Mouth sores are most commonly caused by minor tissue injury, viral infections, or canker sores with no identifiable cause. Less often, they signal a nutritional deficiency or an underlying health condition. The type of sore, where it appears, and how long it lasts all point to different causes.
Canker Sores: The Most Common Culprit
Canker sores are painful white or yellow ulcers that form inside the mouth, typically on the inner cheeks, lips, or tongue. They don’t have a single known cause, but researchers believe a combination of triggers sets them off. Common ones include:
- Minor mouth injuries from dental work, aggressive brushing, sports, or accidentally biting your cheek
- Food sensitivities to chocolate, coffee, strawberries, eggs, nuts, cheese, and spicy or acidic foods like pineapple, grapefruit, and oranges
- Emotional stress
Most canker sores are minor, smaller than a pea, and heal within about two weeks without scarring. The pain typically improves within a few days even without treatment. Major canker sores, which are larger than one centimeter, can take months to heal and often leave scars. A rarer type called herpetiform canker sores appear as clusters of tiny pinpoint ulcers and generally heal within two weeks.
Salty foods, chips, pretzels, and acidic fruits tend to make canker sores worse once they’ve formed. If you get them frequently, keeping a food diary can help you identify your personal triggers.
Cold Sores: A Viral Infection
Cold sores (also called fever blisters) are painful, fluid-filled blisters that form on the outside of the mouth, usually around the border of the lips. They’re caused by herpes simplex virus type 1, or HSV-1. The easiest way to distinguish them from canker sores is location: cold sores appear outside the mouth, canker sores only appear inside.
After the initial infection, the virus stays in your body permanently and can reactivate in response to stress, fever, trauma, or hormonal changes. Before a cold sore appears, the skin often feels itchy or tingly. Multiple small blisters then form, sometimes merging together into a larger sore. An outbreak typically lasts 10 to 14 days.
Herpes can occasionally cause sores inside the mouth as well, but when it does, they appear on the gums or the hard palate rather than on the soft inner cheeks where canker sores show up. These intraoral herpes sores look like small clustered ulcers because the blisters break quickly.
Your Toothpaste May Be Making It Worse
Sodium lauryl sulfate (SLS), a foaming agent found in most commercial toothpastes, can irritate the delicate lining of the mouth and increase the frequency of canker sores. Research published in the British Dental Journal found that SLS can cause peeling of the mouth’s inner lining by stripping away its protective layers. One clinical study found four times fewer soft tissue lesions when people used SLS-free toothpaste compared to a formula containing SLS.
If you deal with recurring canker sores, switching to an SLS-free toothpaste is a simple change worth trying. Several widely available brands now make versions without this ingredient.
Nutritional Deficiencies
Recurring mouth ulcers can be a sign that your body is low on certain nutrients, particularly vitamin B12, iron, and folate. A B12 deficiency, for example, can cause ulcers on the gums or tongue, sometimes alongside a smooth, glossy tongue where the tiny surface bumps have worn away. That smooth tongue may also feel sore.
These deficiencies are treatable, but they won’t resolve on their own. If your sores keep coming back without an obvious trigger like injury or food, a blood test can check your nutrient levels.
Fungal Infections
Oral thrush is a fungal infection caused by Candida, a yeast that normally lives in the mouth in small amounts. It only causes problems when something throws off the balance. Dry mouth, diabetes, a weakened immune system (including from HIV), certain drying medications, poorly fitting dentures, and poor oral hygiene all create conditions where Candida can overgrow. Thrush typically appears as creamy white patches on the tongue, inner cheeks, or roof of the mouth, and the tissue underneath can be raw and sore.
Underlying Health Conditions
When mouth sores recur frequently and don’t have an obvious trigger, they can sometimes point to a systemic condition. Several diseases are known to cause oral ulcers:
- Celiac disease, an autoimmune reaction to gluten, often causes mouth ulcers as one of its earliest or only symptoms
- Crohn’s disease, an inflammatory bowel condition, can produce sores anywhere in the digestive tract, including the mouth
- Behçet’s syndrome, a rare condition that inflames blood vessels throughout the body, characteristically causes recurring oral and genital ulcers
- Lupus, an autoimmune disease, can produce painless ulcers in the mouth
- HIV and other immunodeficiencies make a person more vulnerable to oral infections and ulcers of all types
In children, a condition called PFAPA syndrome causes periodic fevers along with mouth sores, sore throat, and swollen neck glands in recurring cycles.
Sores That Don’t Heal
Most mouth sores resolve within two weeks. A sore that persists beyond that timeline deserves attention. Oral cancer can present as a lip or mouth sore that doesn’t heal, a white or reddish patch inside the mouth, a lump or growth, unexplained mouth pain, ear pain, or difficulty swallowing and chewing. These signs don’t automatically mean cancer, but any sore lasting longer than two weeks should be evaluated by a doctor or dentist.
Easing the Pain While Sores Heal
For ordinary canker sores, over-the-counter topical numbing products containing benzocaine (such as Orajel or Anbesol) can take the edge off pain while the sore heals on its own. For more severe or persistent sores, doctors sometimes prescribe corticosteroid ointments that reduce inflammation and speed healing.
In the meantime, avoiding nuts, chips, pretzels, spicy seasonings, salty foods, and acidic fruits like oranges and pineapple helps prevent further irritation. Rinsing with warm salt water a few times a day can also soothe the area without any special products.