Canker sores form when your immune system overreacts and attacks the thin tissue lining the inside of your mouth. The exact reason this happens varies from person to person, but the triggers fall into a handful of well-understood categories: physical injury, certain foods, stress, nutritional gaps, genetics, and sometimes an underlying health condition. Somewhere between 5% and 66% of people get them, depending on the population studied, making them one of the most common oral complaints worldwide.
Your Immune System Is the Core Problem
Canker sores aren’t infections. They’re the result of your body’s own immune cells turning against the soft tissue inside your mouth. In people who get recurrent canker sores, blood tests show higher levels of inflammatory signaling molecules and lower levels of the anti-inflammatory ones that normally keep the immune response in check. One study found that a specific type of immune cell responsible for preventing the body from attacking its own tissue was present at roughly half the normal level in people with active sores.
In practical terms, this means your immune system loses its ability to ignore harmless bacteria already living in your mouth, or it begins reacting to your own tissue as though it were a threat. The result is a localized inflammatory attack that breaks down the surface layer, creating that painful, shallow ulcer you recognize as a canker sore.
Common Triggers That Start a Sore
Physical Injury
Biting the inside of your cheek, scraping your gums with a chip, or irritating your mouth with orthodontic hardware are some of the most frequent triggers. Rough-textured foods like nuts, seeds, pretzels, and crusty bread can create tiny abrasions that give the inflammatory process a place to start. Even aggressive tooth brushing can do it.
Acidic and Irritating Foods
Citrus fruits, tomatoes, strawberries, and tomato-based sauces are classic offenders because their acidity irritates the delicate lining of the mouth. Coffee, carbonated drinks, and spicy foods (hot peppers, curry, salsa) can also trigger or worsen sores. Some people find that dairy proteins provoke inflammation as well, though this varies widely.
Stress
Studies comparing people with recurrent canker sores to those without them consistently find significantly higher psychological stress levels in the canker sore group. In one study, 17 out of a group of patients reported a direct relationship between stressful life events and the severity of their outbreaks. Stress appears to act as a trigger or amplifier rather than a standalone cause, likely by suppressing the immune regulation that keeps inflammation in check. To make things worse, 16 of those same patients said the sores themselves increased their stress, creating a feedback loop.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), a foaming agent in most commercial toothpastes, is a well-documented irritant. A clinical study found four times fewer oral tissue lesions after switching to an SLS-free toothpaste compared with one containing SLS. If you get frequent sores, switching toothpaste is one of the simplest things to try first.
Nutritional Deficiencies That Increase Risk
Low levels of certain nutrients are consistently linked to recurrent canker sores. In a study of 273 people with recurring outbreaks, about 20% were deficient in iron, nearly 5% were low in vitamin B12, and about 3% were deficient in folic acid. Roughly 21% had anemia. These nutrients all play roles in maintaining healthy mucous membranes and regulating immune function, so when levels drop, the mouth’s lining becomes more vulnerable.
Vitamin B12 deficiency deserves special attention. Case reports describe patients whose recurrent sores resolved after B12 supplementation, and researchers have recommended screening for B12 deficiency in anyone with a chronic pattern of canker sores. If your sores keep coming back and you eat a limited diet, or you follow a vegan or vegetarian diet (which can be low in B12 and iron), a blood test checking these levels is a reasonable step.
Genetics Play a Real Role
If your parents get canker sores, you’re more likely to get them too. Research has found a high frequency of family history among people with recurrent sores, and the connection appears to be rooted in specific genes related to immune function. Certain variations in the human leukocyte antigen (HLA) system, which helps your immune system distinguish your own cells from foreign invaders, are associated with either greater susceptibility or greater resistance to canker sores. Two specific genetic markers have been linked to resistance, meaning people who carry them are less likely to develop sores. This helps explain why some people can eat all the citrus they want and never get a sore, while others seem to get one from minor irritation.
When Canker Sores Signal Something Bigger
Most canker sores are a nuisance, not a sign of serious illness. But frequent or severe outbreaks can occasionally point to an underlying condition. Crohn’s disease, which causes inflammation throughout the digestive tract, lists mouth sores as a recognized symptom because the disease can affect tissue from the mouth all the way to the intestines. Celiac disease, an autoimmune reaction to gluten, is another condition where recurrent oral ulcers show up as a secondary symptom. Immune-suppressing conditions and certain medications can also increase canker sore frequency.
If your sores are unusually large, take longer than two weeks to heal, come in frequent waves, or are accompanied by other symptoms like digestive problems, fatigue, or unexplained weight loss, those patterns are worth investigating with a healthcare provider.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different. Canker sores appear inside the mouth, on the inner cheeks, lips, tongue, or soft palate. They look like single round sores, white or yellow with a red border. They are not contagious and are not caused by a virus.
Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. They’re clusters of small, fluid-filled blisters caused by the herpes simplex virus (usually HSV-1), and they are contagious. If your sore is inside your mouth and isn’t a blister, it’s almost certainly a canker sore.
How Long They Last
Most canker sores are the minor type, which heal on their own within 10 to 14 days without scarring. Major canker sores, which are larger and deeper, can take up to 6 weeks to heal and sometimes leave scars. There’s also a less common variety called herpetiform ulcers (despite the name, unrelated to herpes), which appear as clusters of tiny sores that can merge together.
Pain typically peaks in the first few days and gradually decreases as the sore heals. Over-the-counter topical gels or rinses containing numbing agents can help manage discomfort during that initial stretch. Avoiding acidic, spicy, and crunchy foods while a sore is active helps prevent further irritation.
Reducing How Often You Get Them
Because canker sores have multiple overlapping triggers, managing them usually means addressing several fronts at once:
- Switch to SLS-free toothpaste. This single change reduces sore frequency for many people.
- Track your food triggers. Keep a simple log of what you ate in the 24 hours before a sore appears. Patterns with citrus, tomatoes, or specific snacks often become clear within a few months.
- Check for nutritional gaps. If sores are frequent, ask about testing your iron, B12, and folate levels. Correcting a deficiency can dramatically reduce outbreaks.
- Manage stress proactively. Since stress is both a trigger and a consequence of canker sores, anything that lowers your baseline stress level (sleep, exercise, workload changes) can break the cycle.
- Protect your mouth from injury. Use orthodontic wax on braces, chew carefully with sharp-edged foods, and brush with a soft-bristled toothbrush.