Canker sores are small, round or oval ulcers with a white or yellow center surrounded by a red border. They form on the soft, moist surfaces inside your mouth, not on your lips or skin. Up to 20% of people get them regularly, and while they can look alarming, most heal on their own within two to three weeks.
The Classic Appearance
A typical canker sore starts as a small, flat ulcer less than 1 cm across. The center has a yellow-gray coating, a thin layer of healing tissue that gives the sore its distinctive pale look. The tissue immediately surrounding it is inflamed, creating a well-defined red halo or border. The sore is flat against the surrounding tissue, without any raised bump or lump beneath it.
Most canker sores are round or oval, with smooth, even edges. They appear on the soft, non-attached tissue inside your mouth: the inner cheeks, the inner lips, the underside or sides of the tongue, and the floor of the mouth. They don’t typically show up on the hard palate (the roof of your mouth) or on the gums, since those surfaces have a tougher lining.
Three Types, Three Different Looks
Not all canker sores are identical. They fall into three categories based on size and number.
Minor canker sores account for 80 to 90% of cases. These are the ones most people picture: oval, less than 1 cm across, with a clean red edge. They heal in 10 to 14 days without scarring. You might get one at a time, or a small cluster of up to about ten.
Major canker sores make up roughly 5 to 10% of cases. These are larger than 1 cm, often round with sharply defined borders, and can be deep enough to cause significant pain that interferes with eating and talking. They take longer to heal, sometimes six weeks or more, and can leave scars. Major sores sometimes appear on tougher surfaces like the palate, unlike their smaller counterparts.
Herpetiform canker sores are the least common (5 to 10% of cases) and look quite different from the other two types. Instead of one or a few distinct ulcers, you get numerous tiny sores, each under 1 cm, that can merge together into larger, irregularly shaped ulcers. Despite the name, they have nothing to do with the herpes virus.
How They Change Over Time
Before a canker sore becomes visible, you may notice a tingling or burning sensation in one spot inside your mouth. This prodromal phase lasts a day or two. The area may look slightly red or swollen, but there’s no open sore yet.
Within a day or so, the surface breaks down into an open ulcer. This is when the characteristic white or yellow center appears, and it’s usually the most painful stage. Over the next several days, the red border may look quite inflamed. Eating acidic, salty, or spicy foods tends to intensify the pain during this window.
After about a week, the sore gradually shrinks. The red border fades, the yellow-gray center fills in with new tissue, and pain decreases. Minor sores are typically gone within two weeks. The tissue underneath heals flat, with no lasting mark.
Canker Sores vs. Cold Sores
The most common mix-up is between canker sores and cold sores, but they look and behave quite differently. Cold sores are fluid-filled blisters that appear on the outside of the mouth, typically on or around the lips. They crust over as they heal. Canker sores are open ulcers, never blisters, and they form exclusively inside the mouth. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not contagious and have no viral cause.
When a Sore Looks Different Than Expected
Most canker sores follow the pattern above closely enough that you can identify them at a glance. A few visual features, however, should prompt a closer look from a healthcare provider.
A canker sore is flat. If you feel a small lump or bump beneath a mouth ulcer, that’s not typical. Oral cancers often have a raised area under the lesion that you can detect by running your tongue or finger over it. The edges of a canker sore are red and inflamed, which is actually a reassuring sign of normal inflammation. Cancerous lesions don’t usually have that angry red border.
Shape matters too. A linear or irregularly shaped ulcer, rather than the usual round or oval one, may point toward other conditions that need evaluation. Any ulcer that lasts longer than three to four weeks without healing warrants a professional assessment to rule out other causes. The same goes for a white spot that turns red over time, a small spot that keeps growing larger, or a lesion that begins to bleed when it previously didn’t.
People who get canker sores very frequently, or who develop them alongside joint pain, eye inflammation, genital ulcers, or digestive symptoms, may be dealing with an underlying condition rather than simple recurrent canker sores. In these cases the sores themselves may look identical to ordinary ones, but their pattern and accompanying symptoms tell a different story.