Canker sores develop when your immune system attacks the thin lining inside your mouth, breaking down tissue and creating a small, painful ulcer. About 20% of the general population gets them, most commonly during the teenage years and twenties. The process involves a chain reaction of immune cells and inflammation, but the original trigger varies from person to person.
What Happens Inside Your Mouth
A canker sore starts when something activates your immune system against your own oral tissue. The body’s infection-fighting T cells, which normally target viruses and bacteria, instead begin destroying the cells lining the inside of your mouth. These immune cells release inflammatory signals that recruit even more immune cells to the area, creating a cycle of tissue damage that sustains itself.
People who get recurrent canker sores have an abnormality in one of the immune system’s early-warning pathways, which causes an exaggerated inflammatory response. Their blood shows elevated levels of several inflammatory markers, including molecules that help immune cells stick to blood vessel walls and migrate into tissue. This means the immune system doesn’t just overreact locally. It’s primed for a stronger response throughout the body.
One leading theory points to a specific mouth bacterium. The immune system may mistake proteins in the oral lining for proteins on the surface of this bacterium, essentially launching a friendly-fire attack. The result is a round, white or yellow crater surrounded by a red, inflamed border, typically on the inner cheeks, lips, tongue, or soft palate.
What Triggers the Process
Physical trauma to the mouth is the most common starting point. Biting your cheek, scraping your gums with a sharp chip, aggressive tooth brushing, or irritation from dental work can all breach the mucosal barrier just enough to set off the immune cascade. For most people, a minor bite heals quietly. For those prone to canker sores, that same small injury triggers the exaggerated immune response that turns a tiny wound into a full ulcer.
Other well-established triggers include:
- Nutritional deficiencies. Low vitamin B12 is strongly linked to recurrent canker sores. In one study, half of patients with recurrent ulcers were deficient in B12, compared to none in the control group. Low folate levels were also common, appearing in about 46% of affected patients.
- Stress. Psychological stress suppresses parts of the immune system while amplifying inflammatory responses, making the mouth lining more vulnerable to breakdown.
- Hormonal changes. Some people notice canker sores at specific points in their menstrual cycle, suggesting hormone fluctuations play a role in mucosal vulnerability.
- Certain foods. Acidic or abrasive foods like citrus, tomatoes, and nuts are frequent triggers, likely because they irritate or mildly damage the oral lining.
- Sodium lauryl sulfate. This foaming agent found in many toothpastes can irritate the mouth lining and increase ulcer frequency in susceptible people.
Why Some People Get Them and Others Don’t
Genetics play a clear role. Researchers have identified specific immune-system genes that increase susceptibility, including a gene variant called HLA-DRB1*01:03 and variants near genes that control key inflammatory signals. Several of these same genetic markers also appear in Behçet’s disease, a more serious inflammatory condition where painful mouth ulcers are the most common symptom. If your parents got canker sores, you’re significantly more likely to get them too.
The genetic connection helps explain why canker sores aren’t purely about triggers. Two people can eat the same acidic meal or experience the same mouth injury, but only the genetically predisposed person develops an ulcer. Their immune system is wired to overreact to stimuli that others shrug off.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different conditions. Canker sores form inside the mouth and appear as a single round white or yellow sore with a red border. Cold sores (fever blisters) form outside the mouth, usually around the border of the lips, and look like clusters of small, fluid-filled blisters. Cold sores are caused by herpes simplex virus type 1 and are contagious. Canker sores are not caused by a virus and cannot spread to another person.
The Healing Timeline
Most canker sores are “minor” ulcers, less than about a centimeter across. These follow a predictable pattern: a tingling or burning sensation for a day or two, then the ulcer appears and peaks in pain around days three through five, then gradually heals over one to two weeks without scarring. During the painful phase, eating, drinking, and talking can all be uncomfortable, especially with acidic or spicy foods.
Major canker sores are larger, deeper, and can take six weeks or longer to heal, sometimes leaving scars. A third type, called herpetiform ulcers, appears as clusters of tiny pinpoint sores that can merge together. Despite the name, these have nothing to do with the herpes virus.
Conditions Linked to Frequent Canker Sores
If you’re getting canker sores repeatedly, especially more than three or four times a year, it may reflect something broader going on. Celiac disease can cause recurrent oral ulcers, sometimes as the only noticeable symptom. Inflammatory bowel diseases like Crohn’s are also associated with mouth ulcers. Behçet’s disease, which causes inflammation in blood vessels throughout the body, typically begins with mouth sores that look identical to ordinary canker sores but recur frequently and can appear alongside genital ulcers, skin lesions, and eye inflammation.
Iron deficiency, B12 deficiency, and folate deficiency can all drive recurrent ulcers on their own. In some cases, correcting the deficiency through diet or supplements reduces or eliminates the sores entirely. This is one of the more actionable causes to investigate if you’re dealing with frequent outbreaks, since a simple blood test can identify it.
Reducing How Often They Come Back
Because canker sores result from an immune overreaction rather than an infection, treatment focuses on calming inflammation and avoiding known triggers. Switching to a toothpaste without sodium lauryl sulfate reduces recurrence for many people. Keeping a food diary can help you identify which foods consistently precede an outbreak.
For sores that have already formed, over-the-counter topical gels containing a numbing agent can reduce pain. Rinsing with warm salt water or a baking soda solution (one teaspoon in half a cup of warm water) helps keep the area clean and may speed healing slightly. For severe or very frequent canker sores, prescription options include topical anti-inflammatory pastes and oral medications that modulate the immune response.
If your canker sores are unusually large, last longer than three weeks, spread in clusters, or come with fever, these patterns suggest something beyond a typical canker sore and warrant a closer look from a dentist or doctor.