Canker sores are caused by a T-cell immune response that damages the lining of your mouth, but the triggers that set off that response vary widely, from nutritional gaps to stress to the toothpaste you use. About 20% of people get them repeatedly, and up to 25% of the global population deals with them at some point. Understanding the specific triggers can help you figure out why yours keep coming back.
The Immune Response Behind the Sore
A canker sore isn’t an infection. It’s your own immune system attacking the thin tissue inside your mouth. Specifically, certain white blood cells (T cells) mount an inflammatory response against the oral lining, releasing signaling molecules that break down tissue and create the open ulcer you see and feel. Research on gene activity in canker sore tissue shows a distinct inflammatory profile dominated by molecules like TNF-alpha and interferon-gamma, the same signals your body uses to fight viruses and bacteria, but misdirected at healthy tissue.
This is why canker sores look and behave the way they do: a shallow, painful crater with a white or yellowish center surrounded by a red border. The pain comes from exposed nerve endings in that eroded tissue, and the redness is inflammation. Most heal on their own within one to two weeks once the immune flare-up subsides.
Stress and Cortisol
Stress is one of the most commonly reported triggers, and the connection is measurable. In one study comparing people with recurrent canker sores to healthy controls, salivary cortisol (your body’s primary stress hormone) was more than three times higher in canker sore patients, even during periods when they didn’t have an active sore. Their anxiety scores were also significantly elevated, and the correlation between anxiety and cortisol was nearly perfect statistically.
This doesn’t mean stress directly burns a hole in your mouth. Instead, elevated cortisol appears to dysregulate the immune system in ways that make the mouth lining more vulnerable to that T-cell attack. If you notice canker sores clustering around exams, deadlines, or emotionally difficult periods, the pattern is real and physiologically grounded.
Vitamin and Mineral Deficiencies
Low levels of vitamin B12, folate, and iron are found in a meaningful percentage of people with recurring canker sores. Screening studies have identified nutritional deficiencies in roughly 14 to 18% of these patients, and research consistently shows that people with recurrent canker sores consume significantly less B12 and folate than people who don’t get them.
The good news: correcting these deficiencies through diet or supplementation often reduces how frequently canker sores appear. B12 is found primarily in animal products (meat, fish, eggs, dairy), while folate is abundant in leafy greens, legumes, and fortified grains. If you’re vegetarian, vegan, or eat a limited diet and get frequent canker sores, a deficiency is worth investigating with a simple blood test.
Genetics and Family History
About one-third of people with recurrent canker sores have a family history of them. This genetic component is linked to specific immune system markers (HLA types) that influence how your body distinguishes its own tissue from threats. If one or both of your parents dealt with frequent canker sores, you’re substantially more likely to get them too, and your outbreaks may start earlier in life.
Genetics also help explain why some people can eat all the acidic food they want and never get a sore, while others seem to develop one from a single orange. The underlying susceptibility varies from person to person, and triggers act on top of that baseline vulnerability.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), a foaming agent in most mainstream toothpastes, is a well-documented trigger. A systematic review of clinical trials found that switching to SLS-free toothpaste significantly reduced the number of canker sores, the duration of each episode, the total number of outbreaks, and pain levels. All four measures improved consistently across the studies reviewed.
SLS strips away the protective mucous layer inside your mouth, leaving the tissue more exposed to irritation and immune overreaction. If you get canker sores regularly, switching toothpaste is one of the simplest and most evidence-backed changes you can make. Several widely available brands market themselves as SLS-free.
Physical Trauma to the Mouth
Biting your cheek, poking yourself with a chip, getting jabbed by a toothbrush bristle, or rubbing against a rough orthodontic bracket can all trigger a canker sore in someone who’s predisposed. The mechanical damage breaches the surface of the oral lining, and in susceptible people, the immune system overreacts to the injury site instead of simply repairing it.
Dental work is another common culprit. The stretching, pressure, and minor tissue injuries from a cleaning or procedure can set off sores in the days that follow. If this happens to you consistently, it’s the trauma, not anything the dentist did wrong.
Food Triggers
Acidic foods like citrus fruits, tomatoes, pineapple, and vinegar-based dressings can irritate the delicate lining of the mouth and provoke sores in people who are prone to them. The acid doesn’t cause canker sores on its own, but it lowers the local pH enough to weaken the tissue surface, giving the immune system an opening to overreact.
Some people also report flare-ups from chocolate, coffee, strawberries, nuts, and cheese. These triggers are highly individual. Keeping a simple food diary when sores appear can help you identify your personal patterns over a few months.
Hormonal Fluctuations
Some women notice canker sores tied to their menstrual cycle, typically appearing in the days just before or during a period. This corresponds to the natural rise and fall of progesterone. In rare cases, the body develops a true hypersensitivity to its own progesterone, producing oral erosions that arrive with each cycle and resolve once menstruation ends. More commonly, the hormonal shift simply modulates the immune system enough to lower the threshold for an outbreak.
Pregnancy and menopause can also shift canker sore frequency in either direction. Some women find their sores disappear entirely during pregnancy, while others experience them for the first time.
Underlying Health Conditions
Frequent, severe, or unusually persistent canker sores can sometimes signal a systemic condition. Celiac disease, Crohn’s disease, and other inflammatory bowel conditions are associated with recurrent oral ulcers, sometimes appearing before any digestive symptoms do. In celiac disease, the connection likely involves both nutritional malabsorption (B12, folate, iron) and the broader immune dysregulation the condition causes.
Immune-suppressing conditions and certain medications can also increase canker sore frequency. If your sores are large (over a centimeter), take more than two weeks to heal, come in clusters of many at once, or are accompanied by fever, these patterns point toward something beyond ordinary canker sores and are worth a medical evaluation.
Three Types of Canker Sores
Not all canker sores are the same size or severity. Minor canker sores are the most common type: small (under 1 cm), shallow, and they heal without scarring. Major canker sores are larger and deeper, often taking weeks to resolve and sometimes leaving scars. Herpetiform canker sores appear as clusters of many tiny ulcers that can merge together. Despite the name, they have nothing to do with the herpes virus.
The type you get tends to be consistent. If yours are always small and heal within a week or two, that’s the minor form and the least concerning. Recurring major or herpetiform types are more disruptive and more likely to benefit from targeted treatment.