Canker sores form when your immune system attacks the thin lining inside your mouth, creating small, painful ulcers on the soft tissue of your cheeks, lips, tongue, or gums. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The frustrating reality is that there’s rarely one single cause. Instead, canker sores typically result from a combination of triggers acting on a person whose immune system is already prone to overreacting in the mouth.
What Actually Happens Inside Your Mouth
A canker sore isn’t just surface damage. It’s an immune-mediated attack on your own tissue. Certain immune cells, particularly a type called T cells, begin destroying the cells that make up your oral lining. These T cells release inflammatory signals that recruit even more immune activity to the area, creating a self-sustaining cycle of tissue destruction. The result is a shallow crater, usually white or yellowish with a red border, that exposes the nerve-rich tissue underneath. That’s why they hurt so much, especially when food, drinks, or your tongue touch them.
People who get frequent canker sores show higher levels of inflammatory markers in their blood and an overactive immune signaling pathway that drives an exaggerated response to minor irritation. In other words, something that wouldn’t bother most people’s mouths, like a small scratch from a chip, can set off a full immune response in someone prone to canker sores.
Physical Trauma Is the Most Common Trigger
The single most frequent trigger is minor injury to the inside of your mouth. This includes biting your cheek or lip, poking yourself with a fork, brushing too aggressively, or getting scratched by a sharp piece of food like a tortilla chip or crusty bread. Dental work is another classic trigger: cleanings, fillings, or any procedure that involves instruments pressing against your cheeks or gums can set one off within a day or two.
Braces and other orthodontic appliances are a major culprit, especially in the first few weeks after adjustment, when brackets and wires rub against unfamiliar spots. If you wear braces, orthodontic wax applied over sharp edges can reduce this kind of irritation significantly.
Foods That Trigger Outbreaks
Certain foods are well-known triggers, and they work through different mechanisms. Acidic foods like citrus fruits, tomatoes, and pineapple can irritate the oral lining directly. Coffee, alcohol, and carbonated drinks are also acidic enough to cause problems, and they can dry out your mouth on top of that, removing the protective layer of saliva.
Nuts can act as both a mechanical irritant (their rough texture scratches the tissue) and an allergenic trigger in some people. Chocolate is another common offender due to its acidity and potential to provoke a mild immune reaction. Spicy foods don’t necessarily cause canker sores, but the active compound in hot peppers triggers a pain response that can worsen existing ulcers or irritate tissue that’s already vulnerable.
For some people, the connection is actually an immune reaction to specific foods. Gluten sensitivity and other food allergies can provoke inflammation in the oral lining that leads to ulceration. If you notice a pattern between certain foods and outbreaks, that’s worth paying attention to, because eliminating the trigger food can sometimes stop the cycle entirely.
Stress and Canker Sores
Stress is one of the most commonly reported triggers, and there’s a physiological basis for the connection. Research comparing people with recurrent canker sores to those without found that the canker sore group had higher levels of cortisol (the body’s primary stress hormone) in their saliva, along with higher rates of anxiety and depression. Stress suppresses parts of the immune system while activating others, and that imbalance may tip the scales toward the kind of overreactive immune response that produces ulcers.
There’s also a simpler explanation that likely works alongside the hormonal one: stressed people tend to clench their jaws and bite their cheeks or lips more often, creating the physical trauma that starts the process.
Nutritional Deficiencies
Low levels of iron, vitamin B12, and folate are all linked to recurrent canker sores. These nutrients play essential roles in maintaining healthy tissue and supporting normal immune function. When you’re deficient, the lining of your mouth becomes thinner and more fragile, and your immune system may behave abnormally. Mouth ulcers are actually a recognized symptom of B12 and folate deficiency anemia.
If you get canker sores frequently and can’t identify an obvious trigger, it’s worth having your levels checked with a simple blood test. Correcting a deficiency sometimes resolves the problem.
Your Toothpaste Might Be Part of the Problem
Many popular toothpastes contain a foaming agent called sodium lauryl sulfate (SLS). Research has shown that SLS toothpaste can increase the frequency of recurrent mouth ulcers. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared to one containing SLS. The detergent strips away the protective mucus layer inside your mouth, leaving the tissue more vulnerable to irritation and immune attack. Switching to an SLS-free toothpaste is one of the easiest changes you can make if you’re dealing with frequent outbreaks.
Genetics Play a Significant Role
About 40% of people with recurrent canker sores have a family history of them, and those with a genetic predisposition tend to get them earlier in life and more severely. Researchers have identified specific genetic markers that are more common in people who get canker sores. One study found that a marker called HLA-B51 appeared in 23% of people with recurrent canker sores compared to just 9% of the general population, while another marker, HLA-Cw7, showed up in 23% of affected individuals versus only 5% of controls.
This genetic component explains why some people seem to get canker sores from the slightest provocation while others never get them at all, even with the same diet, stress levels, and oral hygiene habits.
When Canker Sores Signal Something Else
Occasional canker sores are extremely common and almost always harmless. But frequent or unusually severe outbreaks can sometimes point to an underlying condition. Celiac disease, inflammatory bowel disease, and Behçet’s disease all feature recurrent mouth ulcers as a primary symptom. In Behçet’s disease, painful mouth sores that look identical to canker sores are the most common sign, often recurring every few weeks.
Canker sores that are unusually large, heal very slowly (taking more than three weeks), or come with other symptoms like joint pain, eye inflammation, fever, or swollen lymph nodes warrant further investigation. Large, slow-healing ulcers can also be associated with HIV infection or, rarely, oral cancer. A sore that simply won’t heal should always be evaluated to rule out a more serious cause.
How Long They Last
Most canker sores are the minor type: small, shallow, and round. These typically heal on their own within one to two weeks without scarring. Major canker sores are larger, deeper, and more painful. They can take several weeks to heal and may leave a scar. A less common third type, called herpetiform ulcers, appears as clusters of tiny sores that can merge together, though these also tend to heal within a couple of weeks.
During healing, avoiding acidic, spicy, and salty foods reduces pain. Over-the-counter topical gels that coat the sore can provide temporary relief by creating a barrier between the ulcer and whatever touches it. Rinsing with warm salt water or a baking soda solution (one teaspoon per half cup of water) can help keep the area clean and reduce inflammation.