Canker sores form when your immune system attacks the thin tissue lining your mouth, creating small, painful open wounds. Unlike cold sores, they aren’t caused by a virus and aren’t contagious. The triggers range from something as simple as biting your cheek to underlying nutritional deficiencies, and roughly 20% of the general population deals with them repeatedly.
What Actually Happens Inside Your Mouth
A canker sore starts when certain immune cells, specifically a type of white blood cell called T cells, begin destroying the surface layer of your oral tissue. These cells release inflammatory signals that sustain the damage, essentially keeping the wound open and raw. In people who get canker sores frequently, this immune response appears to be abnormally aggressive. Their T cells may be reacting to a common mouth bacterium in a way that cross-reacts with the body’s own tissue, causing the immune system to mistakenly target the mouth lining itself.
This is why canker sores aren’t just surface scratches. They’re the result of an active inflammatory process underneath the tissue, which is also why they hurt so much relative to their size and why they take time to resolve on their own.
Genetics Play a Major Role
If both of your parents get canker sores, you have about a 90% chance of getting them too. If neither parent is affected, that number drops to around 20%. This strong hereditary pattern suggests that susceptibility is largely built into how your immune system is wired. Some people simply have a mouth lining that’s more reactive to irritation and a more hair-trigger inflammatory response in their oral tissue.
This genetic component explains why some people can eat acidic food, bite their cheek, and brush aggressively without ever getting a sore, while others seem to develop one from the slightest provocation.
Physical Triggers That Start the Process
The most common immediate cause is mechanical trauma to the inside of your mouth. This includes accidentally biting your cheek or tongue, irritation from braces or dental appliances, aggressive tooth brushing, or even the friction of a rough chip or crusty bread scraping against your gums or inner lip. These small injuries break through the protective surface layer, and in people who are prone to canker sores, the immune system overreacts to the damage instead of quietly healing it.
Dental work is another frequent trigger. The stretching, pressure, and minor abrasions from a dental visit can set off a sore in susceptible people within a day or two.
Your Toothpaste Could Be Making It Worse
Sodium lauryl sulfate (SLS), a foaming agent found in most toothpastes, is a well-documented trigger. A systematic review pooling data from multiple trials found that switching to an SLS-free toothpaste reduced the number of canker sores by about one ulcer per outbreak period, shortened healing time by roughly two days, and significantly decreased pain. The reduction was consistent across all measures of ulcer severity.
SLS strips away the protective mucous layer inside your mouth, leaving the tissue more vulnerable to irritation. If you get canker sores regularly, switching to an SLS-free toothpaste is one of the simplest changes you can make. Several major brands sell SLS-free versions, and they’re easy to find in most drugstores.
Foods That Trigger or Worsen Sores
Acidic foods and drinks are among the most reliable dietary triggers. Citrus fruits, tomatoes, strawberries, fizzy drinks, and alcohol all lower the pH inside your mouth. This disrupts the protective protein layer coating your oral tissue, leaving it more exposed and more likely to break down into a sore. Spicy and salty foods irritate the delicate mucous membrane directly, which can be enough to trigger an ulcer on its own or intensify one that’s already forming.
If you already have a canker sore, hard, crunchy, or sharp-textured foods like crusty bread, tortilla chips, and raw vegetables will aggravate it mechanically. Softer, cooler, and blander foods are easier on the healing tissue.
Nutritional Deficiencies Behind Recurring Sores
People who get canker sores repeatedly often turn out to be low in specific nutrients. In one study of 40 people with recurrent canker sores, 75% were deficient in vitamin B12, folate, or both. Iron deficiency is another common finding. These nutrients are essential for healthy cell turnover in the mouth lining, and when levels drop too low, the tissue becomes thinner, more fragile, and slower to repair itself.
This connection is worth paying attention to if your canker sores are frequent or seem to come in clusters. A simple blood test can check your B12, folate, and iron levels. In many cases, correcting the deficiency with supplements or dietary changes reduces how often sores appear. Good dietary sources of these nutrients include leafy greens, legumes, eggs, meat, and fortified cereals.
Stress and Hormonal Shifts
Emotional stress is one of the most commonly reported triggers, and it tracks with what we know about how stress hormones affect the immune system. Stress can both suppress and dysregulate immune function, tipping the balance toward the kind of overreactive inflammatory response that produces canker sores. Many people notice sores appearing during exam periods, after a major life event, or during stretches of poor sleep.
Hormonal changes, particularly around menstruation, are another recognized trigger. Some women notice a pattern of sores appearing at a specific point in their cycle, likely related to the way fluctuating hormone levels influence the immune response in mucosal tissue.
When Canker Sores Signal Something Else
Occasional canker sores are normal and not a sign of disease. But frequent, severe, or unusually persistent sores can sometimes point to an underlying condition. Celiac disease, Crohn’s disease, and Behçet’s syndrome all feature recurrent oral ulcers as a symptom. In celiac disease, the connection is often through nutrient malabsorption: the damaged intestine can’t properly absorb B12, folate, and iron, which then shows up as chronic mouth sores along with other symptoms.
If your sores are constant, unusually large, take weeks to heal, or come with other symptoms like digestive problems, joint pain, or skin lesions, it’s worth investigating further. In people with HIV, canker sores can take months to heal rather than the typical week or two.
Three Types and How They Differ
Not all canker sores are the same. Minor canker sores, the kind most people get, are less than a centimeter across and shallow. They heal on their own within one to two weeks without scarring. These account for the vast majority of cases.
Major canker sores are larger, deeper, and considerably more painful. They can take weeks or even months to heal and sometimes leave scars. Herpetiform canker sores are a third type that appear as clusters of many tiny ulcers, sometimes dozens at once, that can merge into larger irregular sores. Despite the name, they have nothing to do with the herpes virus.
Reducing How Often They Come Back
Because canker sores arise from a combination of genetic predisposition and environmental triggers, management is mostly about identifying and minimizing your personal triggers. A few practical steps that consistently help:
- Switch to SLS-free toothpaste to reduce chemical irritation to your mouth lining.
- Track dietary triggers like citrus, tomatoes, spicy foods, and alcohol to see if a pattern emerges.
- Check for nutritional gaps in B12, folate, and iron, especially if sores are frequent.
- Use a soft-bristled toothbrush and brush gently to avoid micro-injuries to your gums and cheeks.
- Manage stress through whatever works for you, since the stress-to-sore connection is real and well-documented.
Over-the-counter topical treatments containing numbing agents can reduce pain while a sore heals. Antimicrobial mouth rinses may also help by keeping the wound clean and reducing secondary irritation. For people with severe or constant outbreaks, prescription options are available that target the inflammatory process more directly.