Canker sores form when your immune system mistakenly attacks the thin lining of your mouth, creating small, painful open wounds. Unlike cold sores, which are caused by a virus and appear on the outside of your lips, canker sores are not contagious and develop inside your mouth on soft tissue like your cheeks, tongue, and the floor of your mouth. The exact reason this happens varies from person to person, but it usually comes down to a combination of genetics, immune overreaction, and environmental triggers.
What Actually Happens Inside Your Mouth
A canker sore isn’t just surface irritation. It’s a localized immune attack on your own tissue. The process starts when certain immune cells in your mouth become overactivated, often in response to a trigger like a small injury or stress. White blood cells flood the area and release inflammatory signals that ramp up the attack instead of calming it down. At the same time, the anti-inflammatory signals that would normally keep things in check are unusually weak in people prone to canker sores.
The result is a destructive cycle. Immune cells kill off the surface cells lining your mouth, creating an open ulcer. Meanwhile, other immune cells release reactive oxygen species, essentially corrosive molecules that cause further damage to surrounding tissue. This is why canker sores can feel disproportionately painful for their size: the tissue destruction exposes nerve endings, and the ongoing inflammation keeps them irritated.
People who get recurrent canker sores also appear to have a mouth lining that is hypersensitive to normal bacteria. Their tissue overreacts to microbial signals that wouldn’t bother most people, which helps explain why sores can seem to appear without an obvious cause.
Common Triggers
Even if your immune system is primed for canker sores, something usually sets off each episode. The most well-documented triggers include:
- Physical injury: Biting your cheek, brushing too hard, dental work, or irritation from braces or a rough tooth edge can break the mucosal surface just enough to start the immune cascade.
- Stress: Emotional or physical stress is one of the strongest triggers. College students, for example, commonly develop canker sores during finals week.
- Certain foods: Chocolate, peanuts, and eggs are among the foods most often linked to flare-ups. Highly acidic foods like citrus fruits and tomatoes can also irritate the mouth lining enough to provoke a sore.
- Hormonal shifts: Some women develop canker sores on a predictable schedule tied to their menstrual cycle, suggesting that hormonal fluctuations affect the mouth’s vulnerability.
Nutritional Deficiencies Play a Bigger Role Than You’d Think
If you get canker sores frequently, a nutritional gap could be part of the picture. One study of patients with recurrent oral ulcers found that about 50% were deficient in vitamin B12, roughly 46% had low folate levels, and about 11% had low iron stores. These nutrients are critical for maintaining healthy mucosal tissue and a well-regulated immune response. When they’re low, the mouth lining becomes thinner and more fragile, and the immune system may behave more erratically.
This doesn’t mean a supplement will cure canker sores for everyone, but if you’re getting them often, it’s worth checking whether your diet is providing enough B12 (found in meat, fish, eggs, and dairy), folate (leafy greens, legumes, fortified grains), and iron.
Genetics and Family History
If your parents got canker sores, you’re more likely to get them too. A large genome-wide study identified 97 genetic variants that alter a person’s odds of developing mouth ulcers. Many of these variants affect genes involved in inflammation, specifically how your body produces and regulates inflammatory signals. Certain genetic profiles lead to higher levels of pro-inflammatory activity and lower levels of the anti-inflammatory signals that would normally prevent your immune system from attacking your own tissue.
Variations in genes related to serotonin transport also appear to influence susceptibility, which may partly explain the strong connection between stress and canker sore outbreaks.
Your Toothpaste Might Be Making It Worse
Sodium lauryl sulfate (SLS) is a foaming agent found in most mainstream toothpastes, and clinical evidence suggests it contributes to canker sores. A meta-analysis of randomized controlled trials found that people who switched to SLS-free toothpaste experienced about one fewer ulcer per outbreak, episodes that were nearly two days shorter, and significantly less pain compared to when they used standard toothpaste.
SLS strips away the protective mucus layer inside your mouth, leaving tissue more exposed to irritation. If you’re prone to canker sores, switching to an SLS-free toothpaste is one of the simplest changes you can make. Several major brands now offer SLS-free options.
Three Types of Canker Sores
Not all canker sores are the same, and the type you get determines how long you’ll be dealing with it.
Minor canker sores are the most common. They’re smaller than a centimeter across (roughly the size of a pea or smaller), shallow, and heal within a few weeks without leaving a scar. Most people who get canker sores only ever experience this type.
Major canker sores are larger than a centimeter, deeper, and significantly more painful. They can take months to heal and often leave scarring behind. These are less common but far more disruptive to eating and speaking.
Herpetiform canker sores are rare. Instead of one or two distinct ulcers, dozens of tiny pinpoint sores cluster together. Despite the name, they have nothing to do with the herpes virus. They typically heal within about two weeks without scarring.
When Canker Sores Signal Something Else
Occasional canker sores are normal and affect roughly 20% of the population at some point. But persistent or severe ulcers can be a sign of an underlying condition. Canker sores are a hallmark feature of Behçet’s disease, a systemic inflammatory disorder, and are part of its diagnostic criteria. They’re also associated with cyclic neutropenia (a condition where immune cell counts drop on a regular cycle), HIV, and PFAPA syndrome, a periodic fever condition most common in children.
Celiac disease and Crohn’s disease can also cause recurrent mouth ulcers, sometimes as the first noticeable symptom before gut problems appear.
Signs that warrant a closer look from a doctor or dentist include sores lasting longer than two weeks, new sores developing before old ones heal, sores that extend to the outer border of your lips, pain severe enough to prevent eating or drinking, or canker sores accompanied by a high fever. Unusually large sores or frequent outbreaks also merit attention, especially if they represent a change from your usual pattern.
Managing and Reducing Outbreaks
Most minor canker sores heal on their own, but there are practical steps to speed recovery and reduce how often they come back. Over-the-counter topical treatments containing a numbing agent can help manage pain during meals. Prescription-strength topical anti-inflammatory gels can reduce healing time for more severe sores, particularly the major type that would otherwise linger for weeks or months.
Beyond treating individual sores, prevention is about reducing your trigger exposure. Switching to an SLS-free toothpaste, managing stress, avoiding your personal food triggers, and ensuring adequate intake of B12, folate, and iron addresses the most controllable risk factors. If braces or a rough tooth edge are causing repeated trauma to the same spot, a dentist can smooth or adjust the hardware to stop the cycle.