A canker sore forms when your immune system attacks the thin lining inside your mouth, destroying a small patch of tissue and leaving behind a shallow, painful ulcer. Unlike cold sores, which are caused by a virus and appear on the outside of the lips, canker sores are not infections. They develop entirely inside the mouth, on the inner cheeks, lips, tongue, or soft palate, and they result from an overactive inflammatory response in tissue that’s already been weakened or irritated.
The Immune Response That Creates the Ulcer
Canker sores begin with an immune trigger. Something irritates or damages the soft tissue inside your mouth, and your body responds disproportionately. Specialized immune cells called T lymphocytes activate against proteins on the surface of your own mucosal cells, essentially treating normal mouth tissue as a threat. Once activated, these immune cells release a cascade of inflammatory signaling molecules that recruit even more immune cells to the area.
One key player in this process is TNF-alpha, an inflammatory molecule that ramps up the immune attack. TNF-alpha causes the surface cells of your mouth lining to display molecular “flags” that mark them for destruction. Other immune cells recognize those flags and launch a cytotoxic response, killing the marked cells. Additional inflammatory molecules amplify the damage: some drive excessive recruitment of immune cells to the site, while others work together with TNF-alpha to intensify the assault on the tissue. The result is a localized area of dead and eroded tissue, which is the ulcer itself.
This is why canker sores hurt so much relative to their size. The tissue destruction exposes nerve endings in the deeper layers of the mouth lining, and the ongoing inflammation keeps those nerves sensitized.
What Triggers the Process
No single cause explains every canker sore. In most people, ulcers result from a combination of a susceptible immune system and a triggering event. The most common triggers fall into a few categories.
Physical trauma is one of the most frequent. Biting the inside of your cheek, scraping your gums with a toothbrush, or rubbing from orthodontic brackets can all break the mucosal surface just enough to set off the immune cascade. For people prone to canker sores, even minor injuries that wouldn’t bother someone else can produce a full ulcer within a day or two.
Chemical irritation also plays a significant role. Sodium lauryl sulfate (SLS), a foaming agent found in many toothpastes, can damage the protective mucin layer that coats the inside of your mouth. SLS denatures the proteins in that layer and dissolves structural fats in epithelial cells, thinning the tissue and making it more vulnerable to irritants. At concentrations above 0.5%, SLS causes direct cell death in the upper layers of the mouth lining. Switching to an SLS-free toothpaste reduces canker sore frequency for many people.
Other recognized triggers include certain foods (citrus, tomatoes, chocolate, and spicy foods are commonly reported), emotional stress, hormonal shifts, and nutritional deficiencies in iron, zinc, B12, or folate. Some people notice ulcers appear during periods of sleep deprivation or illness, when the immune system is already dysregulated.
Why Some People Get Them and Others Don’t
Canker sores affect somewhere between 5% and 66% of the population depending on the group studied, making them one of the most common oral conditions. They tend to start in childhood or adolescence, and most people experience fewer outbreaks as they age.
Genetics clearly play a role, since canker sores run in families. Researchers have investigated specific immune-related genes, particularly one called HLA-B*51, but the results are inconsistent across populations. A study in Israeli patients found the gene in 23% of people with recurrent canker sores compared to 9% of controls, while studies in Turkish and Korean populations found no significant difference at all. This suggests that no single gene is responsible. Instead, susceptibility likely involves a combination of genetic factors that make certain people’s immune systems more reactive to minor oral injuries or irritants.
People with certain systemic conditions, including celiac disease, inflammatory bowel disease, and some vitamin deficiencies, experience canker sores at higher rates. In these cases, treating the underlying condition often reduces or eliminates the ulcers.
How a Canker Sore Progresses
Canker sores follow a predictable timeline with distinct phases. The first sign is a burning or tingling sensation at a specific spot inside the mouth, typically lasting one to two days before any visible ulcer appears. No blister or bump forms during this stage, which helps distinguish a developing canker sore from a cold sore (which starts as a fluid-filled blister on the outside of the mouth).
After the prodromal phase, the tissue breaks down and a shallow ulcer appears. It’s usually round or oval, white or yellowish in the center, and surrounded by a red border. Pain peaks during the first four to seven days and is often surprisingly intense for such a small wound. Eating, drinking, and talking can all aggravate it.
Healing begins on its own without any specific treatment. For the vast majority of canker sores, the entire process from first tingle to fully healed tissue takes about 10 days, with no scarring.
Minor, Major, and Herpetiform Types
Most canker sores are classified as minor. These measure less than one centimeter across (smaller than a pea) and heal within a few weeks without leaving a scar. They account for the large majority of cases.
Major canker sores are bigger than one centimeter, significantly more painful, and can take weeks to months to fully heal. They often penetrate deeper into the tissue and frequently leave scars. People who get major canker sores tend to have them recurrently and may have overlapping ulcers at different stages of healing.
Herpetiform canker sores are the rarest type. Despite the name, they have nothing to do with the herpes virus. They appear as clusters of tiny pinpoint sores that can merge into larger irregular ulcers. They typically heal within about two weeks without scarring.
Canker Sores vs. Cold Sores
The simplest way to tell the difference is location. Canker sores form only inside the mouth, on the inner cheeks, lips, tongue, or soft palate. They appear as white or yellow open sores. Cold sores (fever blisters) form on the outside of the mouth, usually around the border of the lips, and start as fluid-filled blisters before crusting over.
Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not caused by any virus, are not contagious, and cannot spread to other people through kissing or sharing utensils.
Reducing Frequency and Severity
Because canker sores involve an overactive immune response to local irritation, prevention focuses on minimizing triggers. Switching to a toothpaste without sodium lauryl sulfate is one of the most effective steps, since SLS directly damages the protective lining of the mouth. Using a soft-bristled toothbrush and being careful with sharp or crunchy foods helps reduce mechanical trauma.
Keeping a food diary can help identify dietary triggers, since the connection between specific foods and canker sores varies widely between individuals. Addressing any underlying nutritional deficiencies, particularly B12, iron, and folate, can reduce recurrence in people whose levels are low.
For sores that have already formed, over-the-counter topical gels containing numbing agents can reduce pain during the most uncomfortable days. Rinsing with warm salt water several times a day helps keep the area clean and can speed healing slightly. Most canker sores resolve completely on their own, but sores that last longer than three weeks, grow unusually large, or come with fever may warrant a closer look from a healthcare provider.