A typical mouth ulcer is a small, round or oval sore with a white, yellow, or grayish center surrounded by a bright red border. Most are less than 5 mm across, roughly the size of a pencil eraser, and appear on the soft tissue inside your mouth: inner cheeks, inner lips, tongue, gums, or the roof of your mouth. They look like a shallow, punched-out crater, and despite their small size, they can be surprisingly painful.
How a Mouth Ulcer Forms
Before you can see anything, you’ll usually feel it. A burning or stinging sensation develops one to two days before the ulcer becomes visible. During this early phase, a small yellowish raised spot appears, often with redness around it. That spot then breaks open into the characteristic shallow sore covered by a loosely attached membrane. The membrane gives the ulcer its whitish or grayish appearance, while the inflamed tissue around it creates the red halo.
The Three Types and How They Differ
About 80% of mouth ulcers are the minor type. These are under 5 mm in diameter, round or oval, and heal on their own within one to two weeks without leaving a scar. They’re the ones most people picture when they think of a canker sore.
Major ulcers are less common and more aggressive. They measure over 10 mm across (roughly the width of your fingertip), extend deeper into the tissue, and can take weeks or even months to heal. Because they penetrate deeper, major ulcers are more likely to leave a scar once they finally close up.
The third type, called herpetiform ulcers, looks quite different from the other two. Instead of one distinct sore, you get a cluster of tiny pinpoint ulcers, each only 1 to 2 mm wide. A single episode can produce more than 20 of these at once, and neighboring ulcers sometimes merge into larger, irregularly shaped sores. Despite the name, herpetiform ulcers are not caused by the herpes virus.
Where They Usually Show Up
Mouth ulcers form almost exclusively on the soft, non-keratinized tissue inside your mouth. The most common spots are the inner cheeks, the inner surface of your lips, the sides and underside of your tongue, and the soft palate. They rarely appear on the hard gums directly around your teeth or on the outer surface of your lips. If you see a sore in those locations, it’s more likely something else, such as a cold sore or an injury from a sharp tooth.
Mouth Ulcers vs. Cold Sores
These two get confused constantly, but they look and behave differently. A mouth ulcer is a single round or oval sore, white or yellow with a red border, and it only forms inside the mouth. A cold sore is a cluster of small fluid-filled blisters that typically appears on the outside of the mouth, around or on the lips. Cold sores start as visible blisters that eventually burst, crust over, and scab. Mouth ulcers never form blisters or scabs. If you see fluid-filled bumps grouped together on the outer lip, that’s almost certainly a cold sore caused by the herpes simplex virus, not a canker sore.
What a Concerning Ulcer Looks Like
Most mouth ulcers are harmless and heal within two weeks. But not every sore in your mouth is a simple canker sore, and certain visual features suggest something more serious.
An ulcer linked to oral cancer typically has firm, hardened edges that feel thickened or rolled when you press on them, unlike the soft, flat borders of a regular canker sore. The surface may look granular or velvety rather than smooth. Color is another clue: cancerous lesions sometimes present as persistent red patches (a velvety, eroded appearance) or white patches that don’t wipe away. These sores tend to appear on or under the tongue or behind the lower front teeth.
The most important difference is time. A normal mouth ulcer resolves within one to three weeks. In the UK, national cancer referral guidelines flag any unexplained mouth ulcer lasting longer than three weeks as a reason for urgent specialist evaluation. The same principle applies broadly: if a sore doesn’t heal in that window, feels hard or fixed to the tissue beneath it, bleeds easily, or keeps growing, it warrants a professional look. Painlessness can actually be a warning sign too, since cancerous ulcers are often less painful than you’d expect for their size.
What Triggers Ordinary Mouth Ulcers
For most people, mouth ulcers are triggered by minor physical trauma: biting your cheek, brushing too hard, or irritation from braces or a rough tooth edge. Stress and hormonal changes are well-established triggers, and some people notice ulcers flare around certain foods, particularly acidic or spicy items. Nutritional deficiencies in iron, B vitamins, or folate can also make you more prone to recurrent sores. In some cases, frequent mouth ulcers are associated with underlying conditions like celiac disease or inflammatory bowel disease, where the ulcers are one visible sign of a broader inflammatory process.
There’s no single cure for recurrent canker sores, but most minor ulcers heal on their own in 7 to 14 days. Over-the-counter topical gels or rinses can reduce pain while the sore runs its course. Avoiding known triggers, keeping sharp dental work smoothed down, and using a soft-bristled toothbrush all help reduce how often they come back.