A canker sore is a small, open ulcer inside the mouth that’s round or oval with a white or yellow center and a red border. Most are flat against the tissue, roughly 2 to 3 millimeters across, and noticeably painful. They form exclusively on the soft tissues inside the mouth: the inner lips, cheeks, tongue, and soft palate.
Color, Shape, and Size
The hallmark look is a shallow, round or oval crater with a yellowish-gray center surrounded by a bright red halo. That red ring is inflamed tissue, and the pale center is a thin membrane covering the ulcer itself. The sore sits flat against the surrounding tissue rather than rising above it.
The most common type, which accounts for over 70% of cases, is smaller than 10 millimeters, with many measuring just 2 to 3 millimeters. These tend to appear on the inner lips, tongue, or inside of the cheeks. You might get one or a few at a time, and they typically stay superficial, meaning they don’t dig deep into the tissue.
Three Types and How They Differ
Not all canker sores look the same. There are three recognized forms, and the visual differences between them are distinct.
- Minor canker sores are by far the most common. They’re small (under 10 mm), round, shallow, and heal within about two weeks without scarring. Most people who get canker sores get this type.
- Major canker sores are larger than 10 millimeters and can spread to areas like the gums and the back of the throat. They’re deeper, take over six weeks to heal in some cases, and can leave scars.
- Herpetiform canker sores appear as dozens of tiny, deep ulcers that cluster together and merge into larger sores with irregular, uneven edges. Despite the name, they have nothing to do with the herpes virus. They typically heal within a month without scarring.
Where They Form
Canker sores only develop on soft, non-keratinized tissue. That means the inner surface of the lips, the inside of the cheeks, the underside and sides of the tongue, and the soft palate. They never appear on the hard, firm tissue of the gums in mild cases, though major canker sores can extend to the gums and the back of the throat. You will not find a canker sore on the outside of your lips or on your skin.
How They Change Over Time
A canker sore doesn’t appear all at once. It goes through a clear progression that changes how it looks and feels at each stage.
The first sign is a burning or prickling sensation on a patch of tissue inside your mouth. This prodromal stage lasts one to three days. During this time, the spot looks like a raised, reddened area with no visible ulcer yet.
By about the third day, the ulcer fully forms. The center turns yellowish-gray, the red border becomes more defined, and pain typically peaks. This active ulcer stage lasts roughly three to six days, sometimes longer.
Then healthy tissue begins closing over the sore from the edges inward. The red halo fades, the crater fills in, and pain drops off steadily. Minor canker sores complete this entire cycle within about two weeks.
Canker Sores vs. Cold Sores
These two get confused constantly, but they look quite different. A canker sore is an open, flat ulcer inside the mouth. A cold sore is a fluid-filled blister that forms on or around the outside of the lips, caused by the herpes simplex virus. Cold sores start as a cluster of small blisters that eventually burst, crust over, and scab. Canker sores never blister, never crust, and never appear outside the mouth. If you see a fluid-filled bump on your outer lip, that’s not a canker sore.
What Triggers Them
The exact cause varies from person to person, and in many cases no single trigger can be identified. Recurring canker sores are often linked to deficiencies in vitamin B12, folate, or iron. These nutrients support cell repair in the mouth’s lining, and when levels drop, the tissue becomes more vulnerable to breakdown and ulceration.
Other common triggers include minor injuries from biting your cheek or aggressive brushing, stress, hormonal shifts, and certain acidic or spicy foods. Some people notice a pattern, getting a new sore every time they’re under pressure at work or after eating particular foods, while others get them seemingly at random.
When a Sore Could Be Something Else
Most canker sores are harmless and heal on their own. But a sore that doesn’t follow the typical pattern deserves attention. Researchers at MD Anderson Cancer Center highlight several visual differences between a canker sore and something more serious like oral cancer.
Canker sores are painful from the start. Early oral cancers typically are not. Canker sores are flat. Oral cancers often have a small lump or bump beneath the surface you can feel with your tongue or finger. And canker sores heal within two to three weeks. A sore that lingers beyond that window, or one that grows larger, changes from white to red, or starts bleeding when it didn’t before, warrants a visit to a doctor or dentist for evaluation.