Canker sores on the tongue are caused by a combination of physical irritation, immune system activity, and individual triggers that vary from person to person. Unlike cold sores, which are viral, canker sores (called aphthous ulcers in medical settings) result from your own immune cells attacking a small patch of oral tissue. The tongue is one of the most common sites, accounting for about 17% of all traumatic oral ulcers, largely because it’s constantly in contact with teeth, food, and everything else in your mouth.
Physical Injury Is the Most Common Trigger
The single most frequent cause is mechanical trauma. Biting your tongue while eating or talking, scraping it against a chipped tooth, or irritating it with braces, retainers, or ill-fitting dentures can all break through the thin surface layer of tissue and set the stage for a canker sore. Teeth with sharp or jagged edges from decay, fractures, or rough dental fillings are repeat offenders. Even vigorous tooth brushing can do enough damage to start one.
Thermal and chemical injuries count too. Burning your tongue on hot coffee or soup damages the surface cells in a way that can develop into a full ulcer rather than simply healing over.
Certain Foods Irritate Vulnerable Tissue
Acidic foods like citrus fruits, tomatoes, and pineapple can erode the delicate lining of the tongue, especially if you already have a tiny nick or weak spot. Spicy foods containing capsaicin (the compound that makes chili peppers hot) trigger a more complex reaction: capsaicin activates pain receptors on the tongue’s surface, which causes local blood vessels to dilate and releases inflammatory signaling molecules. This creates a cascade of swelling and tissue irritation that, in susceptible people, can tip into ulcer formation.
Chocolate, coffee, cheese, nuts, and wheat are also commonly reported triggers, though the exact mechanism for each isn’t fully understood. If you notice a pattern between a specific food and sores appearing a day or two later, that food is likely a personal trigger worth avoiding.
Your Immune System Does the Actual Damage
Regardless of what starts the process, the sore itself forms because of an overactive immune response. In people prone to canker sores, a trigger sets off a wave of immune cells, primarily T-lymphocytes, that accumulate beneath the surface layer of the tongue. These cells release inflammatory chemicals that destroy the overlying tissue, creating the painful, open crater you recognize as a canker sore. This is why canker sores aren’t contagious: they’re driven by your own immune system, not by a virus or bacteria.
People who smoke actually tend to get fewer canker sores. Nicotine suppresses some of the inflammatory chemicals involved in ulcer formation. This isn’t a reason to smoke, but it illustrates how central the immune response is to the process.
Vitamin Deficiencies Play a Larger Role Than Most People Realize
If you get canker sores repeatedly, a nutritional gap may be involved. Studies consistently find that people with recurrent canker sores are more likely to have low levels of vitamin B12, folate (vitamin B9), and iron. One screening found nutritional deficiencies in roughly 14 to 18% of patients with recurring sores. In some cases, changes in the mouth’s lining, including ulcers, are the only visible sign of an early B12 or folate deficiency, appearing before any other symptoms show up.
Research also shows that people who get frequent canker sores tend to consume less B12 and folate in their diets compared to the general population. This means the issue isn’t always about absorption problems; sometimes it’s simply not eating enough of these nutrients. Good sources of B12 include meat, fish, eggs, and fortified cereals. Folate is found in leafy greens, legumes, and citrus. If your sores keep coming back, a blood test to check these levels is a reasonable step.
Stress and Hormonal Shifts
Stress is one of the most commonly reported triggers, and the connection has a biological basis. Acute stress raises cortisol and other hormones that alter immune function, potentially tipping the balance toward the inflammatory overreaction that produces ulcers. Many people notice sores appearing during exam periods, work deadlines, or emotionally difficult stretches rather than during the stressful event itself.
Hormonal fluctuations during the menstrual cycle also appear to play a role. Progesterone, which rises and falls throughout the cycle, acts as a natural immune suppressor and influences inflammatory pathways in the mouth. Women with recurrent canker sores have been found to have significantly different levels of progesterone and testosterone compared to women without them, suggesting that hormonal shifts can directly influence who gets sores and when.
Toothpaste Ingredient Linked to More Frequent Sores
Sodium lauryl sulfate (SLS), a foaming agent in most mainstream toothpastes, is a well-documented contributor. A systematic review of clinical trials found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, the duration of each ulcer, the number of episodes, and pain levels. SLS strips away the protective mucus layer on oral tissue, making the tongue and cheeks more vulnerable to irritation. If you’re prone to canker sores, switching toothpaste is one of the simplest changes you can make.
Underlying Health Conditions
Recurrent canker sores can sometimes signal a systemic condition. Celiac disease is one of the strongest associations: children with celiac disease develop canker sores at roughly four times the rate of healthy children, with about 42% affected compared to 15% of controls. In adults with celiac disease, the overall prevalence of canker sores is about 34%, compared to 13% in the general population. Importantly, mouth ulcers can appear even when a person has no digestive symptoms, meaning they may be the first clue that celiac disease is present.
Inflammatory bowel diseases like Crohn’s and ulcerative colitis share a similar link. Researchers believe these conditions and recurrent canker sores may share a common root cause: a dysregulation of the immune system’s response in mucosal tissue. Behçet’s disease, a rarer inflammatory disorder, also features recurrent oral ulcers as a hallmark symptom.
Three Types and How Long They Last
Not all canker sores are the same. Minor aphthous ulcers, which account for the vast majority of cases, are smaller than 10 millimeters across. You’ll typically have one to five at a time, and they heal within 10 to 14 days without scarring.
Major aphthous ulcers are larger than 10 millimeters and far more painful. These can persist for up to six weeks and sometimes leave scars. Herpetiform ulcers are a third type, named for their resemblance to herpes lesions (though they’re not caused by a virus). These appear as clusters of 10 to 100 tiny sores that can merge into larger irregular ulcers.
When a Sore Might Be Something Else
Most tongue ulcers are harmless canker sores, but it’s worth knowing the differences that distinguish them from oral cancer. Canker sores are painful from the start, have flat surfaces with red, inflamed edges, and heal within two to three weeks. Early oral cancers tend to be painless, may have a small lump or bump underneath the lesion that you can feel with your tongue or finger, and the surrounding tissue usually isn’t red or inflamed in the same way.
The clearest warning sign is duration. A canker sore that hasn’t healed after three weeks warrants a professional evaluation. The same applies to any sore that keeps growing, bleeds easily, or is accompanied by a hard lump in the tissue beneath it.