Canker sores form when your immune system overreacts against the lining of your mouth, triggering inflammation that breaks down the tissue and creates a shallow, painful ulcer. Between 5% and 25% of people get them, and in some populations the rate climbs as high as 60%. Despite how common they are, there’s no single cause. Instead, a combination of immune dysfunction, nutritional gaps, physical irritation, and stress creates the conditions for these sores to appear.
What Happens Inside Your Mouth
A canker sore isn’t an infection. It’s your own immune system attacking the soft tissue inside your mouth. In people who get recurrent canker sores, the blood contains elevated levels of inflammatory signaling molecules, particularly one called TNF-alpha, which stays elevated even between outbreaks. At the same time, the body produces less of the anti-inflammatory signals (IL-10 and TGF-beta) that would normally keep that response in check.
There’s also a measurable shortage of a specific type of immune cell that acts as a brake on inflammation. In healthy people, these regulatory cells make up about 7.3% of a certain white blood cell population. In people with active canker sores, that proportion drops to roughly 3.6%. With fewer regulatory cells and more inflammatory signals, the immune system essentially turns on the mouth’s own lining, breaking it down into an open ulcer. The result is that familiar round or oval sore, usually less than a centimeter across, with a white or yellowish center and a red border.
Common Triggers
Even if your immune system is primed to overreact, canker sores usually need a trigger to actually appear. The most common ones fall into a few categories.
Physical injury. Biting your cheek, brushing too hard, or scraping your gums on a sharp chip or cracker creates a tiny wound that your immune system then amplifies into a full ulcer. Dental work, braces, and ill-fitting dentures do the same thing.
Stress and fatigue. Emotional stress and sleep deprivation are among the most frequently reported triggers. Both suppress the regulatory arm of the immune system, making an inflammatory overreaction more likely.
Acidic and spicy foods. Coffee, citrus fruits, pineapple, and tomatoes can irritate the delicate tissue inside the mouth. These foods don’t cause canker sores on their own, but they can provoke outbreaks in people who are already susceptible, and they make existing sores significantly more painful.
Hormonal shifts. Some women notice canker sores flare in sync with their menstrual cycle, suggesting that hormonal changes play a role in tipping the immune balance.
Nutritional Deficiencies
About 5% to 10% of people with recurrent canker sores have low blood levels of iron, folate, zinc, or B vitamins (B1, B2, B6, or B12). These nutrients support the health and repair of mucosal tissue, so when levels drop, the mouth lining becomes more vulnerable to breakdown. Iron deficiency is especially common in younger women with frequent sores.
The practical takeaway: if you get canker sores often and can’t identify an obvious trigger, a blood test checking iron, B12, and folate levels is a reasonable step. In cases where a deficiency is found, correcting it with diet or supplements can reduce how often sores return.
Links to Digestive and Autoimmune Conditions
Canker sores can also be a sign of something deeper going on. Celiac disease is the clearest example. In one large study, 22.7% of people with celiac disease had mouth ulcers, compared to just 7.1% of people without it. That’s roughly a fourfold increase in risk. Reported rates in celiac patients range from about 4% to 33%, depending on the study, but the connection is consistent.
Inflammatory bowel diseases like Crohn’s and ulcerative colitis carry a similar association. In these conditions, the same immune dysfunction that damages the gut lining can target the mouth. When mouth ulcers appear alongside digestive symptoms like chronic diarrhea, bloating, or unexplained weight loss, the sores may be pointing toward an undiagnosed condition rather than standing alone.
Genetics and Family History
If your parents got canker sores, you’re more likely to get them too. Studies consistently show a strong familial pattern, though pinning down the exact genes responsible has been difficult. One candidate, a gene variant called HLA-B*51, was tested directly in a study of over 100 patients with recurrent canker sores. It turned up in 45.5% of patients, but also in 46.3% of healthy people, meaning it wasn’t a meaningful predictor. The genetic component appears to involve multiple genes working together rather than any single marker, which makes it harder to test for but no less real in its effect.
Toothpaste and Sodium Lauryl Sulfate
You may have heard that switching to a toothpaste without sodium lauryl sulfate (SLS), a common foaming agent, can prevent canker sores. The idea is that SLS strips away a protective layer of the mouth lining, leaving it more vulnerable. It’s a plausible theory, but the evidence is thin. A 2019 review of the available clinical data found there wasn’t enough evidence to confirm that SLS-free toothpaste reduced the frequency, duration, or pain of canker sores. One double-blind study found no significant change in ulcer patterns at all after switching. Still, some people report improvement, and SLS-free toothpaste is inexpensive and harmless to try.
How Canker Sores Heal
Minor canker sores, the kind most people get, typically heal on their own within 10 to 14 days without scarring. The first day or two usually involves a tingling or burning sensation before the ulcer fully forms. Pain peaks in the first few days after the sore opens, then gradually fades as new tissue grows in from the edges. Avoiding irritants like acidic foods, alcohol-based mouthwash, and rough-textured snacks during this window can speed things along and reduce discomfort.
Major canker sores, which are larger than about a centimeter and extend deeper into the tissue, can take six weeks or longer to heal and may leave a scar. These are much less common but considerably more painful and disruptive to eating and speaking.
Canker Sores vs. Cold Sores
People often confuse the two, but they’re completely different conditions. The simplest distinction is location: canker sores occur inside the mouth, on the soft tissue of the cheeks, tongue, or floor of the mouth. Cold sores (fever blisters) appear outside the mouth, typically on or around the lips. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not caused by a virus and cannot be spread to another person. If you have a sore inside your mouth that isn’t on the gums right at the base of a tooth, it’s almost certainly a canker sore.