Most mouth sores are small, round, and either white or red, but the specific appearance varies depending on what’s causing them. Some are shallow craters with a colored center, others are fluid-filled blisters, and a few look like textured patches or lacy white lines. Knowing what each type looks like helps you figure out what you’re dealing with and whether it needs attention.
Canker Sores
Canker sores are the most common type of mouth sore and the easiest to recognize. They appear as single, round or oval ulcers about 2 to 4 millimeters across, roughly the size of a pencil eraser or smaller. The center is a shallow crater that starts out yellowish and gradually turns gray as it heals. Surrounding the crater is a bright red ring of inflamed tissue.
These sores only show up inside the mouth, specifically on soft, moveable tissue: the inner cheeks, inner lips, the underside of the tongue, the floor of the mouth, or the soft palate. They never appear on the gums right along your teeth or on the roof of your mouth. That location detail is one of the quickest ways to tell them apart from other sores. Canker sores are painful, especially when eating or drinking something acidic, but they typically heal on their own within a week or so.
Cold Sores
Cold sores look completely different from canker sores. They appear as clusters of tiny, fluid-filled blisters, usually right on the border of your lips or just outside your mouth. The blisters sit on a red base and contain clear fluid, not white or yellow pus. Over a few days the blisters break open, merge into a shallow sore, and then form a yellowish or brownish crust that eventually falls off.
When cold sores do show up inside the mouth, they tend to appear on firm tissue that’s attached to bone: the hard palate (roof of the mouth), the gums near your teeth, or the top surface of the tongue. This is the opposite of where canker sores form, so location alone can help you tell them apart. The key visual difference is that cold sores start as a group of small blisters filled with clear fluid, while a canker sore is a single open crater.
Oral Thrush
Thrush doesn’t look like a typical sore at all. It appears as slightly raised white patches with a texture often compared to cottage cheese. These patches can show up on the tongue, inner cheeks, roof of the mouth, or the back of the throat. Unlike some other white lesions in the mouth, thrush patches can be wiped or scraped away, though doing so may cause slight bleeding and reveal raw, red tissue underneath.
Thrush is a yeast overgrowth and tends to affect people with weakened immune systems, those taking antibiotics, or those using steroid inhalers. It’s usually not as painful as a canker sore or cold sore, though it can cause a burning sensation or make food taste slightly off.
Hand, Foot, and Mouth Disease
This viral illness, most common in young children, causes mouth sores that start as small red spots, usually on the tongue and the insides of the cheeks. The spots quickly blister and can become quite painful, making it hard for kids to eat or drink. These oral lesions appear alongside a characteristic rash of red spots or blisters on the palms of the hands and soles of the feet, which is the easiest way to identify what’s going on.
Leukoplakia and Erythroplakia
Leukoplakia shows up as thick, white or gray patches on the inner surfaces of the mouth. Unlike thrush, these patches cannot be scraped or wiped away. The texture varies: some are smooth, others are rough, ridged, or wrinkled. The patches are usually hard or thick to the touch, painless, and may go unnoticed for a while.
Sometimes white patches appear alongside raised red areas, a combination called speckled leukoplakia. Pure red patches on their own are called erythroplakia. Both of these are considered potentially precancerous, meaning they carry a higher risk of developing into oral cancer over time. The fact that they don’t hurt is actually what makes them easy to overlook. Any white or red patch in your mouth that persists for more than two to three weeks and can’t be explained by an obvious cause like biting your cheek deserves a professional evaluation.
Oral Lichen Planus
This condition has a distinctive look that’s hard to confuse with anything else once you know what to look for. It produces lacy, bluish-white lines on the inside of the cheeks, sometimes described as a web or net-like pattern. These lines can also appear on the edges of the tongue or on the gums.
Some people with lichen planus develop an erosive form, where the lacy pattern is accompanied by shallow, painful, recurring ulcers. The combination of the white lace pattern with open sores is the visual signature of this condition. It tends to be chronic, coming and going over months or years.
How Location Helps Identify the Sore
Where a sore appears in your mouth is one of the most useful clues for figuring out what it is. Canker sores stick to soft, loose tissue: inner cheeks, inner lips, the underside of the tongue, and the floor of the mouth. Cold sores that appear inside the mouth favor the opposite, showing up on firm tissue attached to bone like the hard palate, the gums near the teeth, and the top of the tongue. Cold sores outside the mouth cluster along the lip border.
Thrush, leukoplakia, and lichen planus can appear on a wider range of surfaces, but each has preferred spots. Thrush favors the tongue and inner cheeks. Leukoplakia tends to form on the inner cheek surfaces and floor of the mouth. Lichen planus has a strong preference for the inner cheeks.
When a Sore Takes Too Long to Heal
Most benign mouth sores clear up within a week or so without any treatment. The three-week mark is the commonly used threshold for concern. If a sore, patch, or lump in your mouth lasts longer than three weeks, is bleeding, or keeps coming back frequently, it’s worth getting it looked at. Painless sores that won’t heal are particularly worth paying attention to, since oral cancers often present as persistent ulcers or patches that don’t hurt, at least not initially.
A sore that’s clearly healing, even if slowly, is a different story from one that stays the same size or grows. Tracking whether the sore is getting smaller over a week or two gives you useful information to bring to a dentist or doctor if the sore does persist.