Canker sores are caused by an overreaction of your immune system that breaks down the lining of your mouth. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The exact reason one person gets them while another doesn’t involves a mix of genetics, physical triggers, nutritional gaps, and stress, but the underlying mechanism is the same: your body’s own defenses turn against healthy tissue inside your mouth.
Canker sores affect somewhere between 5% and 66% of the population depending on the group studied, making them one of the most common oral complaints. Most are small, heal on their own within two weeks, and never need treatment. But understanding what triggers them can help you get fewer of them.
What’s Actually Happening Inside Your Mouth
A canker sore isn’t just surface irritation. It’s an inflammatory attack driven by your white blood cells. In people who get recurrent canker sores, the immune system produces elevated levels of pro-inflammatory signaling molecules, particularly ones that ramp up tissue destruction. At the same time, the anti-inflammatory signals that would normally keep the reaction in check are suppressed. The result is a small but painful crater in the soft tissue of your cheek, lip, tongue, or gum.
Researchers believe this happens because the immune system mistakes harmless bacteria that naturally live in your mouth for a threat. Certain strains of oral bacteria share surface features with your own tissue, so the immune response that targets them also damages the surrounding lining. People who get frequent canker sores also have fewer regulatory immune cells, the type responsible for calling off the attack once it starts. In one study, people with active canker sores had roughly half the proportion of these regulatory cells compared to healthy controls.
Physical Triggers That Start a Sore
Mechanical trauma is one of the most common and immediate triggers. Biting the inside of your cheek, scraping your gum with a tortilla chip, or burning your mouth on hot food can all set one off. Orthodontic braces are a frequent culprit, especially on the inner cheeks where brackets and wires rub against soft tissue. Ill-fitting dentures, sharp edges on broken teeth, and aggressive toothbrushing can do the same.
Not every mouth injury turns into a canker sore. But if you’re genetically prone to them, even minor trauma can trigger the inflammatory cascade that creates an ulcer instead of a simple scratch that heals quietly.
Stress and Hormonal Shifts
If you’ve noticed canker sores appearing during exam weeks, work deadlines, or emotionally difficult periods, you’re not imagining the connection. Psychological stress alters immune function and increases inflammation throughout the body, including in the mouth. The same stress hormones that suppress your ability to fight off a cold also seem to lower the threshold for canker sore formation.
Hormonal changes play a similar role. Some women notice sores appearing at specific points in their menstrual cycle, suggesting that fluctuations in estrogen and progesterone influence how the oral lining responds to irritation.
Nutritional Deficiencies
Low levels of certain nutrients are strongly linked to recurrent canker sores. The most common deficiencies found in people with frequent outbreaks involve iron, zinc, folate, and vitamins B1, B2, B6, and B12. These nutrients all play roles in maintaining healthy mucous membranes and regulating immune responses.
You don’t need to be severely deficient to notice the effect. Even borderline-low levels can increase your susceptibility. If you get canker sores regularly and can’t identify an obvious trigger, a blood test checking these levels is a reasonable step.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate (SLS) is a foaming agent found in most commercial toothpastes, and it can strip away the protective mucous layer inside your mouth. For people prone to canker sores, SLS-containing toothpaste has been shown to increase the frequency of outbreaks. It essentially thins the barrier that protects delicate oral tissue, making it easier for irritation to escalate into a full ulcer.
Switching to an SLS-free toothpaste is one of the simplest changes you can make. Several brands market themselves as SLS-free, and for some people this alone reduces how often sores appear.
Certain Foods as Triggers
Acidic and abrasive foods are well-known triggers. Citrus fruits, tomatoes, pineapple, strawberries, and vinegar-based foods can irritate the mouth lining enough to spark a sore in someone who’s susceptible. Spicy foods and very salty snacks can do the same, not by causing an allergic reaction but by chemically or physically aggravating tissue that’s already vulnerable.
Some people also have sensitivities to specific food proteins, particularly gluten. In fact, canker sores are a recognized oral symptom of celiac disease, and some people discover their celiac diagnosis only after investigating why they keep getting mouth ulcers.
Genetics and Family History
If both your parents get canker sores, your chances of getting them are significantly higher. There’s a clear hereditary component tied to how your immune system is wired, specifically which versions of certain immune-recognition genes you carry. These genetic variations influence how aggressively your body responds to oral bacteria and minor tissue damage.
This genetic predisposition explains why some people can bite their cheek and heal without incident, while others develop a painful ulcer from the same injury every time.
Links to Underlying Health Conditions
Occasional canker sores are rarely a sign of anything serious. But frequent or unusually severe outbreaks can sometimes signal a systemic condition. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis are associated with mouth ulcers, as the same type of inflammation that damages the gut can affect oral tissue. Behçet’s disease, a rare condition involving blood vessel inflammation, uses recurrent oral ulcers as one of its primary diagnostic criteria.
Lupus and reactive arthritis can also produce oral sores. If your canker sores are large (over 1 cm), last longer than three weeks, occur alongside other unexplained symptoms like joint pain or digestive problems, or leave scars after healing, these patterns are worth investigating further.
The Three Types of Canker Sores
Not all canker sores are the same. Minor canker sores account for 75% to 85% of cases. They’re less than 1 cm across, heal within 7 to 14 days, and don’t leave scars. These are the ones most people are familiar with.
Major canker sores make up 5% to 10% of cases. They’re larger than 1 cm, can persist for up to six weeks, and often leave scars on the tissue where they formed. They’re considerably more painful and can interfere with eating and speaking.
Herpetiform canker sores, despite their name, have nothing to do with herpes. They also represent 5% to 10% of cases and appear as clusters of tiny sores (each just 1 to 3 mm) that can number up to 100 at a time. These small ulcers sometimes merge into larger irregular sores. They typically heal within two weeks, though some cases persist much longer.
Reducing How Often They Come Back
Because canker sores involve multiple overlapping triggers, prevention usually means addressing several factors at once. Switching to SLS-free toothpaste, managing stress, correcting nutritional deficiencies, and avoiding your personal food triggers can all reduce outbreak frequency. If you wear braces or dentures, making sure they fit properly and using dental wax over sharp points helps prevent the mechanical injuries that set off sores.
Keeping a simple log of when sores appear, what you ate, and what was happening in your life at the time can reveal patterns that aren’t obvious otherwise. Many people find that their sores cluster around the same two or three triggers, and once those are identified, outbreaks become much less frequent.