A canker sore is a small, shallow ulcer inside the mouth with a white or yellow center surrounded by a red border. Most are round or oval, roughly 2 to 4 millimeters across (smaller than a pea), and flat against the tissue rather than raised. They appear on soft surfaces like the inner cheeks, the floor of the mouth, the sides or underside of the tongue, the soft palate, and the base of the gums.
What a Typical Canker Sore Looks Like
The classic canker sore has a clearly defined oval shape. The center is white, gray, or yellow, which is the ulcerated tissue itself. The rim around it is noticeably red and inflamed, sometimes described as angry-looking. The surface is flat, not blistered or raised, and the sore sits on the soft, movable tissue inside the mouth rather than on the lips or outside the face.
Before the sore becomes visible, you’ll usually feel a burning or tingling sensation at the spot where it’s forming. Within a day or two, the ulcer appears. Pain tends to peak in the first few days, then gradually fades as the sore heals over about two weeks without leaving a scar.
Three Types and How They Differ
Minor Canker Sores
These account for about 85% of all canker sores. They’re oval, 2 to 4 millimeters in diameter, with a clean red edge. You’ll typically get one at a time, occasionally two or three. They heal within about 10 days and leave no scarring.
Major Canker Sores
About 10% of cases involve major canker sores, which are deeper and larger, usually over 1 centimeter in diameter. They tend to be round with well-defined borders, though very large ones can develop irregular edges. These appear on the lips, soft palate, and throat, and they’re significantly more painful. Major canker sores can take weeks to months to heal and often leave scars behind. Some people also experience fever and general fatigue alongside them.
Herpetiform Canker Sores
The rarest type, making up roughly 5% of cases. Despite the name, these aren’t caused by the herpes virus. They look quite different from the other two types: instead of a single sore, you’ll see clusters of tiny pinpoint ulcers, sometimes as many as 100 at once, sitting on a red base. These small ulcers often merge together into larger, irregularly shaped sores. They typically heal within about two weeks without scarring.
Canker Sores vs. Cold Sores
The easiest way to tell these apart is location. Canker sores only form inside the mouth. Cold sores (fever blisters) appear outside the mouth, usually along the border of the lips or on the skin around them.
They also look fundamentally different. A canker sore is a single, flat, round ulcer with that characteristic white or yellow center. A cold sore is a patch of multiple small, fluid-filled blisters clustered together. Cold sores eventually crust over and scab as they heal. Canker sores never blister and never form a scab.
How a Canker Sore Changes as It Heals
A canker sore doesn’t go through dramatic visual stages the way a cold sore does. It starts as a tingling or burning spot, becomes a visible ulcer within a day or two, and then gradually shrinks from the edges inward. The red border fades first, and the white or yellow center slowly fills in with normal pink tissue. Minor sores typically resolve within two weeks even without treatment. You won’t see crusting, scabbing, or blistering at any point during healing.
Major canker sores follow the same general pattern but much more slowly. Because they penetrate deeper into the tissue, they can leave a visible scar, a small patch of slightly different-colored or textured tissue where the ulcer was.
When a Mouth Sore May Not Be a Canker Sore
A few visual details can help you distinguish a routine canker sore from something that needs medical attention. Canker sores are flat. If you can feel a small lump or bump beneath a mouth ulcer, that’s not typical of a canker sore. Canker sores also hurt, often quite a bit, especially in the first few days. Oral cancers in their earliest stages are usually painless, which can paradoxically make them easier to ignore.
Watch for a small spot that keeps growing larger, a white patch that turns red, or a lesion that starts bleeding when it previously didn’t. A sore that hasn’t healed within two to three weeks is worth having a doctor or dentist examine. Most canker sores resolve well within that window, so persistence is one of the most reliable warning signs that something else may be going on.