Canker sores form when your immune system mistakenly attacks the thin tissue lining your mouth. About 20% of the general population gets them, and the triggers range from biting your cheek to running low on certain vitamins. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. Understanding what sets them off can help you get fewer of them.
What Actually Happens Inside Your Mouth
A canker sore isn’t just a scratch that got irritated. It’s a localized immune reaction. Your body sends specialized immune cells to a spot on the soft tissue inside your mouth, and those cells begin destroying the surface layer of tissue as if fighting off an infection that isn’t there. This process is fueled by inflammatory signaling molecules that sustain the attack, keeping the area raw and open until the immune response winds down on its own.
People who get frequent canker sores show higher levels of certain inflammatory markers in their blood, and the immune pathway involved resembles an overactive inflammatory response. That’s why canker sores tend to cluster during times when your immune system is already stressed or disrupted.
Physical Injuries That Start the Process
The most common trigger is simple trauma to the inside of your mouth. This includes biting your tongue or cheek, scraping your gums while brushing too hard, or irritation from dental work like a cleaning, filling, or braces adjustment. Sharp-edged foods like chips, pretzels, nuts, and seeds can also create tiny abrasions that give your immune system a target.
Orthodontic wires and brackets are a frequent culprit because they create ongoing friction against the cheeks and lips. Even a new toothbrush with stiff bristles can do enough damage to trigger a sore in someone who’s prone to them. The injury itself is usually minor, something you might not even notice, but for susceptible people it’s enough to kick off the immune response that turns a small scratch into an open ulcer.
Foods That Trigger Outbreaks
Certain foods provoke canker sores through acidity, chemical irritation, or physical abrasion. The biggest offenders are acidic foods: citrus fruits, tomatoes and tomato-based sauces, strawberries, and coffee. These don’t cause sores in everyone, but in people who are already prone, the acid irritates the mouth lining enough to start one.
Spicy foods like hot peppers, curry, and salsa can inflame sensitive oral tissue. Chocolate contains a compound called theobromine that triggers oral irritation in some individuals. Carbonated drinks, including sparkling water, can also be harsh on the mouth lining. And some people are sensitive to dairy proteins, which may contribute to inflammation.
Salty snacks deserve a special mention because they work two ways. The rough edges of chips and pretzels create tiny cuts, and the salt then irritates those cuts. If you notice sores appearing a day or two after eating a particular food, that food is worth avoiding for a few weeks to see if the pattern holds.
Vitamin and Mineral Deficiencies
Low levels of certain nutrients are strongly linked to recurring canker sores. In one study comparing people with frequent sores to healthy controls, about half of the canker sore patients had low vitamin B12 levels, while none of the control group did. Nearly 46% had low folate levels, and about 10% were low in iron (measured as ferritin, your body’s iron storage protein).
These deficiencies affect how quickly your mouth tissue repairs itself and how well your immune system regulates inflammation. If you’re getting canker sores frequently, especially several times a year, it’s worth having your B12, folate, and iron levels checked. The fix can be as straightforward as a dietary change or supplement.
Stress and Hormonal Shifts
Stress is one of the most reliable canker sore triggers, though the connection is indirect. When you’re under prolonged stress, your body produces elevated levels of stress hormones that suppress some parts of your immune system while ramping up inflammatory responses. That imbalance creates the conditions for your mouth tissue to come under attack.
Hormonal fluctuations also play a role, particularly for women. Canker sores commonly appear in the days just before a menstrual period, when hormone levels shift rapidly. These hormonally timed sores tend to heal on their own once the period ends. Some women also notice more frequent outbreaks during pregnancy or other times of significant hormonal change.
Genetics and Family History
If your parents get canker sores, you’re significantly more likely to get them too. Researchers have identified specific genetic markers on immune system genes that are associated with higher susceptibility. Certain variants increase your risk, while others appear to offer protection against developing sores at all.
This genetic component explains why some people can bite their cheek, eat a bag of salt-and-vinegar chips, and never develop a sore, while others seem to get one from the slightest provocation. Your baseline susceptibility is largely inherited; what varies is which triggers push you over the threshold.
Other Known Triggers
Sodium lauryl sulfate (SLS), a foaming agent found in most toothpastes, irritates the mouth lining in some people and has been linked to more frequent canker sores. Switching to an SLS-free toothpaste is one of the simplest changes you can make if you get sores often.
Sleep deprivation, illness, and anything else that taxes your immune system can increase your risk during that window. Some people also notice outbreaks after quitting smoking, likely because tobacco suppresses certain immune responses in the mouth, and stopping removes that suppression.
How Long They Last
Most canker sores are the minor type: less than a centimeter across, shallow, and round or oval with a white or yellowish center and a red border. These heal on their own within one to two weeks without scarring. The pain is usually worst in the first few days and gradually fades.
Major canker sores are larger, deeper, and can take six weeks or longer to heal. They sometimes leave scars. A third type, called herpetiform ulcers, appears as clusters of many tiny sores that can merge together. Both of these less common types are worth getting evaluated if they appear, since persistent or unusually large mouth ulcers can sometimes signal an underlying condition like celiac disease, inflammatory bowel disease, or an immune disorder.
Reducing How Often You Get Them
Since canker sores result from a combination of susceptibility and triggers, the most effective prevention strategy is identifying and avoiding your personal triggers. Keep a simple log of what you ate, your stress level, and any mouth injuries in the days before a sore appears. Patterns usually emerge within a few months.
Practical steps that help many people include switching to a soft-bristled toothbrush, using SLS-free toothpaste, cutting back on acidic and spicy foods, and managing stress through sleep and exercise. If deficiencies are part of the picture, foods rich in B12 (meat, fish, eggs, dairy), folate (leafy greens, legumes), and iron (red meat, beans, fortified cereals) can help close the gap. Over-the-counter mouth rinses and topical gels containing a numbing agent can ease pain during an active outbreak, though they don’t speed healing dramatically.