Canker sores form when the immune system attacks the thin lining of your mouth, breaking down tissue and creating a small, painful ulcer. Between 5 and 25% of the general population gets them, with some studies reporting rates as high as 60% depending on the group studied. The process unfolds over several days, starting well before the visible sore appears, and involves a combination of immune activity, physical irritation, and individual susceptibility factors that researchers are still piecing together.
The Three Stages of Formation
A canker sore doesn’t appear overnight. It develops through a predictable sequence that takes roughly a week from first sensation to full ulcer.
The first phase is the prodromal stage, lasting one to three days before any sore is visible. You’ll feel a burning or prickling sensation in a specific spot inside your mouth, and the area becomes raised and reddened. Nothing looks particularly alarming yet, but the tissue is already under attack beneath the surface.
By around the third day, the ulcer has fully formed. The damaged tissue breaks down into the classic canker sore: a shallow, yellow-gray crater ringed by a red halo. This ulcer stage lasts three to six days and is typically the most painful period. The exposed tissue underneath is rich in nerve endings, which is why even mild contact with food or your tongue can sting intensely.
The healing stage follows, as healthy tissue gradually closes over the sore. Pain fades as the ulcer shrinks. For minor canker sores (the most common type, under 1 cm across), the whole cycle from first tingle to full healing takes about 10 to 14 days without treatment.
What Happens Inside the Tissue
The core problem is an overreaction by your immune system. White blood cells, particularly certain T cells, become activated and target the cells lining the inside of your mouth. These immune cells release inflammatory signals that damage the thin oral mucosa, essentially dissolving a patch of tissue and leaving the raw ulcer behind.
Psychological stress appears to be one pathway into this process. Stress hormones can alter immune function by shifting the balance of T cells, B cells, natural killer cells, and key chemical messengers like adrenaline and dopamine. A study measuring cortisol in saliva found that canker sore patients showed higher rates of anxiety and depression, though cortisol levels themselves weren’t consistently elevated. The connection between stress and canker sores likely runs through multiple nervous and immune system pathways rather than a single hormone.
Triggers That Start the Process
Most canker sores result from a combination of a susceptible person and something that tips their oral tissue over the edge. The triggers fall into a few broad categories.
Physical damage is the most straightforward. Biting the inside of your cheek, brushing too hard, orthodontic wires scraping against tissue, or even rough-textured food can break the mucosal surface just enough to trigger the immune cascade in someone prone to these sores.
Chemical irritation plays a role too. Sodium lauryl sulfate (SLS), a foaming agent in many toothpastes, strips away the protective mucin layer that coats the inside of your mouth. Research has documented that SLS causes sloughing of oral tissue, increased permeability of the lining, separation of surface cell layers, and protein breakdown in the mucosa. By damaging this protective barrier, SLS exposes the underlying tissue and can initiate ulcer formation. Switching to an SLS-free toothpaste is one of the simpler interventions people try, and some find it reduces how often sores recur.
Nutritional gaps weaken the body’s ability to maintain healthy oral tissue. Deficiencies in iron, vitamin B12, and folate are all linked to higher rates of canker sores. In one study comparing patients with recurrent mouth ulcers to healthy controls, significantly more ulcer patients had low levels of hemoglobin, iron, vitamin B12, and folic acid. These nutrients are essential for healthy cell turnover, and the mouth’s lining replaces itself every one to two weeks, making it especially sensitive to shortfalls.
Why Some People Get Them Repeatedly
Genetics is a major factor. Researchers have identified specific immune-related gene variants that increase susceptibility. Certain HLA alleles (the genes that help your immune system distinguish your own tissue from foreign invaders) are significantly associated with developing recurrent canker sores. Other HLA variants appear to protect against them. If your parents got frequent canker sores, you’re substantially more likely to as well.
This genetic component explains why the condition peaks in the second decade of life and why some people get sores monthly while others never experience one. Your immune system’s baseline programming determines how aggressively it responds to minor oral trauma or irritation. For people with the susceptible gene variants, even small provocations can set off the tissue-destroying immune response.
Three Types, Different Severity
Not all canker sores are equal. They fall into three categories based on size and behavior.
- Minor canker sores account for the vast majority of cases. They’re small (under 1 cm), shallow, and heal within 10 to 14 days without scarring.
- Major canker sores are larger and penetrate deeper into the tissue. They can take up to a month to heal and are more likely to leave scars.
- Herpetiform canker sores appear as clusters of many small sores and can persist anywhere from 10 to 100 days. Despite the name, they have nothing to do with the herpes virus.
Canker Sores vs. Cold Sores
People frequently confuse these two conditions, but they’re fundamentally different. Canker sores are not caused by a virus. They form only on the soft, non-keratinized tissue inside the mouth: inner cheeks, inner lips, the floor of the mouth, the soft palate, and the sides or underside of the tongue. Cold sores, caused by the herpes simplex virus, typically appear on the lips, hard palate, gums, and outer mouth, and they’re contagious. Canker sores are not.
There’s no lab test or biopsy that definitively confirms a canker sore. Diagnosis is based on appearance and ruling out other causes. If sores always appear inside the mouth on soft tissue, don’t spread to others, and follow the prodromal-to-healing pattern described above, they’re almost certainly canker sores. Herpes lesions tend to appear on one side only, affect the gums and hard palate, and may come with fever or swollen lymph nodes, especially during a first outbreak.