bringing to a doctor’s attention.
Relieving the Pain While It Heals
Most minor ulcers don’t need treatment beyond patience. But when they’re in a spot that makes eating or talking miserable, over-the-counter numbing gels containing benzocaine can take the edge off. Antiseptic mouth rinses with hydrogen peroxide help keep the area clean and may speed healing slightly. Rinsing with warm salt water several times a day is a low-tech option that reduces bacteria around the sore and soothes inflammation.
For severe or frequent outbreaks, prescription options include steroid mouth rinses that reduce inflammation and prescription-strength numbing solutions. A topical treatment called Debacterol chemically cauterizes the ulcer and can shorten healing time to about a week. Avoiding rough or acidic foods during an outbreak also makes a real difference in day-to-day comfort.
How to Tell It’s Not Something Serious
A standard canker sore is flat, painful from the start, and surrounded by a red, inflamed border. It gradually becomes less painful as it heals. Oral cancer, by contrast, typically starts painless. Cancerous lesions often have a small lump or bump underneath that you can feel with your tongue or finger, and they lack the angry red border that canker sores have. A spot that starts small and grows larger, a white patch that turns red, or a lesion that begins bleeding when it previously didn’t are all reasons to get checked.
The key timeline to remember is two weeks. If a mouth ulcer hasn’t healed within that window, or if you use tobacco or drink alcohol regularly and notice any persistent sore, have it evaluated by a dentist or doctor. This is especially true for ulcers that keep recurring in the exact same spot, since benign canker sores tend to appear in different locations each time.