Mouth sores have dozens of possible causes, ranging from something as simple as biting your cheek to underlying health conditions that need treatment. Most are harmless and heal on their own within one to three weeks, but understanding what triggers them helps you avoid repeat episodes. Canker sores alone affect between 5% and 25% of the general population, with some studies estimating rates as high as 60% in certain groups.
Canker Sores: The Most Common Type
Canker sores (aphthous ulcers) are small, shallow ulcers that form inside the mouth, on the tongue, inner cheeks, or gums. They’re not contagious. The exact mechanism behind them isn’t fully understood, but several well-established triggers set them off.
Physical trauma is one of the most frequent. Biting the inside of your cheek, brushing too aggressively, or wearing braces or ill-fitting dental appliances can damage the soft tissue enough to spark an ulcer, especially in people who are genetically prone. About one-third of people with recurring canker sores have a family history of them, and specific inherited immune markers appear to increase susceptibility.
Stress is another reliable trigger. Studies have documented spikes in canker sore outbreaks during high-pressure periods like exam seasons. Hormonal shifts also play a role: some women develop mouth ulcers during the second half of their menstrual cycle, when progesterone levels drop. Interestingly, quitting smoking is also a known trigger, though the reasons aren’t entirely clear.
Nutritional Deficiencies
If you get mouth sores frequently and can’t pin down an obvious trigger, a nutritional gap may be the cause. Low levels of vitamin B12, folate (vitamin B9), iron, and zinc are all linked to recurring oral ulcers. A B12 or folate deficiency can also cause a sore, red tongue alongside the ulcers. These deficiencies are common enough that doctors often check for them when someone reports chronic mouth sores. Correcting the deficiency, whether through diet or supplements, typically reduces or eliminates the problem.
Cold Sores and Other Viral Causes
Cold sores look and feel different from canker sores. They appear on or around the lips as clusters of small, fluid-filled blisters and are caused by herpes simplex virus type 1 (HSV-1). The first outbreak tends to be the worst, sometimes causing widespread sores across the gums and mouth in a condition called gingivostomatitis. After that, the virus stays dormant in your body and can reactivate periodically, often triggered by illness, sun exposure, or stress.
Other viruses cause mouth sores too. Coxsackieviruses, which spread through contaminated hands and surfaces, are responsible for hand, foot, and mouth disease. This is especially common in young children and produces painful sores inside the mouth along with a rash on the hands and feet. Bacterial infections from streptococcus (the same bacteria behind strep throat) can also cause inflammation and sores in the mouth.
Foods and Allergens That Irritate the Mouth
Certain foods are well-known mouth sore triggers. Highly acidic fruits like oranges, lemons, and tomatoes can irritate the lining of your mouth, particularly if you already have a small wound or are prone to canker sores. Spicy foods do the same.
Some people experience a condition called oral allergy syndrome, where eating certain raw fruits or vegetables causes tingling, swelling, or sores in the mouth. This happens because proteins in those foods resemble pollen proteins your immune system already reacts to. If you’re allergic to birch pollen, for example, apples, cherries, peaches, carrots, and almonds can trigger a reaction. Grass pollen allergies are linked to reactions from kiwi, melon, and tomatoes. Ragweed allergies can make bananas, cucumbers, watermelon, and cantaloupe problematic.
Ingredients in oral care products can also be culprits. Flavorings like cinnamon (cinnamaldehyde), menthol, and peppermint oil found in toothpastes and mouthwashes occasionally cause a reaction called allergic contact stomatitis, producing sores or irritation inside the mouth. Sodium lauryl sulfate, a foaming agent in many toothpastes, is another common irritant for people who get frequent canker sores.
Medications That Cause Mouth Sores
Several categories of medication can produce sores or other damage inside the mouth. Chemotherapy is the most well-known cause, frequently leading to oral mucositis, a painful inflammation of the mouth lining that can make eating and drinking difficult during treatment.
Beyond chemotherapy, common pain relievers like aspirin and other anti-inflammatory drugs can irritate oral tissue, particularly if tablets are held against the gums or cheek rather than swallowed. Blood pressure medications, including certain ACE inhibitors, sometimes cause swelling of the lips or mouth. Some anti-seizure medications and antibiotics carry a small risk of triggering Stevens-Johnson syndrome, a serious immune reaction that causes widespread blistering of the skin and mucous membranes, including inside the mouth. Bone-strengthening drugs used for osteoporosis can, in rare cases, cause breakdown of the jawbone tissue that becomes visible through the mouth lining.
Thrush and Yeast Overgrowth
Oral thrush is a fungal infection that produces creamy white patches on the tongue, inner cheeks, and roof of the mouth. It’s caused by an overgrowth of yeast that normally lives in your mouth in small amounts. Thrush is most common in babies, older adults, people with weakened immune systems, and anyone taking antibiotics or inhaled corticosteroids (such as asthma inhalers). The patches can be sore and may bleed slightly when scraped.
Autoimmune and Chronic Conditions
Recurring mouth sores that don’t respond to typical treatments can sometimes signal a systemic health condition. Behçet’s disease causes painful ulcers that look like canker sores but tend to be larger and come back frequently, typically healing in one to three weeks before reappearing. Celiac disease, an immune reaction to gluten, often produces mouth ulcers as one of its earliest symptoms. Crohn’s disease and other inflammatory bowel conditions can cause sores anywhere along the digestive tract, including the mouth. Lupus can also produce oral ulcers, often on the roof of the mouth.
In these cases, the mouth sores are a symptom of the broader condition, and treating the underlying disease is what brings lasting relief.
How to Tell if a Mouth Sore Needs Attention
Most mouth sores heal within three weeks. If yours lasts longer than that, it’s worth getting evaluated. You should also pay attention if new sores keep appearing before old ones heal, if pain doesn’t respond to over-the-counter treatments, or if you develop a fever or diarrhea alongside the sores.
Oral cancer is rare, but it can start as a sore that simply won’t heal. The key differences: canker sores hurt from the beginning and get better, while cancerous lesions often start painless and develop pain over time that doesn’t go away. Watch for red, white, or mottled patches in your mouth, a lump or thickening in your cheek or under the skin near a sore, texture changes like rough or crusty areas, unexplained loosening of teeth, numbness in the tongue, or persistent swelling in the neck. Unintentional weight loss and chronic bad breath despite good hygiene are also warning signs. Any sore that changes in appearance but doesn’t heal within two to three weeks deserves a professional look.