Canker sores on the tongue are caused by an overactive immune response that breaks down the thin tissue lining your mouth. Unlike cold sores, they’re not caused by a virus and aren’t contagious. The frustrating reality is that no single cause explains every canker sore. Instead, a combination of triggers, from biting your tongue to running low on certain vitamins, sets off an immune reaction that eats through the surface tissue and leaves behind that painful, crater-like ulcer.
Your Immune System Turns on Your Own Tissue
At the cellular level, canker sores happen when your body’s immune cells mistakenly attack the lining of your mouth. Specific white blood cells called T cells destroy the surface tissue of your tongue, and the damage is kept going by a flood of inflammatory signaling molecules. People with active canker sores carry a higher proportion of a particular type of T cell compared to people without them, and these cells appear to drive a sustained assault on the oral lining.
One leading theory is that this immune attack is triggered by a common mouth bacterium. Your immune system may confuse proteins on the bacterium with proteins in your own mouth tissue, launching a friendly-fire strike. People prone to canker sores also show abnormal activity in one of the body’s key pathways for detecting bacteria, which may explain why their immune response overreacts in the first place.
Physical Triggers That Start the Process
The immune response needs a starting point, and physical trauma to the tongue is one of the most common. Accidentally biting your tongue, scraping it against a sharp tooth edge, or irritating it with braces or dental appliances can all create a small wound that spirals into a full canker sore in someone who’s prone to them. Even aggressive tooth brushing or eating hard, crunchy foods can be enough.
This is why canker sores tend to show up on the sides and tip of the tongue rather than the top surface. These are the areas most likely to rub against teeth or get caught between them during chewing.
Vitamin B12 Deficiency Is a Major Factor
Nutritional gaps play a surprisingly large role. In one study comparing people with recurrent canker sores to healthy controls, about 50% of those with canker sores were deficient in vitamin B12, while none of the control group were. That’s a striking difference. The deficient group had average B12 levels of just 124 pg/mL, well below the 220 pg/mL threshold considered normal.
Low folate levels have also been linked to canker sores, though the connection is less clear-cut. In the same study, folate deficiency appeared at similar rates in both groups. Iron deficiency is another commonly cited cause, but the evidence is mixed. If you’re getting canker sores frequently, a blood test checking B12 levels is a reasonable step. Vegetarians, vegans, and older adults are especially likely to run low on B12.
Your Toothpaste May Be Making It Worse
Sodium lauryl sulfate, or SLS, is a foaming agent found in most commercial toothpastes. It’s the ingredient that makes toothpaste lather. The problem is that SLS strips away the protective mucus layer inside your mouth, leaving the tissue underneath exposed and vulnerable. It’s also directly toxic to the cells responsible for tissue repair.
A 2019 review of four clinical trials involving 124 participants found that switching to SLS-free toothpaste significantly reduced the number of canker sores, how long they lasted, and how much they hurt. One of the trials reported a 64% reduction in canker sore occurrence just from changing toothpaste. If you’re dealing with recurring sores, this is one of the simplest changes you can make. SLS-free toothpastes are widely available at most pharmacies and grocery stores.
Food Sensitivities and Acidic Foods
Certain foods are well-known triggers for people who are susceptible. Citrus fruits, tomatoes, vinegar-based foods, and spicy dishes can all irritate the tongue’s lining enough to provoke a sore. These foods don’t cause canker sores in everyone, but if you notice a pattern between eating something acidic and developing a sore a day or two later, the connection is likely real. Avoiding these foods during an active outbreak also helps sores heal faster.
Stress and Hormonal Shifts
Emotional stress is one of the most frequently reported triggers, and it lines up with what we know about stress suppressing and then dysregulating immune function. Many people notice canker sores appearing during exam periods, work deadlines, or other high-stress stretches. Hormonal changes, particularly those tied to menstrual cycles, can also trigger outbreaks in some people. The pattern tends to be consistent: sores appear at the same point in the cycle each month.
Underlying Health Conditions
Frequent or severe canker sores can sometimes signal a deeper health issue. Celiac disease, an autoimmune reaction to gluten, is one of the most common underlying conditions. Some people discover they have celiac disease only after investigating why they keep getting mouth ulcers. Crohn’s disease and other inflammatory bowel conditions are also associated with recurrent canker sores, likely because of the same kind of immune system misfiring that affects the gut lining.
Behçet’s disease, a rarer condition that causes inflammation in blood vessels throughout the body, frequently presents with mouth ulcers as one of its earliest symptoms. HIV and other conditions that disrupt immune function can also cause persistent or unusually severe oral ulcers.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different. Canker sores appear inside the mouth, on your tongue, inner cheeks, or gums. Cold sores (fever blisters) appear outside the mouth, typically around the border of your lips. Canker sores look like a single round white or yellow sore with a red border. Cold sores are clusters of small, fluid-filled blisters. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are an immune response and cannot be passed to anyone else.
How Long They Last
Most canker sores on the tongue are the minor type, measuring less than about a centimeter across. These heal within two to three weeks without leaving a scar. Major canker sores are larger, deeper, and can take months to heal, sometimes leaving scar tissue behind. If you’re getting a canker sore that hasn’t healed within three weeks, that’s worth getting checked out.
When a Tongue Sore Isn’t a Canker Sore
Most tongue sores are harmless canker sores, but oral cancer can look similar in its early stages. There are a few key differences. Canker sores are usually painful from the start. Early oral cancers typically are not. Canker sores tend to be flat, while cancerous lesions often have a small lump or bump underneath that you can feel with your tongue or finger. The edges of a canker sore look red and inflamed, which isn’t usually the case with cancer.
Pay attention to any sore that grows larger over time, a white spot that turns red, or a lesion that starts bleeding when it didn’t before. A sore that persists beyond three weeks without healing deserves a professional evaluation, according to MD Anderson Cancer Center.