Canker sores form when your immune system attacks the thin lining inside your mouth, creating small, painful ulcers. Unlike cold sores, which are caused by a virus and appear on the outside of the lips, canker sores develop only on interior surfaces like the inner cheeks, tongue, or soft palate. The exact reason the immune system targets this tissue isn’t fully understood, but researchers have identified a clear chain of events and a long list of triggers that set it off.
What Happens Inside Your Mouth
The process starts when something provokes an immune response in the soft tissue lining your mouth. Your body sends specialized immune cells, particularly a type of T cell that destroys tissue on contact, to a specific spot on that lining. These cells begin breaking down the surface layer of tissue, and the damage is amplified by signaling molecules that recruit even more immune activity to the area. People with recurring canker sores have elevated blood levels of several of these inflammatory signals, which helps explain why some people are far more prone to them than others.
The result is a shallow crater in the tissue, typically less than 5 millimeters across, with a grayish-white center and a red border. That white or yellow film is a layer of protein and dead cells covering the raw tissue underneath, which is why it hurts so much when food, drinks, or your teeth make contact.
Common Triggers
No single cause has been identified, but several well-documented triggers can start the process.
Mouth injuries are one of the most common. Biting your cheek, scraping your gums on a sharp chip, or irritation from braces or ill-fitting dental appliances can all damage the tissue enough to trigger the immune overreaction. Even aggressive toothbrushing can do it.
Stress plays a measurable role. Psychological stress activates your body’s hormonal stress response, raising cortisol levels and shifting the balance of immune cells in your mouth. That combination of elevated stress hormones and disrupted local immunity creates conditions where ulcers are more likely to develop and progress.
Nutritional deficiencies show up frequently in people with recurring canker sores. In one study of 273 patients with recurrent ulcers, about 21% were anemic, 20% had low iron, nearly 5% were deficient in vitamin B12, and close to 3% were low in folic acid. Correcting these deficiencies often reduces how frequently ulcers return.
Your Toothpaste May Be a Factor
Sodium lauryl sulfate (SLS), a foaming agent in most commercial toothpastes, has a surprisingly direct connection to canker sore formation. SLS dissolves the protective mucus layer that coats the inside of your mouth. It denatures the proteins in that layer, strips away structural fats from cell surfaces, and penetrates into deeper tissue. The end result is an oral lining that’s thinner, more permeable, and more vulnerable to irritants.
In a double-blind clinical study, patients who brushed with SLS-containing toothpaste developed significantly more canker sores than those who used SLS-free alternatives. Several studies have also linked SLS toothpaste to greater pain intensity and longer healing times. Switching to an SLS-free toothpaste is one of the simplest changes you can make if you get canker sores regularly.
How They Develop and Heal
Most canker sores fall into one of three categories, and the timeline depends on which type you have.
- Minor ulcers are the most common. They’re under 5 millimeters wide, round, and heal within 10 to 14 days without scarring.
- Major ulcers exceed 1 centimeter in diameter and can appear on tougher tissue surfaces in the mouth. These can take up to 6 weeks to heal and often leave scars.
- Herpetiform ulcers (unrelated to herpes despite the name) start as clusters of tiny sores, sometimes up to 100 at once, that merge into large, irregularly shaped ulcers.
Most people experience the minor type. You’ll usually notice a tingling or burning sensation a day or two before the sore becomes visible. The pain peaks in the first few days, then gradually fades as the tissue rebuilds from the edges inward.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re fundamentally different. Cold sores (fever blisters) are caused by herpes simplex virus infection and appear as fluid-filled blisters on the outside of the mouth, usually around the lip border. They’re contagious. Canker sores form only inside the mouth, are not caused by a virus, and cannot be passed from person to person. The simplest way to tell them apart is location: outside the mouth means cold sore, inside means canker sore.
When Canker Sores Signal Something Deeper
Occasional canker sores are extremely common and rarely indicate a larger problem. But frequent or severe recurrences can be associated with systemic conditions. Celiac disease, Crohn’s disease, and Behçet syndrome all feature recurring mouth ulcers as a symptom. Immune deficiencies, including HIV, can also present this way. In children, a condition called PFAPA syndrome causes cyclical fevers alongside recurring canker sores and sore throats.
If you’re getting canker sores frequently, especially alongside other symptoms like digestive problems, joint pain, or recurring fevers, that pattern is worth investigating. Blood work to check for iron, B12, and folate deficiencies is a reasonable starting point, since those are both common and easy to correct.
Reducing Pain and Healing Time
There’s no cure for canker sores, but several approaches can shorten how long they last. Anti-inflammatory pastes applied directly to the sore are considered the most effective option for minor ulcers, reducing both pain and healing time. In one large study of a prescription paste, 21% of treated patients had complete healing at three days, compared to just 8% of untreated patients.
For prevention, the most practical steps are switching to SLS-free toothpaste, addressing any nutritional deficiencies, and minimizing mouth injuries by fixing dental appliances that rub or scrape. Managing stress won’t eliminate canker sores on its own, but given the documented hormonal pathway connecting stress to ulcer formation, it’s one more variable you can influence.