Canker sores develop when your immune system attacks the thin lining inside your mouth, creating small, painful ulcers on your cheeks, lips, tongue, or gums. Unlike cold sores, they aren’t caused by a virus, and they aren’t contagious. The frustrating truth is that no single cause has been identified, but researchers have pinpointed a range of triggers that set off the process, from biting your cheek to the toothpaste you use every morning.
What’s Actually Happening Inside Your Mouth
A canker sore forms when certain immune cells, particularly a type called T cells, begin destroying the surface tissue inside your mouth. These T cells release chemical signals that recruit more immune cells to the area, creating a cycle of inflammation and tissue breakdown. The result is that familiar shallow, round ulcer with a white or yellowish center and a red border.
People who get frequent canker sores have higher levels of a specific subtype of T cell (gamma-delta T cells) compared to people who rarely get them. Their immune systems also show abnormal activity in a signaling pathway that controls how the body responds to perceived threats, essentially overreacting to triggers that wouldn’t bother someone else. This is why some people get canker sores constantly while others go their entire lives without one.
Physical Injury to the Mouth
The most straightforward trigger is mechanical damage to the soft tissue inside your mouth. Biting your cheek, scraping your gums with a tortilla chip, getting poked by a loose orthodontic wire, or brushing too aggressively with a hard-bristled toothbrush can all create the initial break in tissue that your immune system then escalates into a full ulcer. Dental work is another common culprit, since instruments and appliances can scratch or irritate the lining of your mouth.
If you’re prone to canker sores, switching to a soft-bristled toothbrush and waxed floss can reduce the minor injuries that trigger outbreaks.
Your Toothpaste May Be a Trigger
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, is one of the best-documented canker sore triggers. In a clinical study, patients who switched from an SLS-containing toothpaste to an SLS-free version saw their ulcer count drop from an average of 14.3 to just 5.1 over a three-month period. That’s a 64% reduction from changing nothing but toothpaste.
A 2019 systematic review of four clinical trials involving 124 participants confirmed the pattern: SLS-free toothpaste consistently reduced the number of ulcers, how long each one lasted, and the pain they caused. If you get canker sores regularly, checking your toothpaste ingredient list is one of the simplest things you can try. SLS-free options are widely available at most drugstores.
Nutritional Deficiencies
Low levels of certain vitamins and minerals are surprisingly common in people with recurrent canker sores. In one study of patients with frequent oral ulcers, about half were deficient in vitamin B12, 46% had low folate levels, and roughly 11% had low iron stores. These nutrients all play roles in maintaining healthy mucosal tissue and supporting normal immune function.
This doesn’t mean every canker sore signals a deficiency, but if you’re getting them frequently, it’s worth considering whether your diet provides enough leafy greens, legumes, eggs, meat, and fortified grains. A blood test can confirm whether a deficiency is contributing to your outbreaks.
Foods That Irritate the Mouth
Acidic and abrasive foods are common triggers. Citrus fruits (oranges, grapefruits, lemons, limes, pineapple), tomatoes, strawberries, and coffee all contain acids that can irritate the soft tissue inside your mouth and either cause new ulcers or worsen existing ones. Spicy foods containing hot peppers have a similar effect. Sodas combine high acid content with corn syrup, both of which promote inflammation in oral tissue.
Physically rough foods matter too. Chips, pretzels, nuts, and anything sharp or crunchy can scratch the inside of your mouth, creating the kind of minor injury that develops into an ulcer in susceptible people.
Food sensitivities play a role as well. Chocolate, gluten, and dairy are among the more common culprits. People with celiac disease or gluten sensitivity are particularly prone to canker sores, and for some, recurring ulcers are one of the first signs of an undiagnosed sensitivity.
Stress and Hormonal Changes
Emotional stress is one of the most frequently reported triggers, likely because stress hormones suppress parts of the immune system while activating others, tipping the balance toward the kind of inflammatory overreaction that produces ulcers. Many people notice canker sores appearing during exam periods, work deadlines, or other high-pressure stretches. Hormonal shifts during menstrual cycles also trigger outbreaks in some women, with sores appearing at predictable points in their cycle.
Genetics and Family History
If your parents get canker sores, your chances of getting them are significantly higher. The tendency toward recurrent canker sores runs in families, suggesting a genetic component to the immune response that drives them. Researchers have investigated specific genetic markers, but so far no single gene or marker has been definitively linked to canker sore susceptibility in the general population. The hereditary component is likely a combination of multiple genes influencing how your immune system behaves in the mouth.
When Canker Sores Signal Something Else
Occasional canker sores are extremely common and usually nothing to worry about. But frequent, severe, or unusually persistent ulcers can sometimes point to an underlying condition. Celiac disease, inflammatory bowel diseases like Crohn’s, and Behçet’s disease all feature mouth ulcers as a symptom. In Behçet’s disease, painful mouth sores that look identical to canker sores are the most common sign, typically healing in one to three weeks but returning repeatedly.
If your canker sores are unusually large, last longer than two to three weeks, come with other symptoms like joint pain or digestive issues, or are so frequent that you rarely have a break between outbreaks, those patterns are worth bringing up with a doctor.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different. Canker sores form inside the mouth, on the soft tissue of your cheeks, lips, tongue, or gums. Cold sores (fever blisters) appear on the outside of your mouth, typically around the lips. Cold sores are caused by herpes simplex virus (usually HSV-1), are contagious, and often start as a cluster of tiny blisters. Canker sores have no known infectious cause and cannot be spread to another person.