Canker sores form when your immune system attacks the thin tissue lining the inside of your mouth. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The triggers range from biting your cheek to eating certain foods to simply being stressed out, but the underlying process is the same: your body’s own immune cells turn against healthy oral tissue, creating those painful shallow ulcers on your gums, tongue, or inner cheeks.
What Actually Happens Inside Your Mouth
A canker sore isn’t just a scratch or burn. It’s an immune-driven event. When something triggers the process, certain white blood cells called T cells begin destroying the surface layer of your oral tissue. These immune cells are normally reserved for fighting infections, but in this case they target healthy cells instead. The damage is fueled by a cascade of inflammatory signaling molecules that sustain the attack, keeping the area inflamed and raw until the cycle winds down on its own.
One theory is that this misfired immune response may be aimed at a common mouth bacterium called Streptococcus sanguinis. The immune system may confuse proteins on this bacterium with proteins in your own oral tissue, essentially attacking both. This would explain why canker sores tend to recur in some people: their immune systems are primed to overreact to something that’s always present in the mouth.
Physical Injury to the Mouth
One of the most common triggers is simple mechanical trauma. Biting the inside of your cheek, poking your gums with a tortilla chip, brushing too aggressively, or getting jabbed by orthodontic brackets can all set off the immune response described above. Dental work is another frequent culprit. The friction from a new retainer or the scrape of a dental instrument can damage enough tissue to start the process. If you’re someone who gets canker sores regularly, you’ve probably noticed they tend to appear right where you bit yourself or where a rough edge rubbed against your cheek.
Foods That Trigger Sores
Acidic and abrasive foods are well-known triggers. Citrus fruits, tomatoes, strawberries, and coffee all contain acids that irritate the delicate tissue inside your mouth. Spicy foods containing hot peppers can do the same. Sodas combine high acid content with sugars that promote inflammation.
Food sensitivities play a role too. Chocolate, for instance, triggers sores in some people not because of acidity but because of an immune-related sensitivity. Gluten is another trigger, particularly for people with celiac disease or gluten sensitivity. When the intestines can’t properly absorb nutrients, the resulting vitamin deficiencies can make oral tissue more vulnerable. In fact, recurring mouth ulcers are sometimes the only visible sign of undiagnosed celiac disease.
Sharp or crunchy foods like chips, crusty bread, and pretzels can physically scrape the inside of your mouth, combining mechanical injury with whatever chemical irritants are in the food itself.
Your Toothpaste May Be a Factor
Many commercial toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that strips away the protective mucus layer coating your oral tissue. This mucus layer acts as a barrier between your mouth’s surface and everything it contacts. When SLS dissolves that barrier, the exposed tissue becomes more vulnerable to irritants and allergens that can set off an ulcer.
The effect is measurable. In one study, people prone to canker sores who switched to an SLS-free toothpaste saw their ulcer count drop by about 60 to 70 percent. If you’re getting frequent sores, switching toothpaste is one of the easiest changes you can make. SLS-free options are widely available at most pharmacies.
Stress and Immune Suppression
Stress is one of the most commonly reported triggers, and the connection is straightforward. When you’re under prolonged stress, your immune system weakens. That weakened state makes the soft tissue in your mouth more susceptible to breakdown and infection. Many people notice canker sores appearing during exam weeks, after poor sleep, or during emotionally difficult periods. It’s not that stress directly causes the sore. Rather, it lowers the threshold, making it easier for other triggers to take hold.
Genetics and Family History
If both of your parents get canker sores, you’re significantly more likely to get them too. The genetic component is strong, though researchers haven’t pinpointed a single gene responsible. Studies looking at specific immune-system markers like HLA-B*51 (a gene variant linked to other inflammatory conditions) found no difference between people with recurrent canker sores and healthy controls. This suggests the genetic influence is more complex, likely involving multiple genes that shape how your immune system responds to everyday oral irritants.
Underlying Health Conditions
For most people, canker sores are annoying but harmless. In some cases, though, they signal something deeper. Celiac disease, Crohn’s disease, Behcet’s disease, and lupus can all cause recurring oral ulcers. Acid reflux is another culprit: stomach acid reaching the mouth can erode tissue and trigger sore formation.
The key distinction is how the sores respond to treatment. Canker sores caused by an underlying condition often don’t improve with standard over-the-counter remedies. If you’re getting frequent ulcers that resist typical treatments, that pattern itself can be a clue that something systemic is going on. Celiac disease is worth particular attention here, since mouth sores or dental enamel defects may be its only outward symptoms for years before digestive issues appear.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re fundamentally different. According to the National Institute of Dental and Craniofacial Research, the distinctions are clear:
- Location: Canker sores appear inside the mouth. Cold sores appear outside, typically around the lips.
- Cause: Cold sores are caused by herpes simplex virus type 1. Canker sores have no viral cause.
- Contagion: Cold sores are contagious. Canker sores are not. You cannot give someone a canker sore by sharing a drink or kissing.
- Appearance: Canker sores are round or oval with a yellowish-gray center and a red border. Cold sores are fluid-filled blisters that crust over.
Both types typically heal on their own within one to two weeks without treatment.
Reducing How Often They Come Back
Since canker sores result from a combination of triggers rather than a single cause, prevention means reducing your exposure across multiple fronts. Switch to an SLS-free toothpaste. Cut back on acidic foods during periods when you’re already stressed or run down. Use orthodontic wax over any sharp edges on braces. Avoid brushing so hard that you abrade your gums.
Pay attention to patterns. If sores consistently appear after you eat a specific food, your body is telling you something. Keeping a simple log of what you ate and when sores appeared can help you identify triggers you wouldn’t have suspected, like cinnamon, certain nuts, or even a particular brand of chips. For people who get sores frequently despite these precautions, screening for nutritional deficiencies (especially iron, B12, and folate) or celiac disease can uncover a treatable root cause.