Canker sores develop when your immune system attacks the thin tissue lining your mouth, creating small, painful ulcers on the inner cheeks, gums, tongue, or soft palate. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The triggers range from something as simple as biting your cheek to underlying nutritional deficiencies, and some people are far more prone to them than others based on genetics alone.
Physical Injury Is the Most Common Trigger
The single most frequent cause is minor trauma to the inside of your mouth. Biting your cheek, scraping your gums with a tortilla chip, or catching soft tissue on a sharp tooth edge can all set one off. Dental work is a classic trigger, whether it’s a routine cleaning that irritates your gums or a new braces wire that rubs against your cheek. Even brushing too hard with a stiff-bristled toothbrush can damage the delicate mucosal lining enough to start an ulcer.
Sports injuries, accidentally jabbing yourself with a fork, or wearing ill-fitting dentures all fall into the same category. The injury itself may be so minor you don’t notice it at the time, but within a day or two a sore appears at that exact spot. If you notice sores consistently forming near a sharp tooth surface or dental appliance, that’s worth mentioning to your dentist.
Your Toothpaste May Be Contributing
A foaming agent called sodium lauryl sulfate (SLS), found in most major toothpaste brands, is a known soft tissue irritant. It’s the same compound used in shampoos and household cleaners to create lather. In your mouth, it can dry out and inflame the protective mucous layer, making the tissue more vulnerable to ulceration. If you get canker sores frequently, switching to an SLS-free toothpaste is one of the simplest changes you can make, and many people see a noticeable reduction in outbreaks.
Foods That Trigger Sores
Certain foods can irritate mouth tissue or provoke an immune response that leads to canker sores. The most common culprits fall into a few categories:
- Acidic fruits: Pineapple, oranges, lemons, limes, grapefruit, and strawberries. The acid directly irritates soft tissue.
- Hard, abrasive foods: Chips, pretzels, crusty toast, raw vegetables, and nuts. These can scratch or poke the lining of your mouth, creating the minor injuries that trigger sores.
- Spicy foods: Curries, hot sauces, jalapeños, and spicy chips, especially those with high salt content.
- Salty foods: Salt dries the mouth and inflames the lining. Salted nuts are a particularly common offender.
- Chocolate: Contains an alkaloid called theobromine that can contribute to ulcer formation in sensitive people.
- Coffee and alcohol: Both are highly acidic and can irritate already-vulnerable tissue.
- Dairy: Some people react to proteins in cow’s milk, cheese, and yogurt, though this varies widely between individuals.
Not everyone reacts to the same foods. Paying attention to what you ate in the 24 to 48 hours before a sore appears can help you identify your personal triggers.
Nutritional Deficiencies
Your body needs certain nutrients to maintain healthy oral tissue, and running low on them makes canker sores more likely. In a study of 273 people with recurrent canker sores, about 20% were deficient in iron, roughly 5% were low in vitamin B12, and nearly 3% had folic acid deficiency. Around 21% of the group had anemia. These deficiencies weaken the mucosal lining and impair the immune system’s ability to regulate inflammation in the mouth.
If you get canker sores regularly and can’t pin them to an obvious trigger like injury or food, a blood test checking iron, B12, and folate levels is a reasonable step. Correcting a deficiency often reduces or eliminates recurring outbreaks.
Stress and Hormonal Changes
Emotional stress is one of the most widely reported triggers, though the exact mechanism isn’t fully mapped. Stress shifts immune function in ways that make the body more reactive to minor irritants, and many people notice sores appearing during exams, work deadlines, or periods of anxiety.
Hormonal fluctuations play a similar role. Some women develop canker sores in the days just before their period starts, driven by the rise in progesterone that occurs during that phase of the menstrual cycle. These hormonally triggered sores typically clear up shortly after the period begins.
Genetics Play a Bigger Role Than You’d Think
If both of your parents get canker sores, you have roughly a 90% chance of getting them too. If neither parent is affected, that probability drops to about 20%. This strong hereditary pattern suggests that the underlying immune response, specifically how aggressively your immune system reacts to minor mouth injuries or irritants, is largely inherited. It also explains why some people seem to get canker sores from almost anything while others never get them at all.
What Happens Inside the Sore
Canker sores aren’t infections. They’re an overreaction by your own immune system. When something triggers the process, certain white blood cells called T cells attack the cells lining your mouth as though they were a threat. These T cells release inflammatory signals that recruit more immune cells to the area, creating a cycle of tissue destruction that produces the characteristic crater-shaped ulcer with a white or yellowish center and red border.
People who get recurrent canker sores have higher levels of specific inflammatory markers in their blood and a higher ratio of certain immune cells compared to people who don’t get them. There’s also evidence that bacteria naturally present in the mouth may trigger a cross-reactive immune response, where the immune system mistakes mouth tissue proteins for bacterial proteins and attacks both.
Underlying Health Conditions
Frequent canker sores can sometimes signal a systemic health problem. Celiac disease is one of the most well-established connections. Oral ulcers are among the top five most common ways celiac disease first shows up, and in some cases, mouth sores or dental enamel defects are the only symptoms. The link likely involves both the nutritional deficiencies that celiac causes (particularly iron and B vitamins from impaired absorption) and the autoimmune activity itself.
Crohn’s disease, other forms of inflammatory bowel disease, Behçet’s disease, and lupus can all cause recurrent oral ulcers. Acid reflux is another contributor: stomach acid reaching the mouth can erode tissue and trigger sore formation. If you’re getting canker sores frequently, especially alongside digestive symptoms, fatigue, or joint pain, the sores may be pointing to something deeper.
Minor vs. Major Canker Sores
Most canker sores, about 75% to 85% of cases, are the minor type. These are less than 1 centimeter across, heal on their own within one to two weeks, and don’t leave scars. They’re painful and annoying, but they resolve without intervention.
Major canker sores are less common, accounting for 5% to 10% of cases, but significantly more disruptive. They’re larger than 1 centimeter, can persist for up to six weeks, and often leave scarring on the oral tissue. If you develop a sore that’s unusually large, lasts more than two weeks, or keeps coming back in the same spot, that warrants a closer look from a doctor or dentist to rule out other conditions.