Canker sores form when the thin lining inside your mouth breaks down, creating small, painful open wounds on your cheeks, lips, or tongue. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The frustrating truth is that no single cause explains every canker sore. Instead, a combination of physical triggers, nutritional gaps, stress, genetics, and immune responses can set them off.
Physical Trauma to the Mouth
The most straightforward trigger is injury to the soft tissue inside your mouth. Biting your cheek, poking yourself with a chip, or burning your mouth on hot food can all damage the delicate lining enough to start an ulcer. Dental work is another common culprit: rough edges on fillings, fractured teeth, poorly fitting dentures, and orthodontic brackets create repeated friction against the same spot. Even vigorous toothbrushing can do it.
These trauma-triggered sores usually heal on their own within a week or two once the source of irritation is removed. But if a sharp tooth edge or wire keeps rubbing the same area, the ulcer can persist or keep coming back in the same location.
Genetics Play a Bigger Role Than You’d Think
If you get canker sores frequently, look at your family. Children whose parents both get recurrent canker sores have about a 90% chance of developing them too. When neither parent has a history of them, that number drops to around 20%. This strong hereditary pattern suggests that some people inherit an immune response that overreacts to minor triggers, making their mouth lining more prone to breaking down.
Nutritional Deficiencies
Low levels of certain vitamins and minerals are linked to recurrent outbreaks. Deficiencies in iron, folate, vitamin B12, and other B vitamins (B1, B2, and B6) show up in 18 to 28% of people with recurring canker sores, compared to about 8% in healthy individuals. That’s a meaningful gap, though it also means the majority of canker sore sufferers have perfectly normal levels.
If your sores keep returning and you suspect a nutritional cause, a simple blood test can check for these deficiencies. Correcting them through diet or supplements often reduces the frequency of outbreaks for the people who are genuinely low.
Stress and Your Immune System
Stressful periods, whether from exams, work pressure, or emotional strain, are one of the most commonly reported triggers. The connection isn’t just psychological. Stress raises cortisol levels and increases the production of damaging molecules called reactive oxygen species in your body. Both of these weaken the protective barrier of your oral lining and make it easier for small irritations to turn into full ulcers.
This also explains why canker sores tend to cluster during difficult life periods and ease up when things calm down. Sleep deprivation, which raises stress hormones independently, compounds the problem.
Food and Drink Triggers
Certain foods don’t cause canker sores on their own, but they can irritate the mouth lining enough to trigger one in someone who’s already prone. The most common offenders are citrus fruits, tomatoes, strawberries, spicy foods, salty snacks, fizzy drinks, and alcohol. These foods are either acidic enough to erode the mucosal surface or abrasive enough to create tiny injuries that develop into sores.
Keeping a food diary when sores appear can help you identify your personal triggers. Not everyone reacts to the same foods, and avoiding a specific trigger can significantly reduce how often you get outbreaks.
Toothpaste Ingredients
Sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, has been studied as a possible trigger. A double-blind crossover study found that people who brushed with SLS-containing toothpaste developed significantly more canker sores than those using SLS-free alternatives. Two additional studies found that switching to SLS-free toothpaste reduced the number of ulcers, shortened healing time, and decreased pain intensity.
The evidence isn’t unanimous. A couple of studies found no difference between SLS and SLS-free formulas. Still, if you get frequent sores, trying an SLS-free toothpaste for a few months is a low-risk experiment worth running. Many brands now offer SLS-free options.
Underlying Health Conditions
Occasional canker sores are normal. But frequent, severe, or slow-healing ulcers can sometimes signal a deeper health issue. Conditions associated with recurrent canker sores include celiac disease, Crohn’s disease, Behçet’s syndrome, other auto-inflammatory conditions, and immune deficiencies. In these cases, the sores tend to be more persistent, more numerous, and less likely to resolve on their own with age.
Celiac disease is worth highlighting because it’s underdiagnosed. Damage to the gut from gluten sensitivity can impair absorption of iron, folate, and B12, which circles back to the nutritional deficiency pathway. Some people discover their celiac disease only after investigating why they keep getting canker sores.
How Canker Sores Differ From Cold Sores
People often confuse canker sores with cold sores, but they are completely different conditions. Canker sores form only inside the mouth, on the inner cheeks, lips, tongue, or soft palate. Cold sores (fever blisters) appear outside the mouth, typically around the border of the lips. Cold sores are caused by herpes simplex virus type 1 and are highly contagious. Canker sores have no known viral cause and cannot be passed from person to person.
If your sore is inside your mouth, looks like a shallow white or yellowish oval with a red border, and hurts when you eat or talk, it’s almost certainly a canker sore.
What Helps Them Heal Faster
Most canker sores heal on their own within one to two weeks. Over-the-counter topical pastes can speed that process. In one large study of a prescription paste applied two to four times daily, 21% of patients were fully healed by day three, compared to just 8% of untreated patients. That’s a real acceleration, even if most people will still need several days to a week.
Rinsing with warm salt water or a baking soda rinse several times a day can reduce pain and keep the area clean. Avoiding acidic, spicy, and crunchy foods while the sore is active prevents further irritation. For people who get frequent outbreaks, addressing the underlying triggers (stress management, nutritional correction, switching toothpaste, identifying food sensitivities) tends to be more effective than treating each sore individually.