Canker sores develop 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 something as simple as biting your cheek to nutritional deficiencies, stress, hormonal shifts, and certain foods. Most people get one occasionally, but for some, they keep coming back due to a combination of genetics and ongoing triggers.
What Actually Happens Inside Your Mouth
A canker sore forms when certain immune cells, particularly a type of white blood cell called T cells, begin destroying the surface layer of your oral tissue. These cells release inflammatory signals that break down the delicate lining of your cheeks, lips, gums, or tongue, creating a shallow, painful crater. The tissue damage is real, which is why even a tiny sore can hurt so much when you eat or talk.
Researchers believe this process involves a case of mistaken identity. A common mouth bacterium produces a protein that closely resembles proteins in your own oral tissue. In some people, the immune system can’t tell the difference and launches an attack on both. This cross-reactivity, combined with an overactive inflammatory response, is what drives the ulcer to form and persist for days.
Physical Injury to Your Mouth
The most straightforward trigger is mechanical damage. Biting your cheek, scraping your gums with a chip, or burning the roof of your mouth with hot food can all set the stage for a canker sore to develop at the injury site. Ill-fitting dental appliances like braces, retainers, or dentures are common culprits because they create repeated friction against the same spot.
Even your toothbrush matters. A hard-bristled brush or aggressive brushing technique can irritate the soft tissue enough to trigger an ulcer. Using a soft-bristled toothbrush and waxed floss reduces this kind of minor trauma.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), a foaming agent found in most major toothpaste brands, is a known soft tissue irritant. It’s the same compound used in shampoos, soaps, and household cleaners. For people prone to canker sores, switching to an SLS-free toothpaste can reduce the frequency of outbreaks. Check the ingredients list on your tube; if sodium lauryl sulfate appears, it may be worth trying a brand without it.
Foods That Trigger Outbreaks
Certain foods provoke canker sores through two different pathways: direct irritation and immune-mediated sensitivity.
Acidic foods like citrus fruits, tomatoes, and vinegar-based dressings can irritate the mouth lining directly. Coffee and alcohol do the same while also drying out the mouth, which leaves tissue more vulnerable. Higher coffee intake has been linked to more frequent sores in some studies.
Food sensitivities are a separate issue. In susceptible people, specific proteins trigger a localized immune reaction in the mouth that leads to ulceration. Common offenders include:
- Gluten from wheat, barley, and rye
- Dairy proteins like casein and whey
- Chocolate, likely due to cocoa or the milk and additives paired with it
- Nuts, especially peanuts, almonds, walnuts, and cashews, which act as both physical irritants (their rough texture scrapes tissue) and potential allergens
- Eggs, shellfish, and certain food additives like artificial colors and flavor enhancers
If you notice sores appearing a day or two after eating a particular food, that pattern is worth paying attention to. Keeping a simple food diary alongside your outbreaks can help you identify your personal triggers.
Vitamin and Mineral Deficiencies
Low levels of certain nutrients are strongly linked to recurrent canker sores. In one study comparing 57 patients with recurrent ulcers to 57 healthy controls, half of the canker sore patients were deficient in vitamin B12, while none of the controls were. The difference was striking: patients with low B12 averaged around 124 pg/ml, compared to nearly 900 pg/ml in healthy individuals.
Iron deficiency (measured through ferritin levels) and low folate are also associated with recurrent sores, though the connection isn’t as dramatic as with B12. If you’re getting canker sores frequently, a blood test checking these three levels is a reasonable step. Correcting a deficiency through diet or supplementation can reduce how often sores appear.
Stress and Hormonal Changes
Emotional and physical stress are well-established triggers. Stress suppresses parts of the immune system while ramping up inflammation, creating conditions where canker sores are more likely to develop. Many people notice outbreaks during exam periods, after major life events, or during stretches of poor sleep.
Hormonal fluctuations play a role too, particularly for women. Canker sores are more likely to appear in the days just before a menstrual period, when hormone surges increase gum sensitivity and oral tissue inflammation. These hormonally triggered sores typically heal on their own once the period ends.
Genetics and Family History
Canker sores run in families. More than 40% of people with recurrent sores have a family history of them, and children whose parents both get canker sores have roughly a 90% chance of developing them too. This genetic component helps explain why some people get sores constantly while others never do, even when exposed to the same triggers. If your parents dealt with frequent canker sores, your immune system is likely wired to overreact to the same kinds of provocations.
Underlying Medical Conditions
Recurrent canker sores can sometimes signal a deeper health issue. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis frequently cause mouth ulcers alongside their intestinal symptoms. Celiac disease, an autoimmune reaction to gluten, is another common cause. Lupus and reactive arthritis can also produce oral ulcers, though they may look slightly different from typical canker sores.
If you’re getting canker sores frequently (several times a year or more), they’re unusually large, or they take longer than two weeks to heal, the sores may be a symptom of something systemic rather than a standalone problem.
Types and Healing Timelines
Most canker sores are minor aphthous ulcers, small round or oval craters about 2 to 4 millimeters across. These heal on their own in 7 to 10 days without scarring. Major aphthous ulcers are larger, deeper, and can take weeks or even months to resolve, sometimes leaving scars. A third type, herpetiform ulcers, appear as clusters of very small sores that can merge into larger irregular shapes. Despite the name, they have nothing to do with the herpes virus.
The vast majority of canker sores fall into the minor category. Pain typically peaks in the first few days and gradually fades as the tissue regenerates. Avoiding spicy, acidic, and crunchy foods during healing keeps irritation to a minimum and lets the sore close faster.