Mouth sores most commonly turn out to be canker sores, which affect about 20% of the population and often appear without a clear reason. The good news is that most mouth sores are harmless and heal on their own within two weeks. But several different conditions can cause them, and knowing what you’re dealing with helps you manage the pain and recognize when something needs attention.
Canker Sores: The Most Likely Culprit
Canker sores (aphthous ulcers) are the most common type of mouth sore. They form inside the mouth, usually on the inner cheeks, lips, or tongue, and look like a single round white or yellow sore with a red border. They’re not contagious, and doctors still don’t fully understand why some people get them more than others.
Known triggers include biting your cheek or tongue, eating acidic foods like citrus or tomatoes, and emotional stress. Deficiencies in iron, vitamin B12, or folate also play a role. If your body is low on these nutrients, you may notice sores appearing more frequently, sometimes alongside fatigue or a sore, reddened tongue.
Minor canker sores typically heal in 10 to 14 days without treatment. Larger ones, called major aphthous ulcers, can take up to a month to resolve and tend to be significantly more painful. A third, rarer type appears as clusters of tiny sores that can merge into larger ulcers, making eating and speaking difficult. These can last anywhere from 10 days to over three months.
Cold Sores Are a Different Problem
Cold sores and canker sores are easy to confuse, but they’re completely different. Cold sores form on the outside of your mouth, usually on or around the lips, and appear as clusters of small, fluid-filled blisters. They’re caused by herpes simplex virus type 1 (HSV-1) and are highly contagious, especially when the blisters are open. Canker sores, by contrast, form inside the mouth, aren’t caused by a virus, and can’t be spread to anyone else.
If your sore is inside your mouth and looks like a shallow, round ulcer, it’s almost certainly a canker sore. If it’s a cluster of blisters on your lip, it’s likely a cold sore. The distinction matters because the treatments are different. Cold sores respond to antiviral treatments, while canker sores are managed with pain relief and topical products.
Your Toothpaste Might Be Making It Worse
One overlooked trigger is sodium lauryl sulfate (SLS), a foaming agent found in most toothpastes at concentrations between 1% and 3%. SLS works as a detergent to clean your teeth, but it can irritate the delicate tissue inside your mouth even in small amounts. It strips away the protective mucus layer lining your cheeks and gums, leaving the tissue more exposed to irritants from food and the toothpaste itself.
Research published in the American Journal of Dentistry found that SLS damages the outer layer of mouth tissue in a dose-dependent way. At concentrations above 0.5%, it causes visible destruction of surface cells. At 1% and 1.5%, it produced significantly more peeling of the inner mouth lining compared to SLS-free toothpaste. For people who already get canker sores, this erosion of the protective barrier makes the tissue even more vulnerable, potentially increasing how often sores appear.
If you get frequent canker sores, switching to an SLS-free toothpaste is one of the simplest changes you can try. Several brands market themselves as SLS-free, and they clean just as well without the foaming action.
Oral Thrush Looks Different
Not all mouth sores are ulcers. Oral thrush is a fungal infection caused by an overgrowth of yeast that naturally lives in your mouth. It produces red patches and creamy white sores that can appear on your tongue, inner cheeks, or the roof of your mouth. The white patches sometimes look like cottage cheese and may bleed slightly if you scrape them.
Thrush commonly shows up after a course of antibiotics, which kill off bacteria that normally keep yeast in check. It’s also more common in people with weakened immune systems, people using steroid inhalers for asthma, and those with dry mouth. Unlike canker sores, thrush requires antifungal treatment to clear up.
Conditions That Cause Recurring Sores
If mouth sores keep coming back, they could be a symptom of something happening elsewhere in your body. Several systemic conditions include recurrent oral ulcers as a hallmark feature:
- Celiac disease can cause mouth ulcers as one of its many symptoms, even before digestive problems become obvious. The nutrient malabsorption it causes, particularly of iron, folate, and B12, contributes to sore formation.
- Crohn’s disease and other inflammatory bowel conditions sometimes produce mouth sores as part of widespread inflammation affecting the digestive tract from end to end.
- Behçet’s disease is a rarer inflammatory condition where painful mouth sores resembling canker sores are the most common sign. These sores start as raised, round bumps that quickly become ulcers, typically healing in one to three weeks but returning frequently.
Recurring sores that appear alongside other symptoms, like joint pain, digestive issues, genital sores, or unexplained weight loss, are worth investigating with a doctor rather than writing off as ordinary canker sores.
Managing Pain While Sores Heal
Most canker sores heal without treatment, but that doesn’t mean you need to suffer through 10 to 14 days of pain. Over-the-counter topical products applied directly to the sore can numb the area and speed healing. Look for products containing benzocaine (sold as Anbesol or Orabase), which temporarily numbs the surface tissue. Antiseptic rinses with hydrogen peroxide can also help keep the sore clean and reduce irritation.
While you’re waiting for a sore to heal, avoid spicy, acidic, or crunchy foods that scrape against the ulcer. Rinsing with warm salt water several times a day can reduce inflammation and keep the area clean. Applying any topical product works best when you catch the sore early, ideally as soon as you feel that first tender spot forming.
Sores That Don’t Heal Need Attention
The American Dental Association recommends that any mouth sore lasting beyond 10 to 14 days without showing signs of healing should be evaluated by a dentist or doctor. Most sores that linger are still benign, but a non-healing sore is one of the earliest signs of oral cancer, and catching it early makes a significant difference in outcomes. This is especially important if the sore is painless, feels hard or thickened, or you have risk factors like tobacco use or heavy alcohol consumption. A sore that keeps growing, bleeds easily, or comes with unexplained numbness in your mouth or face also warrants a prompt evaluation.