Canker sores are small, shallow ulcers that appear inside the mouth with a white or yellow center and a red border. They’re round or oval, usually less than 5 mm across, and painful to the touch. Unlike cold sores, they never form on the outside of your lips or on the skin around your mouth.
What a Canker Sore Looks Like at Each Stage
A canker sore doesn’t start as a visible ulcer. In the first stage, you’ll feel a burning or prickling sensation, and the area becomes a raised, reddened spot on the soft tissue inside your mouth. There’s nothing white or open yet, just irritation and tenderness.
Within a day or two, the characteristic ulcer forms: a shallow, clearly defined crater with a yellow-gray center surrounded by a bright red halo. This is when pain peaks. The sore looks like a small, punched-out hole in the lining of your cheek, lip, or tongue. It’s flat or slightly sunken, not raised or bumpy.
During healing, the red border fades and healthy tissue gradually closes over the sore from the edges inward. Pain decreases as the ulcer shrinks. Most canker sores heal completely within one to two weeks without leaving a scar.
Where They Show Up
Canker sores appear exclusively on the soft, moist tissue inside your mouth. The most common spots are the inside of your cheeks, the inside of your lips, under your tongue, and the back of your throat. They can also form on the soft palate or along the gums. You won’t find them on the hard palate (the roof of your mouth toward the front) or on the outside of your lips, which is a key way to tell them apart from other mouth sores.
Three Types and How They Differ
Not all canker sores look the same. There are three distinct types, and each has a different appearance.
Minor canker sores are by far the most common. They’re small, oval or round, less than 5 mm in diameter, and heal within a couple of weeks without scarring. Most people who get canker sores only ever experience this type.
Major canker sores exceed 1 cm in diameter. They’re deeper, often have irregular edges, and can take weeks or even months to fully heal. These are the ones most likely to leave a scar. If you’ve never had a sore this large inside your mouth before, it can look alarming, but major canker sores are a recognized, if uncommon, variation.
Herpetiform canker sores look dramatically different from the other two types. Instead of a single round ulcer, they appear as clusters of tiny pinpoint sores, sometimes as many as 100 at once. Over time, these small ulcers merge together into a large, irregularly shaped sore. Despite the name, they have nothing to do with the herpes virus. This type is rare and typically heals within about two weeks without scarring.
Canker Sores vs. Cold Sores
People often confuse canker sores with cold sores (fever blisters), but they look quite different once you know what to check. Cold sores are clusters of small, fluid-filled blisters that form on the outside of the mouth, usually around the lips or on the skin nearby. They eventually burst, ooze, and crust over. Canker sores never contain fluid, never blister, and never appear outside the mouth. A canker sore is a single round or oval ulcer (white or yellow with a red border) sitting on the soft tissue inside your mouth. If you see a blister on your lip, it’s not a canker sore.
Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not caused by a virus and can’t be passed from person to person.
What Triggers Them
The exact cause of canker sores isn’t fully understood, but several factors make them more likely. Biting the inside of your cheek, irritation from braces or rough dental work, stress, and hormonal changes are all common triggers. Nutritional deficiencies play a role too, particularly low levels of vitamin B12, vitamin D, folate, iron, or zinc. People who get frequent canker sores sometimes find that addressing a deficiency reduces how often they recur.
Some people notice canker sores after eating acidic or spicy foods, though this varies from person to person. Certain toothpastes containing sodium lauryl sulfate have also been linked to more frequent outbreaks.
When a Mouth Sore Isn’t a Canker Sore
Most canker sores are harmless and heal on their own, but certain features suggest something else is going on. Oral cancer can sometimes look like a mouth ulcer, so it’s worth knowing the differences.
A typical canker sore hurts from the start, has a clean white or yellow center with a smooth red border, and heals within two weeks. Oral cancer, by contrast, often doesn’t hurt initially. The pain develops gradually and then doesn’t go away. Other warning signs include a sore that lasts longer than two weeks without healing, texture changes like rough patches or crustiness, a bump or thickening under the skin where the ulcer sits, red or white mottled patches in the mouth, visible lumps or swelling in your neck or jaw, numbness in part of your mouth, or difficulty chewing and moving your tongue.
A sore that keeps changing but never heals is the most important red flag. If any mouth ulcer persists beyond two weeks, it’s worth having a dentist or doctor take a look.