A canker sore is a shallow, round or oval ulcer inside the mouth with a white or yellowish center and a red border. They’re small, flat, and open, not fluid-filled blisters. Most are under 5 millimeters across, roughly the size of a pencil eraser or smaller.
The Classic Appearance
The typical canker sore has a well-defined shape: a round or oval crater with a center coated in a yellowish-gray film, surrounded by a ring of inflamed, reddish tissue. The edges are slightly raised. The sore sits on the surface of the soft tissue and is flat to the touch, unlike a bump or blister.
You’ll find them on the inside of your cheeks, the inside of your lips, under your tongue, on the soft palate, or in the back of your throat. They stick to the soft, movable tissue of the mouth and don’t appear on the gums over bone or the hard palate. They also never show up on the outside of the mouth or on the skin around the lips.
How a Canker Sore Develops Over Time
Before you can see anything, you’ll likely feel a tingling or burning sensation in a specific spot inside your mouth. This prodromal stage lasts a day or two. During this phase, the area may look slightly red or feel tender when your tongue touches it, but there’s no visible ulcer yet.
Within a day or so, the spot breaks open into a full ulcer. This is when the characteristic white or yellow center appears with its red border. The first few days of the open sore are typically the most painful, especially when eating acidic, salty, or spicy food. Pain generally starts improving within a few days, and the sore heals on its own within about two weeks without leaving a scar.
Three Types, Three Different Looks
Not all canker sores look the same. There are three distinct types, and they vary quite a bit in size and number.
Minor canker sores are the most common. They measure less than 5 millimeters across, appear one or a few at a time, and heal within one to two weeks. These are the ones most people picture when they think of a canker sore.
Major canker sores exceed 1 centimeter in diameter, making them noticeably larger and deeper. They take longer to heal, sometimes several weeks, and can leave scars. These are less common but significantly more painful due to their size.
Herpetiform canker sores look different from the other two. Instead of one or two distinct ulcers, they start as clusters of many tiny sores, sometimes up to 100 at once. Over time, these small ulcers fuse together into larger, irregularly shaped sores. Despite the name, they have nothing to do with the herpes virus.
Canker Sore vs. Cold Sore
This is the most common mix-up, but the two look and behave quite differently once you know what to check.
- Location: Canker sores appear inside the mouth. Cold sores (fever blisters) appear on the outside of the mouth, typically on or around the lips.
- Structure: A canker sore is a single, flat, open ulcer with a white or yellow center. A cold sore is a cluster of small, fluid-filled blisters that eventually burst and crust over.
- Contagiousness: Canker sores are not contagious. Cold sores are caused by the herpes simplex virus and spread through direct contact.
If you see a cluster of tiny blisters on the outer edge of your lip that eventually form a scab, that’s a cold sore. If you see a single round white sore on the inside of your cheek or lip, that’s a canker sore.
When a Sore Isn’t a Canker Sore
Most canker sores are harmless and heal on their own, but some mouth sores deserve a closer look. A few visual and behavioral differences help separate a routine canker sore from something more serious, like oral cancer.
Canker sores tend to be flat with inflamed, red edges. Oral cancers often have a small lump or bump beneath the lesion that you can feel with your tongue or finger. Canker sores are painful from the start, while early-stage oral cancers are typically painless. And canker sores heal within two to three weeks. If a sore lasts longer than that, it warrants medical attention.
Other signs to watch for: a small spot that grows larger over time, a white patch that turns red, or a lesion that starts bleeding when it didn’t before. Any of these patterns break the normal canker sore cycle and should be evaluated by a doctor or dentist.