A canker sore is a small, shallow ulcer that forms inside your mouth. These painful spots appear on the inner lips, cheeks, tongue, or floor of the mouth, and they’re one of the most common oral complaints. They are not contagious, not caused by a virus, and in most cases heal on their own within about a week.
What a Canker Sore Looks and Feels Like
Canker sores are round or oval ulcers with a yellowish-gray center and a red, inflamed border. They form on the soft, non-keratinized tissue inside the mouth, which is why you’ll find them on the inner cheeks, inner lips, tongue, soft palate, or floor of the mouth. They never appear on the outer lips or skin.
Before one becomes visible, you’ll often feel a tingling or burning sensation for a few hours. Within one to three days, that spot develops into the characteristic whitish ulcer. Pain tends to peak during the first few days, especially when eating, drinking, or talking. After about a week, most canker sores disappear completely without leaving a mark.
Three Types, Three Different Experiences
Not all canker sores behave the same way. They fall into three categories based on size, number, and severity.
Minor canker sores account for 75% to 85% of all cases. They’re smaller than 1 centimeter across, last 7 to 14 days, and heal without scarring. These are the ones most people picture when they think of a canker sore.
Major canker sores make up about 5% to 10% of cases. They’re larger than 1 centimeter, can persist for up to six weeks, and often leave scars on the tissue where they formed. These are significantly more painful and can interfere with eating.
Herpetiform canker sores also account for roughly 5% to 10% of cases. Despite the name, they have nothing to do with the herpes virus. They appear as clusters of up to 100 tiny ulcers, each only 1 to 3 millimeters wide. They heal within about two weeks.
Common Triggers
The exact cause of canker sores remains unknown, but several triggers are well established:
- Mouth injuries: Biting your cheek, scrapes from braces or ill-fitting dental appliances, or brushing too hard
- Certain foods: Acidic fruits, spicy dishes, coffee, eggs, cheese, peanuts, and almonds
- Stress: Emotional or physical stress is one of the most commonly reported triggers
- Hormonal shifts: Some people get canker sores around their menstrual period
- Toothpaste ingredients: Sodium lauryl sulfate (SLS), a foaming agent in many toothpastes, can irritate the mouth’s protective lining
SLS works as a detergent that strips away the mucous layer protecting your cheeks and gums, leaving the tissue exposed to irritants. Studies on people who switched to SLS-free toothpaste found that their sores healed faster and were less painful. Some also experienced fewer outbreaks overall. If you get canker sores frequently, switching toothpaste is one of the simplest things to try.
Nutritional Deficiencies and Recurring Sores
People who get canker sores repeatedly sometimes have low levels of specific nutrients, particularly vitamin B12, iron, folic acid, and vitamin C. In clinical case reports, patients with chronic outbreaks were found to have vitamin B12 levels well below the normal range, and their sores improved after supplementation. If your canker sores keep coming back, a blood test checking these levels can help identify whether a deficiency is contributing.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re entirely different conditions. Canker sores form inside the mouth on soft tissue. Cold sores (fever blisters) appear on the outside of the mouth, typically around the lips. Cold sores are caused by herpes simplex virus type 1 and are contagious through direct contact. Canker sores have no viral cause and cannot be spread to another person.
Cold sores also look different. They start as fluid-filled blisters that eventually crust over, while canker sores are open ulcers from the start, with that distinct whitish center.
Treatment and Pain Relief
Most minor canker sores don’t require treatment and resolve within a week or two. But that week can be miserable, so several options can ease the pain and speed things along.
Over-the-counter gels and pastes containing benzocaine numb the sore on contact. Applying them as soon as you notice a sore forming tends to work best. Antiseptic mouth rinses with hydrogen peroxide can also help keep the area clean and reduce irritation. For more severe or persistent sores, a doctor or dentist may prescribe a topical paste that reduces inflammation, or a medicated mouth rinse containing a numbing agent.
Simple home measures also help: avoid acidic and spicy foods until the sore heals, use a soft-bristled toothbrush, and rinse with warm salt water a few times a day.
When Canker Sores Signal Something Bigger
Occasional canker sores are normal and nothing to worry about. But frequent or unusually severe outbreaks can sometimes point to an underlying health condition. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis are associated with recurrent mouth ulcers. So is celiac disease, where gluten triggers an immune response that can manifest in the mouth. Diabetes, liver disease, and kidney disease have also been linked to recurring sores.
Behçet’s disease, a chronic inflammatory condition, is particularly notable. Recurrent canker sores are one of the primary criteria used to diagnose it. If you develop mouth ulcers alongside ulcers on other parts of your body (particularly the genitals), joint pain, or eye inflammation, that combination warrants medical evaluation.
Sores that last longer than two weeks, are unusually large, come with fever, or spread rapidly also deserve a closer look. In rare cases, persistent oral ulcers can be a sign of HIV infection or other immune system conditions.