A typical mouth ulcer is a small, round or oval sore with a white, yellow, or grayish center and a bright red border. Most are under 5 millimeters across, roughly the size of a pencil eraser, and appear on the soft, moist surfaces inside your mouth. They look like a shallow crater or “punched-out” hole in the tissue and are often painful to the touch.
The Classic Appearance
A mouth ulcer usually starts as a raised, reddened bump on the inner lining of your cheek, lip, tongue, or the soft part of the roof of your mouth. Within a day or two, that bump breaks open into a shallow, well-defined sore. The center is covered by a loosely attached membrane that looks white, yellowish, or grayish. Surrounding this center is a distinct red halo of inflamed tissue. The edges are clean and smooth, not ragged or lumpy.
You’ll almost always find these ulcers on the softer surfaces inside the mouth: the inner cheeks, inner lips, the sides or underside of the tongue, and the soft palate. They rarely form on the gums directly attached to bone or on the hard palate. If you see a sore in one of those less common spots, it may be something other than a standard canker sore.
Three Types and How They Differ
Not all mouth ulcers look the same. There are three recognized types, and each has a distinct size and pattern.
- Minor ulcers are the most common. They measure up to 5 millimeters in diameter and appear as a single sore or a small handful of them. These heal within 7 to 14 days without scarring.
- Major ulcers are larger than 10 millimeters and can reach up to 3 centimeters, which is bigger than a coin. They’re deeper, more painful, and can take several weeks to heal. These sometimes leave scars.
- Herpetiform ulcers look very different from the other two. They start as tiny, pinpoint-sized dots that appear in clusters of 10 to 100 at once. These clusters can merge into one large, irregular ulcer. Despite the name, they have nothing to do with the herpes virus. They typically heal in 10 to 14 days.
How a Mouth Ulcer Changes Over Time
A mouth ulcer goes through three visible stages. Knowing what to expect at each phase can help you gauge whether yours is healing normally.
In the first stage, which lasts one to three days, you won’t see much. You’ll feel a tingling or burning sensation in a specific spot, and the area may look slightly swollen and red. There’s no open sore yet.
By about the third day, the ulcer fully forms. The tissue breaks open into that characteristic white or yellow crater with a red rim. This active stage is the most painful and lasts roughly three to six days, sometimes longer for larger ulcers. Eating, drinking, and brushing your teeth near the sore will sting.
Then the healing stage begins. Healthy tissue gradually closes over the sore from the edges inward. The red border fades, the white center shrinks, and pain decreases as the ulcer gets smaller. For minor ulcers, the whole process from first tingle to fully healed takes about one to two weeks.
Mouth Ulcer vs. Cold Sore
People often confuse these two, but they look quite different once you know what to check. A mouth ulcer (canker sore) is always inside your mouth. It’s a flat, open crater with a white or yellow center. It never forms a blister and never develops a crust.
A cold sore, caused by the herpes simplex virus, usually appears on or around your lips. It starts as a cluster of small, fluid-filled blisters that eventually burst and form a yellowish crust. Cold sores can occasionally show up inside the mouth, but the blistering and crusting pattern is the giveaway. Cold sores are also contagious, while mouth ulcers are not.
When a Sore Might Be Something Else
The vast majority of mouth ulcers are harmless and heal on their own. But certain visual features suggest a sore needs professional attention. Oral cancer in its earliest stages is usually painless, which is the opposite of a canker sore. A cancerous lesion may start as a small flat spot that slowly grows larger rather than shrinking over two weeks. A white patch that turns red, or a sore that starts bleeding when it didn’t before, also warrants a closer look.
The simplest rule is time. A normal mouth ulcer should be noticeably improving within two weeks and fully healed within three. If a sore stays the same size, keeps getting bigger, or changes color or texture over that window, have it evaluated. Irregular, ragged borders and a hard or raised edge are also features you won’t see on a typical canker sore.