A canker sore forms when your immune system attacks the thin lining inside your mouth, breaking down healthy tissue and leaving behind a shallow, painful ulcer. The process unfolds over several days, starting with a tingling sensation and progressing through tissue destruction before the body repairs the damage. Unlike cold sores, which are caused by a virus and appear on the outside of the lips, canker sores develop inside the mouth and are not contagious.
What Happens Inside Your Mouth
The formation of a canker sore is driven by your own immune cells turning against the tissue lining your cheeks, gums, or tongue. In people prone to canker sores, something triggers a specific type of white blood cell, called a T cell, to target the surface cells of the oral lining as though they were foreign invaders.
Once activated, these immune cells release signaling molecules that amplify the inflammatory response. This cascade increases the visibility of mouth-lining cells to the immune system, essentially painting a bigger target on them. A particular subset of T cells then directly kills the surface cells, creating the open wound you recognize as a canker sore. The destruction is directed and sustained by ongoing chemical signaling at the site, which is why the sore continues to develop over several days rather than appearing all at once.
The Three Stages of Formation
A canker sore goes through a predictable sequence from first sensation to full healing.
Prodromal Stage (Days 1 to 3)
Before any visible sore appears, you’ll feel a burning or prickling sensation in one spot inside your mouth. The area becomes tender and slightly raised, with redness developing on the surface. This is the immune response ramping up beneath the tissue. No ulcer is visible yet, but the process of tissue breakdown has already begun below the surface.
Ulcer Stage (Days 3 to 9)
By around the third day, the tissue breaks open into a characteristic yellow-gray ulcer surrounded by a red halo. The sore continues to enlarge over the next three to four days before stabilizing at its final size. Pain typically peaks during this stage, especially when eating acidic or salty foods. The ulcer stage lasts roughly three to six days, though it can stretch longer in some cases.
Healing Stage
Healthy tissue gradually closes over the sore from the edges inward. Minor canker sores, the most common type at 5 millimeters or smaller, heal within 7 to 14 days total. Major canker sores (larger than 10 millimeters, sometimes reaching 3 centimeters) can take several weeks. A less common type, called herpetiform canker sores, appears as clusters of pinpoint ulcers that may merge into a larger sore and typically heals in one to two weeks.
What Triggers the Process
The exact cause of canker sores remains unknown, but several triggers are well established. Most people who get recurring canker sores can identify at least one pattern.
Physical injury: Biting your cheek, scraping your gums with a chip, or irritation from braces or dental work can set off an ulcer in susceptible people. The minor wound itself isn’t the canker sore. Rather, the trauma activates the immune overreaction that produces one.
Toothpaste ingredients: Sodium lauryl sulfate, a foaming agent in many toothpastes, strips away the natural protective mucous layer inside the mouth. This leaves the tissue more vulnerable to irritation and small injuries that can trigger ulcer formation. Switching to an SLS-free toothpaste reduces outbreak frequency for some people.
Nutritional deficiencies: Low levels of certain nutrients are strikingly common in people with recurrent canker sores. In one study of 57 patients with recurring ulcers, about half were deficient in vitamin B12, nearly 46% had low folate levels, and 42% were anemic. Iron deficiency was less common at around 10%. These nutrients play roles in maintaining healthy mucosal tissue and immune regulation.
Stress: Emotional and physical stress are consistently linked to outbreaks, likely through their effects on immune function. Many people notice canker sores appearing during exam periods, after poor sleep, or during illness.
Hormonal shifts: Some women develop canker sores on a recurring schedule tied to their menstrual cycle. The pattern suggests that fluctuations in hormone levels affect the mouth’s vulnerability to immune-driven tissue breakdown.
Why Some People Get Them Repeatedly
Roughly 20% of the population experiences canker sores at some point, but a smaller group deals with them on a recurring basis. Genetics plays a role: if both your parents get canker sores, your chances are significantly higher. Researchers have investigated specific genetic markers, but findings have been inconsistent. One marker associated with a related condition called Behçet’s disease showed no increased prevalence in people with ordinary recurrent canker sores compared to the general population.
What does seem clear is that susceptibility involves an immune system that overreacts to minor provocations inside the mouth. For people without this tendency, biting their cheek produces a sore spot that heals uneventfully. For those prone to canker sores, the same minor injury can trigger the full inflammatory cascade that destroys surface tissue and creates an ulcer.
Canker Sores vs. Cold Sores
These two conditions are frequently confused, but they differ in almost every way. Canker sores form inside the mouth, on soft tissue like the inner cheeks, tongue, or soft palate. Cold sores appear outside the mouth, typically around the border of the lips. Cold sores are caused by herpes simplex virus type 1 and are contagious through direct contact. Canker sores have no known infectious cause and cannot be spread to another person. If your sore is inside your mouth and not on the gums near the teeth, it is almost certainly a canker sore.
Reducing Your Risk
Because canker sores stem from an immune response rather than an infection, prevention focuses on minimizing triggers. Switching to an SLS-free toothpaste removes one common irritant. Eating adequate B12, folate, and iron, whether through diet or supplements, addresses the nutritional gaps found in many recurrent sufferers. Leafy greens, legumes, eggs, and fortified cereals cover most of these bases.
Avoiding mechanical trauma helps too. If you’re prone to biting your cheek or tongue, orthodontic wax over braces or sharp tooth edges can reduce injury. Managing stress through sleep, exercise, or other strategies won’t eliminate canker sores entirely, but many people notice a drop in frequency when stress levels improve. For women who see a menstrual pattern, tracking the cycle and being extra cautious with oral irritants during vulnerable days can help.