Canker sores are caused by your immune system attacking the soft tissue lining of your mouth. Unlike cold sores, which are viral, canker sores aren’t infectious. They result from an overactive inflammatory response that breaks down the oral mucosa, leaving a shallow, painful ulcer. Somewhere between 5% and 66% of people get them, depending on the population studied, and the wide range reflects how strongly genetics, nutrition, and lifestyle factors influence who’s affected.
Your Immune System Turns on Your Own Tissue
The core mechanism behind a canker sore is immune-mediated destruction. Specific white blood cells, particularly a type of T cell, become activated against the thin tissue inside your cheeks, lips, or under your tongue. These immune cells release inflammatory signals that damage and erode the surface layer, creating the characteristic round or oval ulcer with a white or yellowish center and red border.
The fact that canker sores respond well to anti-inflammatory medications confirms that immune activation drives the process. But what triggers that immune response varies widely from person to person, which is why canker sores have so many apparent “causes.” The triggers below are really triggers of the underlying immune overreaction.
Genetics Play a Major Role
If both of your parents get canker sores, you have roughly a 90% chance of getting them too. If neither parent is affected, that drops to about 20%. This is one of the strongest predictors, and it helps explain why some people get canker sores constantly while others never experience a single one despite similar diets and stress levels.
The genetic component likely involves inherited variations in how your immune system regulates inflammation at mucosal surfaces. Some people’s immune systems are simply more prone to overreacting to minor irritation inside the mouth.
Nutritional Deficiencies Are Surprisingly Common
Low levels of certain nutrients are found at high rates in people with recurring canker sores. In one study comparing 57 patients with recurrent ulcers to 57 healthy controls, about half of the ulcer patients were deficient in vitamin B12, nearly 46% had low folate levels, and roughly 11% had low iron stores.
These nutrients are essential for maintaining healthy mucosal tissue and normal immune function. B12 and folate help your body produce new cells to replace the lining of your mouth, which turns over rapidly. When levels are low, the tissue becomes more fragile and more vulnerable to breakdown. If you get canker sores frequently, it’s worth having your B12, folate, and iron levels checked. Correcting a deficiency can reduce or eliminate outbreaks for some people.
Foods That Irritate the Mouth
Certain foods are well-known triggers, not because they cause canker sores directly, but because they irritate already-sensitive oral tissue enough to set off the immune response. The most common culprits include citrus fruits, tomatoes, chocolate, coffee, and nuts. Acidic foods are particularly problematic because they lower the pH inside your mouth, which can erode the protective mucous layer.
Tracking which foods precede your outbreaks is one of the most practical things you can do. The specific triggers vary from person to person. Some people can eat oranges without issue but flare up after chocolate, and vice versa.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate, a foaming agent found in most major toothpaste brands, is a known soft tissue irritant. It strips away the protective mucous coating inside your mouth, leaving the tissue more exposed and more likely to ulcerate. The same ingredient is used in shampoos, soaps, and household cleaners for its degreasing properties.
Switching to an SLS-free toothpaste is one of the simplest changes you can make. Several brands market themselves as SLS-free specifically for people prone to canker sores. For some people, this single switch noticeably reduces the frequency of outbreaks.
Stress and Hormonal Shifts
Physical and emotional stress are among the most commonly reported triggers. Stress hormones alter immune function in ways that can tip the balance toward the kind of inflammatory overreaction that causes ulcers. Many people notice outbreaks during exam periods, after poor sleep, or during particularly demanding stretches at work.
Hormonal changes also play a role. Some women notice canker sores appearing at specific points in their menstrual cycle, typically linked to shifts in progesterone and estrogen levels. The American Dental Association lists canker sores alongside swollen gums and bleeding gums as oral symptoms that can be hormone-related. These hormonally driven sores tend to follow a predictable pattern, which can help distinguish them from other triggers.
Underlying Health Conditions
Recurring canker sores can be an early sign of certain systemic diseases, particularly those involving immune dysfunction or chronic inflammation. Crohn’s disease is one of the most notable. Researchers estimate that 20% to 50% of people with Crohn’s experience mouth ulcers at some point, and these ulcers are considered an “extra-intestinal manifestation,” meaning Crohn’s inflammation isn’t limited to the gut. People with Crohn’s are more likely to develop mouth ulcers than people with other inflammatory bowel conditions like ulcerative colitis.
Celiac disease is another condition strongly associated with recurrent canker sores. In celiac disease, the immune reaction to gluten can damage mucosal tissue throughout the digestive tract, including the mouth. For some people, persistent canker sores are what leads to a celiac diagnosis in the first place. Other conditions linked to frequent mouth ulcers include Behçet’s disease and various immune deficiencies.
How Long They Last
Most canker sores are the “minor” type: small, round, and self-healing within 10 to 14 days without treatment. These account for the vast majority of cases and, while painful, don’t leave scars.
Major canker sores are larger, deeper, and can persist for up to a month. They’re less common but significantly more disruptive, sometimes making it difficult to eat or speak. A third type, called herpetiform ulcers (despite having nothing to do with herpes), appear as clusters of tiny sores that can merge together and last anywhere from 10 to 100 days.
What Helps Them Heal
For most minor canker sores, no treatment is necessary beyond avoiding irritants and waiting. But when sores are frequent or painful enough to interfere with daily life, several options can shorten the duration and reduce pain.
Topical anti-inflammatory pastes applied directly to the sore are the standard first-line approach. These reduce pain and can shorten healing time, though they don’t prevent future outbreaks. Antiseptic mouth rinses can reduce the severity and pain of active ulcers but similarly don’t change how often they occur. For severe or persistent cases, stronger systemic medications exist, but these carry more significant side effects and are reserved for situations where simpler measures fail.
The most effective long-term strategy is identifying and managing your personal triggers. That might mean correcting a nutritional deficiency, switching toothpaste, adjusting your diet, or investigating whether an underlying condition is driving the outbreaks. Since canker sores have multiple overlapping causes, reducing even one or two triggers can make a noticeable difference in how often they appear.