A canker sore is a small, shallow ulcer that forms inside the mouth, typically on the inner cheeks, lips, tongue, or soft palate. Unlike cold sores, canker sores are not contagious and are not caused by a virus. They affect anywhere from 5% to 66% of the population depending on the group studied, making them one of the most common oral complaints. Most heal on their own within one to two weeks, but they can make eating, drinking, and talking genuinely miserable in the meantime.
What a Canker Sore Looks and Feels Like
Canker sores are white or yellow oval-shaped spots surrounded by a red, inflamed border. They form only on the soft tissues inside your mouth, never on the outer lips or skin. Before one appears, you’ll often notice a tingling or burning sensation in that spot for a day or two.
There are three types. Minor canker sores, the most common, are less than 1 cm across and shallow. They heal without scarring. Major canker sores are larger and penetrate deeper into the tissue, sometimes leaving scars as they heal. Herpetiform canker sores are clusters of many tiny sores that can merge together. Despite the name, herpetiform sores have nothing to do with the herpes virus.
Canker Sores vs. Cold Sores
People often confuse these two, but they’re completely different conditions. Cold sores (fever blisters) are fluid-filled blisters that form on the outside of the mouth, around the lips. They’re caused by herpes simplex virus type 1 and are very contagious, spreading through direct contact. Canker sores form only inside the mouth, have no known viral cause, and cannot be passed from person to person.
What Causes Them
The exact cause isn’t fully understood, but the leading explanation involves an overreaction of the immune system. Certain white blood cells (T-cells) appear to attack the mouth’s own tissue, breaking down the lining and creating the ulcer. What triggers that immune response varies widely from person to person, which is part of why canker sores have been so hard to pin down.
Common triggers include:
- Minor mouth injuries: biting your cheek, aggressive brushing, dental work, or sharp food like chips
- Stress and lack of sleep
- Certain foods: citrus fruits, tomatoes, chocolate, coffee, and spicy dishes are frequent culprits
- Toothpaste containing sodium lauryl sulfate (SLS): this foaming agent can irritate the mouth lining in sensitive people
- Hormonal changes: some women notice sores tied to their menstrual cycle
Nutritional deficiencies also play a role. Low levels of vitamin B12, folate, iron, or vitamin C can weaken the mouth’s mucosal lining. B12 is essential for cellular repair in the mouth, and when it’s deficient, the tissue becomes more vulnerable to breakdown. Folate deficiency has a similar effect, disrupting the normal turnover of the cells that line your cheeks, gums, and tongue. If you get canker sores frequently, a simple blood test can check for these deficiencies.
When Canker Sores Keep Coming Back
Occasional canker sores are normal. Recurrent ones, especially if they’re large, slow to heal, or appear in clusters, can sometimes signal an underlying health condition. Celiac disease, an autoimmune reaction to gluten, is one of the better-studied links. The nutrient malabsorption that comes with celiac (particularly of iron, folate, and B12) creates exactly the kind of deficiencies that promote mouth ulcers. Crohn’s disease and other inflammatory bowel conditions can also cause recurring oral sores.
Behçet’s disease, a rarer condition involving blood vessel inflammation throughout the body, characteristically produces canker sores as one of its earliest symptoms. In these cases the sores tend to be large, frequent, and painful. Immune-suppressing conditions and certain medications can also trigger recurrent outbreaks.
How to Treat a Canker Sore
Most minor canker sores don’t need treatment and resolve within 7 to 14 days. But you can reduce the pain and potentially speed healing with a few approaches.
Over-the-counter topical products containing benzocaine numb the area on contact and can make eating more comfortable. Antiseptic mouth rinses with hydrogen peroxide help keep the sore clean and may reduce secondary irritation. These work best when applied as soon as the sore appears, before it fully develops. Covering the sore with a protective paste creates a barrier against food and saliva that can irritate it further.
At home, rinsing with a simple saltwater solution (about half a teaspoon of salt in a cup of warm water) several times a day can soothe inflammation. Some people find that dabbing a small amount of milk of magnesia directly on the sore helps. Avoiding acidic, spicy, or rough-textured foods while the sore is active makes a noticeable difference in day-to-day comfort.
If you’re prone to canker sores and use a toothpaste with SLS, switching to an SLS-free brand is one of the simplest changes you can try. Some people see a meaningful reduction in how often sores appear.
When a Mouth Sore Needs Attention
A typical canker sore heals within two weeks. If a sore persists beyond that point, it’s worth getting evaluated. A mouth sore that won’t heal is one of the signs of oral cancer, along with unexplained lumps, white or reddish patches, persistent mouth pain, ear pain, or difficulty swallowing. Any of these symptoms lasting more than two weeks should be checked by a doctor or dentist, who can rule out more common causes like infection first.
You should also seek care if your canker sores are unusually large, come in frequent waves, make it too painful to drink fluids, or are accompanied by fever. These patterns suggest something beyond a routine sore and may point to a nutritional deficiency or systemic condition that can be treated once identified.