Most mouth ulcers appear as small, round sores with a white or yellowish center and a red border. They form on the soft tissue inside your mouth, including the inner cheeks, lips, tongue, and floor of the mouth. While that description covers the most common type, mouth ulcers can vary quite a bit in size, shape, and color depending on what’s causing them.
The Typical Canker Sore
The most common mouth ulcer is a canker sore, known clinically as recurrent aphthous stomatitis. These account for the vast majority of oral ulcers, and they have a distinctive look: a symmetrically round sore covered in a thin fibrin layer that gives it a pale, whitish-yellow appearance, surrounded by a bright red halo of inflamed tissue. They usually show up as a single sore, though you can get more than one at a time.
Canker sores come in three sizes. Minor canker sores, which make up 75% to 85% of cases, are under 10 millimeters across (roughly the size of a pencil eraser or smaller) and heal within 7 to 14 days without scarring. Major canker sores exceed 10 millimeters, can last up to six weeks, and often leave scars on the tissue when they finally heal. The third type, herpetiform ulcers, are tiny pinpoint sores less than a millimeter across that appear in clusters of up to 100 at a time, with neighboring sores sometimes merging together into larger irregular patches.
Canker Sores vs. Cold Sores
These two get confused constantly, but they look quite different once you know what to watch for. Canker sores are single round sores with that characteristic white center and red border, and they only appear inside the mouth. Cold sores (fever blisters) are patches of several small fluid-filled blisters that cluster together on the outside of the mouth, typically around the lips. Cold sores are caused by the herpes simplex virus, while canker sores are not contagious and their exact cause is still debated.
One reliable way to tell them apart: location. If the sore is on the outer lip or skin around the mouth and looks blistery, it’s almost certainly a cold sore. If it’s a single open sore on the inside of your cheek, lip, or tongue, it’s most likely a canker sore.
What Injury-Related Ulcers Look Like
Biting your cheek, scraping your gums with a chip, or irritating tissue with a rough bracket or denture edge can all produce traumatic ulcers. These sores have a yellowish-white surface similar to a canker sore, but their shape gives them away. While canker sores are reliably round, traumatic ulcers can be symmetrical or asymmetrical, often matching the shape of whatever caused the injury. Their borders tend to be slightly raised and reddish.
If the source of irritation persists, like a rough dental appliance that keeps rubbing the same spot, the ulcer can become a sustained traumatic ulcer. These develop a more pronounced raised, rolled border with a thickened surface, which can sometimes mimic the appearance of something more serious. Removing the source of irritation usually resolves the sore within a couple of weeks.
White Patches That Aren’t Ulcers
Not every white spot in your mouth is an ulcer. Two common conditions can look similar at a glance but are distinct from open sores.
Oral lichen planus produces symmetric, lacy white lines on the inner cheeks called Wickham striae. These delicate, web-like patterns sit on the surface of the tissue without forming an open wound, though some forms of lichen planus can eventually lead to erosion and ulceration.
Leukoplakia appears as a well-defined white plaque that cannot be wiped away. Unlike the removable white coating on some ulcers, leukoplakia is a change in the tissue itself. It’s painless and doesn’t look like an open sore, but it warrants professional evaluation because a small percentage of cases can become precancerous over time.
When Ulcers Signal Something Systemic
Recurring mouth ulcers that keep coming back in waves can sometimes be a sign of a broader inflammatory condition. In Behçet’s disease, oral ulcers occur in 97% to 100% of patients. The tricky part is that these ulcers look virtually identical to ordinary canker sores: pale, round, with red borders, typically under 10 millimeters. What sets them apart is the pattern. They recur frequently and are usually accompanied by other symptoms like genital ulcers, skin lesions, or eye inflammation.
The ulcers themselves tend to favor non-keratinized tissue (the softer, more flexible surfaces inside the mouth like the inner cheeks, lip lining, tongue borders, and floor of the mouth) rather than the harder tissue of the gums or roof of the mouth. If you’re getting frequent mouth ulcers alongside symptoms elsewhere in your body, the ulcers may be part of a larger picture worth investigating.
What a Concerning Ulcer Looks Like
Most mouth ulcers are harmless and resolve on their own within 10 to 14 days. But certain visual features distinguish ulcers that need professional attention.
Oral cancer can present as an ulcer, and it has a specific look. Ulcerative squamous cell carcinoma typically forms a crater-like sore with rolled, firm borders and a velvety base. Unlike a canker sore, which is flat and soft, a cancerous ulcer often feels hard or indurated when pressed. The borders are irregular rather than neatly round, and the sore doesn’t heal.
The two-week rule is the most important guideline. If any mouth ulcer hasn’t healed within two weeks, it needs to be evaluated. This is especially true if you use tobacco or drink alcohol regularly, both of which increase oral cancer risk. Other red flags include an ulcer that’s growing rather than shrinking, one that bleeds easily, or a sore accompanied by a firm lump in the tissue. A dentist or doctor will visually examine the area and feel for firm lesions in the tissue, and may recommend a biopsy if anything looks suspicious.
Signs of a Secondary Infection
An ordinary canker sore can occasionally become infected, especially if bacteria from teeth or gums enter the open tissue. Signs of infection include increasing redness spreading outward from the ulcer’s border, visible swelling in the surrounding tissue, and worsening pain rather than the gradual improvement you’d expect as a sore heals. In more severe cases, a bacterial infection can lead to a localized pocket of pus (an abscess) or more widespread inflammation of the soft tissue. If a mouth ulcer seems to be getting worse after the first few days rather than better, that’s a signal something beyond normal healing is happening.