Canker sores form when your immune system mistakenly attacks the cells lining the inside of your mouth. About 20% of people get them repeatedly, and while the sores themselves are harmless, they can be surprisingly painful. The triggers range from biting your cheek to running low on certain vitamins, but the underlying process is the same: an overactive immune response destroys a small patch of tissue, leaving behind a shallow, open ulcer.
How Your Immune System Creates the Sore
In people prone to canker sores, something triggers the body to produce excess TNF-alpha, a signaling protein that kicks off inflammation. This protein sets off a chain reaction: it activates surface markers on the cells lining your mouth, essentially flagging those cells as targets. A specific type of immune cell, called a CD8+ T-cell, then attacks and destroys the flagged tissue. The result is a round or oval ulcer with a white or yellowish center and a red border.
This is why canker sores aren’t contagious. They aren’t caused by a virus or bacteria. They’re the product of your own immune system misfiring against healthy tissue. The process is similar to what happens in other autoimmune-like conditions, just localized to a tiny area of your mouth.
Common Triggers
Physical Trauma
The most straightforward cause is mechanical injury. Biting the inside of your cheek, poking your gum with a chip, or getting scratched by a toothbrush bristle can all set off the immune cascade in susceptible people. Orthodontic braces are a well-documented trigger, particularly during the first weeks of treatment when brackets and wires rub against soft tissue that hasn’t toughened up yet. Dental work, ill-fitting dentures, and even aggressive brushing can do the same thing.
Stress and Cortisol
Psychological stress is one of the most consistent triggers researchers have identified. When you’re stressed, your body produces more cortisol through a hormonal chain that starts in the brain and ends at the adrenal glands. Elevated cortisol suppresses and disrupts normal immune function, altering the balance of T-cells, B-cells, and natural killer cells. In people prone to canker sores, this disruption appears to be enough to trigger an outbreak. Studies measuring cortisol in saliva have found that levels rise during active ulcer episodes, and that stress and anxiety consistently accelerate recurrence.
This helps explain why canker sores tend to cluster around exam periods, work deadlines, illness, and sleep deprivation. It’s not just “being stressed” in a vague sense. The hormonal shift physically changes how your immune system behaves.
Nutritional Deficiencies
Running low on certain nutrients makes your mouth lining more vulnerable. The most well-established deficiencies linked to recurrent canker sores are vitamin B12, folate (vitamin B9), and iron. The NHS lists mouth ulcers as a recognized symptom of B12 and folate deficiency anemia. Zinc deficiency has also been associated with more frequent outbreaks. If you’re getting canker sores often and can’t identify an obvious trigger, a blood test checking these levels is a reasonable step.
Toothpaste Ingredients
Sodium lauryl sulfate, or SLS, is a foaming agent found in most major toothpaste brands. It’s the same detergent used in shampoos and household cleaners. SLS irritates the soft tissue inside your mouth, and for some people this irritation is enough to provoke ulcer formation. Switching to an SLS-free toothpaste is one of the simplest changes you can make if you get frequent sores. Several brands market themselves specifically as SLS-free, and they clean your teeth just as effectively.
Food Sensitivities
Certain foods are commonly reported as triggers, particularly citrus fruits, tomatoes, chocolate, coffee, and nuts. There’s no strong evidence that spicy food directly causes canker sores, but it can significantly worsen the pain of an existing one. Highly acidic foods like oranges, lemons, and pineapple are more likely to irritate the mucous membrane enough to provoke a new sore in someone who’s already prone to them. Keeping a food diary for a few weeks can help you identify your personal triggers.
Hormonal Changes
Some people notice canker sores appearing at predictable points in their menstrual cycle, typically in the days just before a period. Hormonal fluctuations affect immune regulation in the same general way that stress does, shifting the balance of inflammatory signaling. This pattern is common enough that researchers consider hormonal changes a recognized trigger, though it affects some people far more than others.
When Canker Sores Signal Something Else
Occasional canker sores are almost always harmless. But frequent or severe outbreaks can sometimes point to an underlying condition. Celiac disease, an autoimmune reaction to gluten, commonly causes recurrent mouth ulcers. In some people, mouth sores are the first or most noticeable symptom of celiac disease before any digestive issues appear.
Crohn’s disease, an inflammatory bowel condition, also causes mouth ulcers in a significant number of patients. Behçet’s syndrome, a rarer inflammatory disorder, lists mouth ulcers as its most universal symptom. Nearly 100% of people with Behçet’s experience them, and a diagnosis requires at least three episodes of oral ulceration within a 12-month period alongside other symptoms.
If you’re getting canker sores frequently (more than a few times a year), if they’re unusually large, or if they come with other symptoms like joint pain, digestive problems, skin lesions, or fatigue, these patterns are worth bringing up with a doctor.
Three Types and How They Differ
Not all canker sores are the same size or severity. Minor canker sores are by far the most common. They’re small (under a centimeter), oval-shaped, and heal within about two weeks without leaving a scar. Pain typically improves within a few days, even though the sore itself is still visible.
Major canker sores are larger, deeper, and can take six weeks or more to heal. They sometimes leave scars. Herpetiform canker sores (despite the name, they have nothing to do with herpes) appear as clusters of tiny pinpoint ulcers that can merge into larger irregular shapes. Both major and herpetiform varieties are much less common than minor sores.
What You Can Do About Triggers
Because canker sores involve an immune response you can’t fully control, prevention is really about reducing the triggers that set that response off. The most practical steps are switching to SLS-free toothpaste, managing stress and sleep, eating a balanced diet that covers your B12, folate, iron, and zinc needs, and being mindful of foods that seem to precede your outbreaks. If you have braces or a dental appliance that’s rubbing, orthodontic wax over the offending bracket can prevent the repeated trauma that leads to sores.
For sores that have already formed, they generally heal on their own within two weeks. Over-the-counter numbing gels or antiseptic rinses can reduce pain during that window. Avoiding acidic and spicy foods while the sore is open helps keep irritation down. Drinking cool liquids through a straw can also bypass the sore entirely.