Canker sores are triggered by a combination of immune system overreaction, physical irritation, dietary factors, stress, and nutritional deficiencies. Between 5 and 25% of the general population gets them repeatedly, with some studies estimating rates as high as 60% in certain groups. Most are small, heal on their own within two weeks, and are not contagious, but understanding what sets them off can help you reduce how often they appear.
How Canker Sores Form
At the cellular level, canker sores are the result of your immune system attacking the lining of your own mouth. White blood cells called T lymphocytes become activated, releasing inflammatory signals that damage the thin layer of tissue inside your cheeks, lips, gums, or tongue. This creates a shallow, painful crater, typically white or yellowish with a red border.
What makes canker sores frustrating is that no single cause explains them. Researchers describe them as a “multifactorial” problem, meaning several triggers stack on top of a genetic tendency. You might tolerate one trigger on its own but develop a sore when two or three overlap, like a stressful week combined with a mouth injury and poor sleep.
Physical Injury to the Mouth
Biting the inside of your cheek, poking your gum with a chip, brushing too aggressively, or getting scraped by braces or a rough dental appliance can all start the process. The minor wound doesn’t heal normally. Instead, your immune system overreacts to the damaged tissue, turning a small nick into a full ulcer. This is one of the most common and immediate triggers, and it explains why canker sores often show up in the exact spot where you bit yourself a day or two earlier.
Toothpaste With Sodium Lauryl Sulfate
Sodium lauryl sulfate (SLS) is a foaming agent in most toothpastes, and it irritates the delicate tissue inside your mouth. In one study, people who switched from a standard SLS-containing toothpaste to an SLS-free version saw their canker sore frequency drop by about 60%. During the three months before the study, participants averaged roughly 18 ulcers. After switching to the SLS-free product, that number fell to about 5.
If you get canker sores regularly, switching toothpaste is one of the simplest changes you can make. SLS-free options are widely available at most pharmacies and grocery stores.
Foods That Irritate Mouth Tissue
Certain foods provoke canker sores through direct chemical irritation or through sensitivity reactions. The most common culprits fall into a few categories:
- Citrus fruits like oranges, lemons, and grapefruits contain citric acid that can erode and irritate the soft tissue lining your mouth.
- Coffee contains salicylic acid, which can irritate your gums, cheeks, and tongue enough to trigger sore formation.
- Chocolate, nuts, and certain spices cause reactions in some people through food sensitivities rather than acidity alone.
- Tomatoes, strawberries, and pineapple combine acidity with rough texture, doubling the irritation potential.
Tracking which foods consistently precede your outbreaks is more useful than avoiding everything on a generic list. Many people find they can tolerate most acidic foods but have one or two specific triggers.
Stress and Hormonal Shifts
Emotional stress is one of the most frequently reported triggers, and the connection is biological, not imagined. Stress hormones like cortisol suppress parts of the immune system while activating others, creating the kind of immune dysregulation that drives canker sore formation. Students, for example, commonly report outbreaks during exam periods.
Hormonal changes during the menstrual cycle also play a role. Some women develop canker sores in the days before or during their period, along with swollen gums or increased gum bleeding. These symptoms typically resolve once the period ends. If they persist beyond that, the cause is likely something else worth investigating.
Nutritional Deficiencies
Your mouth lining turns over rapidly, replacing itself every one to two weeks, and it needs a steady supply of specific nutrients to do that properly. Deficiencies in iron, folate (vitamin B9), and vitamin B12 are the most closely linked to recurrent canker sores. Low zinc levels have also been implicated. These deficiencies can slow the repair of damaged tissue and alter immune function in ways that make ulcers more likely to form and slower to heal.
If your canker sores are frequent and you haven’t been able to identify an obvious trigger like food or toothpaste, a blood test checking these levels is a reasonable next step. Correcting a deficiency often reduces outbreak frequency significantly.
Underlying Health Conditions
Recurrent canker sores can be an early or even sole symptom of certain systemic diseases. About 5% of people with celiac disease develop canker sores as their only noticeable symptom, with no digestive complaints at all. Among people who get frequent canker sores, studies have found celiac disease in anywhere from 0 to 31% of cases, depending on the population studied.
Inflammatory bowel diseases also carry a strong connection. An estimated 20 to 30% of people with Crohn’s disease and about 10% of those with ulcerative colitis experience recurrent mouth ulcers. Behçet’s disease, a rarer inflammatory condition, lists canker sores as one of its hallmark symptoms.
This doesn’t mean frequent canker sores indicate a serious illness. Most of the time, they don’t. But if you’re getting large or unusually persistent sores, especially alongside other symptoms like fatigue, digestive issues, or joint pain, screening for these conditions is worthwhile.
Three Types, Three Different Experiences
Not all canker sores behave the same way. Minor aphthous ulcers, the most common type, measure less than 1 centimeter across and heal on their own in 10 to 14 days without scarring. These are the ones most people picture when they think of canker sores.
Major aphthous ulcers are larger than 1 centimeter, can last weeks to months, and often leave scars when they finally heal. These are significantly more painful and can interfere with eating and speaking. Herpetiform ulcers are a third type that appears as clusters of tiny pinpoint sores. Despite the name, they have nothing to do with the herpes virus. They typically resolve within 30 days and rarely scar.
Why Some People Get Them and Others Don’t
Genetics plays a meaningful role. If both of your parents get canker sores, your likelihood of getting them is substantially higher. The inherited component appears to involve how your immune system responds to minor tissue damage and irritation. Some people’s T cells simply overreact to stimuli that wouldn’t bother someone else’s mouth at all.
This genetic baseline helps explain why two people can eat the same acidic food, use the same toothpaste, and experience the same stress, yet only one develops a sore. Your triggers matter, but they operate on top of an underlying susceptibility that varies widely from person to person.