A canker sore is a small, round or oval ulcer with a white or yellow center surrounded by a red border. It forms only on the soft tissues inside your mouth, most commonly on the inner cheeks, inner lips, or tongue. If you’re looking at a spot in your mouth and trying to figure out what it is, the combination of that pale center with a bright red halo is the hallmark appearance.
The Classic Appearance
The center of a canker sore is typically white, yellowish, or gray. This pale area is the ulcer itself, where the surface tissue has broken down. Surrounding that center is a distinct red ring, almost like a halo. The sore is usually shallow, meaning it doesn’t look like a deep hole in the tissue, but more like a flat, slightly sunken patch. Most are round or oval with clean, well-defined edges.
The size varies, but the most common type (called a minor canker sore) is less than 1 centimeter across, roughly the size of a pencil eraser or smaller. These make up the vast majority of cases.
How It Changes Over Time
A canker sore doesn’t appear all at once. Before you can see anything, you’ll typically feel a burning or prickling sensation on the spot where it’s about to form. This early stage can last a day or two, and during this time the area may look like a small, raised, reddened bump on the inside of your mouth.
Within a day or so, that red bump breaks open into the characteristic ulcer with its yellow-gray center and red border. This is when pain tends to peak. Eating, drinking, or accidentally brushing the sore with your tongue or teeth can make the pain sharper.
After several days, the healing stage begins. Healthy tissue gradually closes over the sore from the edges inward, and the pain fades as the ulcer shrinks. Most minor canker sores heal completely within one to two weeks without leaving a scar.
Three Types and How They Differ
Not all canker sores look the same. There are three distinct types, and knowing which one you’re looking at helps set expectations for healing.
- Minor canker sores are the most common. They’re small (under 1 cm), shallow, round or oval, and heal within a couple of weeks without scarring.
- Major canker sores are larger, deeper, and have less defined edges. They can take up to a month to heal and are more likely to leave a scar in the tissue afterward.
- Herpetiform canker sores look different from the other two. Instead of a single ulcer, they appear as clusters of tiny sores, each only 1 to 2 millimeters across. A cluster can contain more than 20 individual ulcers, and they sometimes merge together into one larger, irregularly shaped sore.
Despite the name, herpetiform canker sores are not caused by the herpes virus. They’re called that simply because the cluster pattern resembles a herpes outbreak visually.
Canker Sores vs. Cold Sores
This is one of the most common mix-ups. The simplest way to tell the difference is location. Canker sores form only inside the mouth, on soft, non-keratinized tissue like the inner cheeks, inner lips, tongue, or soft palate. Cold sores (fever blisters) form on the outside of the mouth, typically around the border of the lips or on the skin near the mouth.
Their appearance differs too. Cold sores start as small fluid-filled blisters that cluster together and eventually break open and crust over. Canker sores never form blisters and never crust. They’re open ulcers from the start, with that distinctive white or yellow center. Cold sores are also contagious, while canker sores are not.
When a Mouth Sore Looks Different
A typical canker sore has clean, symmetrical edges, stays roughly the same size once it forms, and heals within two weeks. If what you’re seeing doesn’t match that pattern, it’s worth paying closer attention.
A sore that lasts longer than three weeks, keeps growing, has irregular or ragged borders, or doesn’t hurt at all could be something other than a canker sore. Oral cancers can sometimes mimic the look of a mouth ulcer early on, but they don’t follow the same healing timeline. A firm lump under or around the sore, unusual color changes in the surrounding tissue, or worsening symptoms over time are all signs that warrant a professional evaluation. A dentist or doctor can assess the sore visually and feel for any firmness in the tissue that might suggest something beyond a routine canker sore.