Canker sores form when your immune system attacks the thin tissue lining the inside of your mouth. They affect roughly 20 to 25 percent of the population, and for many people they come back repeatedly throughout life. The underlying cause isn’t a single factor but a combination of immune system behavior, nutritional gaps, physical irritation, and individual triggers that varies from person to person.
Your Immune System Turns on Your Own Tissue
A canker sore isn’t an infection. It’s an immune response gone wrong. Specific immune cells, particularly a type called T cells, attack and destroy the thin layer of tissue inside your mouth. Once the process starts, those T cells release signaling molecules that sustain the inflammation, keeping the area raw and painful. People with active canker sores show elevated levels of these inflammatory signals in their blood compared to people who don’t get them.
One leading theory points to a case of mistaken identity. A common bacterium that lives harmlessly in your mouth may share structural similarities with proteins in your own tissue. Your immune system, primed to fight the bacterium, accidentally targets your mouth lining instead. This would explain why canker sores tend to recur: the trigger is always present, and in some people the immune system periodically overreacts to it.
Nutritional Deficiencies That Play a Role
Low levels of certain vitamins and minerals are strongly linked to recurrent canker sores, especially vitamin B12. In one study comparing people with recurrent oral ulcers to healthy controls, over 50 percent of the canker sore group was deficient in B12, while none of the control group was. Folate deficiency was also common, appearing in about 46 percent of patients, though interestingly it showed up at the same rate in the control group, making its role less clear-cut.
Iron deficiency is another commonly cited factor, though the evidence is mixed. The practical takeaway: if you get canker sores frequently, a blood test to check your B12, folate, and iron levels is a reasonable step. Correcting a deficiency sometimes reduces how often sores appear.
Foods That Trigger Outbreaks
Certain foods don’t cause canker sores on their own, but they can set one off in people who are already prone. The biggest culprits fall into two categories: acidic foods that irritate the tissue chemically, and rough or sharp foods that cause tiny physical injuries.
Acidic triggers include citrus fruits (oranges, lemons, grapefruit), tomatoes and tomato-based sauces, strawberries, coffee, and carbonated drinks. These lower the pH inside your mouth and can irritate sensitive tissue enough to spark an immune response. Spicy foods, including hot peppers, curry, and salsa, can inflame the mouth lining in a similar way.
On the physical side, sharp-edged foods like nuts, seeds, chips, and pretzels can create small abrasions inside your mouth. Those tiny cuts give canker sores a foothold. Some people also react to dairy proteins or chocolate, which contains a compound called theobromine that can cause oral irritation in sensitive individuals. Tracking which foods precede your outbreaks is the most reliable way to identify your personal triggers, since the list varies widely from person to person.
Your Toothpaste Might Be Making It Worse
Most commercial toothpastes contain a foaming agent called sodium lauryl sulfate (SLS). For people prone to canker sores, this ingredient can be a significant and easily fixable trigger. SLS strips away the protective mucus layer inside your mouth, denatures proteins in the surface cells, and dissolves structural fats that hold the tissue together. The result is a mouth lining that’s more vulnerable to damage and immune overreaction.
In a double-blind crossover study, patients who brushed with SLS-containing toothpaste developed significantly more canker sores than when they used an SLS-free alternative. Two additional studies found that switching to SLS-free toothpaste reduced both the number and duration of outbreaks and lowered pain intensity. SLS-free toothpastes are widely available and cost about the same, making this one of the simplest changes you can try.
Physical Trauma and Dental Work
Biting the inside of your cheek, scraping your gums with a chip, or getting poked by a sharp piece of dental hardware can all set off a canker sore in the injured spot. Braces are a common culprit because brackets and wires create ongoing friction against the cheeks and lips. The tissue damage itself isn’t the sore; rather, the injury triggers the same immune overreaction that produces the ulcer.
This is why canker sores often appear at the exact site of a cut or scrape rather than randomly. Aggressive tooth brushing, ill-fitting dentures, and recent dental procedures can all create the kind of localized trauma that starts the process.
Hormonal Shifts and Stress
Many women notice canker sores appearing around their menstrual period. The American Dental Association notes that hormonal fluctuations during the menstrual cycle can trigger canker sores, swollen gums, and other oral changes. These sores typically resolve after the period ends. Puberty is another hormonally volatile time when canker sores often first appear.
Emotional stress is one of the most commonly reported triggers, though the exact pathway is harder to pin down. Stress alters immune function broadly, and for people whose immune systems are already prone to attacking oral tissue, that shift may be enough to tip the balance toward an outbreak.
Genetics and Family History
If your parents get canker sores, you’re more likely to get them too. The genetic component involves variations in genes that regulate your immune system, particularly the group of genes that help your body distinguish its own tissue from foreign invaders. Certain variations in these immune-signaling genes appear to make the oral lining a more frequent target of misdirected immune attacks.
In the related condition Behçet’s disease, which causes severe, recurring oral ulcers along with other symptoms, a specific genetic variation called HLA-B51 increases risk by about sixfold. One-third to two-thirds of people with Behçet’s carry this variation. For ordinary canker sores the genetics are less dramatic, but the hereditary pattern is consistent enough that family history is one of the strongest predictors of who will be affected.
When Canker Sores Signal Something Deeper
Most canker sores are a nuisance, not a warning sign. They heal on their own within 10 to 14 days without treatment. But frequent or unusually severe outbreaks can sometimes point to an underlying condition. Celiac disease, Crohn’s disease, and Behçet’s disease all feature recurrent oral ulcers as a symptom. HIV and other conditions that suppress immune function can also cause persistent mouth sores.
If your canker sores are unusually large, last longer than three weeks, come with fever, or appear alongside symptoms in other parts of your body (joint pain, eye inflammation, genital sores, digestive problems), those patterns warrant investigation. A doctor can run blood tests to check for nutritional deficiencies, celiac antibodies, and markers of autoimmune activity to rule out systemic causes.
How to Reduce How Often They Come Back
There’s no cure for recurrent canker sores, but you can reduce their frequency by addressing the controllable triggers. Switch to an SLS-free toothpaste. Keep a food diary to identify your personal dietary triggers, and limit acidic, spicy, or rough-textured foods during periods when you’re most vulnerable. If you wear braces or dentures, use dental wax to reduce friction against your cheeks.
Get your B12 and folate levels checked, especially if you follow a vegetarian or vegan diet, since B12 comes almost exclusively from animal products. Managing stress through whatever methods work for you (exercise, sleep, meditation) may also help, given the well-established link between stress and outbreaks. None of these steps guarantees you’ll never get another canker sore, but together they can make a meaningful difference in how often you deal with them and how severe they are when they do appear.