Mouth ulcers are small, shallow sores that form inside the mouth. They typically appear as round or oval white or yellow patches surrounded by a red border. Most are 2 to 3 millimeters across, roughly the size of a pencil eraser or smaller, and they develop on soft tissue like the inner cheeks, inner lips, tongue, or the floor of the mouth.
What a Typical Mouth Ulcer Looks Like
The classic mouth ulcer, called a canker sore, is a single round or oval sore with a yellowish-gray center and a bright red ring around the edge. The center often looks slightly sunken compared to the surrounding tissue, creating a shallow crater. The surface is smooth, not bumpy or raised, and there’s no fluid or blister involved.
Before the sore itself appears, you’ll often notice a burning or prickling sensation along with a raised, reddened area. This prodromal stage lasts one to three days. By about the third day, the full ulcer forms with its characteristic yellow-gray center and red halo. Pain tends to peak during this stage, which lasts another three to six days. After that, healthy tissue gradually closes over the sore and the pain fades. Most mouth ulcers heal completely in 10 to 14 days without leaving a scar.
Small, Large, and Clustered Types
Not all mouth ulcers look the same. They fall into three broad categories based on size and pattern.
- Minor ulcers are the most common type. They measure less than 8 millimeters across, typically 2 to 3 millimeters. They heal on their own within a couple of weeks.
- Major ulcers are deeper, larger (over 1 centimeter), and can last weeks to months. These are more painful, may have irregular edges, and sometimes leave scars when they heal.
- Herpetiform ulcers appear as clusters of many tiny sores, sometimes up to 100 at once, each under 1 centimeter. They sit on a reddened base and can merge together into larger, irregular shapes. Despite the name, they aren’t caused by the herpes virus.
Mouth Ulcers vs. Cold Sores
One of the most common mix-ups is confusing a canker sore with a cold sore. The easiest way to tell them apart is location. Canker sores only form inside the mouth. Cold sores (fever blisters) form on the outside of the mouth, typically around the border of the lips.
They also look different. A canker sore is usually a single round white or yellow sore with a red border. A cold sore is a cluster of several small, fluid-filled blisters grouped together on the skin. Cold sores eventually break open, crust over, and scab before healing. Canker sores never blister or scab because they’re on moist inner tissue.
Mouth Sores From Viral Infections
Some viral illnesses produce mouth sores that look different from typical canker sores. Hand, foot, and mouth disease, common in young children, causes painful blister-like lesions to form on the tongue, gums, and inside of the cheeks, usually one to two days after a fever starts. These sores tend to appear alongside a rash on the hands and feet, which helps distinguish them. A related illness called herpangina produces similar sores concentrated in the back of the mouth and throat rather than toward the front.
When a Mouth Sore Could Be Something Serious
Most mouth ulcers are harmless and heal on their own. But some visual features are worth paying attention to. A sore that doesn’t heal after three weeks is the single most important warning sign. Oral cancers can appear as a persistent sore on the lip or inside the mouth, a white or reddish patch on the inner tissue, or a growth or lump that won’t go away.
Cancerous sores often look different from a typical canker sore. They may have hardened, raised edges rather than a soft red border. They may not be particularly painful at first, which can make them easy to ignore. The key distinction is time: if any mouth sore persists beyond two to three weeks without showing signs of healing, or if it keeps coming back in the same spot, that warrants a closer look from a healthcare provider.