Mouth sores have dozens of possible causes, ranging from something as simple as biting your cheek to signs of a systemic disease. Most are harmless and heal on their own within one to two weeks, but a sore that lasts longer than 14 days warrants a dental or medical evaluation. Understanding the most common triggers can help you figure out what’s behind yours and whether you need to act on it.
Canker Sores
Canker sores (aphthous ulcers) are the most common type of mouth sore. They appear as small, round ulcers with a white or yellowish center and a red border, typically on the inner cheeks, lips, tongue, or soft palate. Unlike cold sores, they form inside the mouth and are not contagious.
The exact cause isn’t fully understood, but several well-established triggers can set them off. Psychological stress is one of the strongest, with flare-ups clustering around high-pressure periods like exams or work deadlines. Minor mouth injuries, like accidentally biting your tongue or irritation from braces, can spark one in people who are genetically prone. Acidic or spicy foods, food sensitivities, and toothpaste containing sodium lauryl sulfate (a common foaming agent) are also frequent culprits. Most canker sores resolve within 7 to 10 days without treatment, though larger ones can take longer and may scar.
Viral Infections
Several viruses target the mouth directly. The two you’re most likely to encounter are herpes simplex and the viruses behind hand, foot, and mouth disease.
Cold Sores (Herpes Simplex)
Cold sores are caused by herpes simplex virus type 1 (HSV-1), which most people pick up in childhood. The virus stays dormant in nerve cells and reactivates periodically, producing fluid-filled blisters that usually cluster on or around the lips. Triggers for reactivation include stress, sun exposure, fever, and a weakened immune system. The blisters break open, crust over, and heal in roughly 7 to 10 days. Cold sores are contagious from the tingling stage until the scab falls off.
Hand, Foot, and Mouth Disease
This illness, caused by coxsackievirus, is most common in children under five but can affect adults. It produces painful mouth sores that start as small red spots on the tongue and insides of the mouth, then blister and become quite painful. A rash on the hands and feet usually appears at the same time. The sores typically clear within a week to 10 days.
Fungal Infections
Oral thrush is a yeast overgrowth caused by Candida, a fungus that normally lives in your mouth in small amounts. When the balance tips, it produces creamy white patches on the tongue, inner cheeks, roof of the mouth, or gums. Scraping the patches can reveal raw, red tissue underneath that may bleed slightly.
Thrush is most common in babies, older adults, people with weakened immune systems, and anyone taking antibiotics or inhaled corticosteroids (which suppress the local immune environment in the mouth). In severe cases, the infection can spread to the throat and make swallowing painful. Treatment typically involves antifungal lozenges, tablets, or a liquid rinse. For breastfeeding pairs, both mother and baby usually need treatment at the same time to prevent passing the infection back and forth.
Nutritional Deficiencies
Your mouth’s lining renews itself quickly, so it’s one of the first places to show signs when your body is low on key nutrients. Deficiencies in iron, vitamin B12, and folate are the most closely linked to recurrent mouth sores and a sore, red tongue. These deficiencies can develop from a restrictive diet, absorption problems (like celiac disease), or heavy menstrual periods in the case of iron.
If you get mouth sores frequently and can’t pin them on an obvious trigger like stress or injury, it’s worth checking your nutrient levels through a simple blood test. Correcting the deficiency, whether through diet or supplements, often reduces or eliminates the sores entirely.
Medications and Cancer Treatment
Certain medications list mouth sores as a side effect. The most significant offender is cancer treatment. Oral mucositis, painful inflammation and ulceration of the mouth lining, is one of the most common side effects of chemotherapy drugs. It can make eating, drinking, and swallowing extremely difficult during treatment cycles. Many targeted cancer therapies also cause mouth inflammation, dry mouth, taste changes, and gum problems.
Outside of cancer treatment, medications that suppress the immune system (used for autoimmune conditions and organ transplants) can make the mouth more vulnerable to sores and infections. Some anti-inflammatory drugs and even certain blood pressure medications occasionally cause oral ulcers. If mouth sores appeared shortly after starting a new medication, that timing is worth mentioning to your prescriber.
Autoimmune and Systemic Diseases
Recurring mouth sores that don’t respond to typical treatments can sometimes signal a broader health condition. A few diseases are particularly known for this.
Behcet’s disease is a rare condition involving inflammation of blood vessels throughout the body. Painful mouth ulcers that look like canker sores are its most common symptom. They begin as raised, round lesions that quickly become deep, painful ulcers and tend to recur frequently. Behcet’s usually also causes genital ulcers, eye inflammation, and skin problems.
Celiac disease, an autoimmune reaction to gluten, frequently causes mouth sores as one of its non-digestive symptoms. People with undiagnosed celiac disease may get recurrent canker sores for years before the underlying condition is identified, especially if they don’t have obvious gut symptoms.
Lupus, inflammatory bowel disease (Crohn’s and ulcerative colitis), and reactive arthritis can also produce oral ulcers. In these cases, the mouth sores are a visible sign of the immune system’s broader misfiring, and they tend to improve when the underlying disease is managed.
Physical Trauma and Irritation
Sometimes the cause is purely mechanical. Braces, ill-fitting dentures, a sharp tooth edge, or even aggressive tooth brushing can create sores on the cheeks, gums, or tongue. Burns from hot food or drinks are another common culprit. These traumatic sores are usually easy to identify because they appear right at the point of contact, and they heal once the source of irritation is removed or the tissue adapts.
Habitual cheek biting or lip biting, often done unconsciously during stress, can create chronic sore spots that keep getting re-injured before they fully heal.
Relieving the Pain
Most mouth sores heal on their own, but the pain can make eating and talking miserable in the meantime. Over-the-counter numbing gels containing benzocaine (sold under brand names like Orajel and Anbesol) provide temporary relief by numbing the area directly. You apply the gel to the sore as needed, following the product’s directions. For canker sores specifically, prescription-strength topical corticosteroid pastes can reduce inflammation and speed healing.
Rinsing with warm salt water (about half a teaspoon of salt in a cup of warm water) several times a day helps keep the area clean and can soothe irritation. Avoiding acidic, spicy, or rough-textured foods while the sore is present makes a noticeable difference in comfort. For thrush or viral infections, the sores won’t resolve until the underlying infection is treated.
When a Mouth Sore Needs Attention
The 14-day rule is the key benchmark. A mouth sore that hasn’t healed after two weeks, or one that keeps coming back in the same spot, should be evaluated by a dentist or doctor. Persistent sores, especially painless ones, white or red patches that don’t scrape off, and unexplained lumps are all part of oral cancer screening criteria. This doesn’t mean every lingering sore is cancer, but it does mean it’s worth a professional look. A biopsy, where a tiny tissue sample is examined under a microscope, can rule out or confirm anything serious.
Other signs worth getting checked include sores accompanied by a high fever, difficulty swallowing, sores that spread rapidly, or frequent recurrences (more than three or four times a year) with no obvious cause.