A canker sore on your gums appears as a small, round or oval ulcer with a white or yellowish center and a distinct red border. It sits flat against the tissue or slightly below the surface, unlike a blister or bump. Most are smaller than a pea, and they show up on the inside of the mouth only, never on the outer lip or skin.
Exactly What You’ll See
In its earliest stage, a canker sore starts as a slightly raised, yellowish spot surrounded by a red halo. You might feel a tingling or burning sensation a day or two before anything is visible. Within a day or so, that spot breaks open into a shallow, punched-out ulcer covered by a loosely attached white, yellow, or grayish membrane. The tissue around the edges stays red and inflamed, sometimes slightly swollen.
The sore itself has a smooth surface. It won’t look patchy, textured, or crusty, and it won’t bleed or ooze fluid. As it heals over the next one to two weeks, the center often shifts from white to a grayish tone before the tissue closes over completely.
Where on the Gums They Show Up
Canker sores strongly prefer the softer, movable tissue inside the mouth. The most common spots are the inner cheeks, the floor of the mouth, and the sides or underside of the tongue. On the gums specifically, they tend to appear on the loose tissue near the base of the teeth or along the gum line where it meets the inner cheek, rather than on the firm, pale-pink tissue directly covering the bone.
That distinction matters because the standard minor canker sore (which accounts for 75% to 85% of all cases) almost always stays on softer surfaces. The more severe major type, which makes up about 5% to 10% of cases, can break this rule and appear on firmer gum tissue as well.
Size Differences Between Types
There are three types of canker sores, and size is the quickest way to tell them apart:
- Minor canker sores are the most common. They measure less than 1 centimeter across (smaller than a pea) and heal within one to two weeks without leaving a scar.
- Major canker sores are larger than 1 centimeter, noticeably deeper, and can take weeks or even months to fully heal. These often leave a scar behind.
- Herpetiform canker sores look very different from the other two. Instead of one round ulcer, you’ll see a cluster of tiny pinpoint sores, sometimes dozens at once, each only 1 to 3 millimeters wide. Despite the name, they have nothing to do with herpes. They typically heal within a month.
If you’re seeing a single small white sore with a red ring around it, you almost certainly have the minor type.
How They Differ From Cold Sores
The confusion between canker sores and cold sores (fever blisters) is common, but the two look and behave quite differently. Cold sores are clusters of small, fluid-filled blisters that appear outside the mouth, usually along the border of the lips. They eventually burst, crust over, and scab. A canker sore is a single open ulcer inside the mouth with no fluid, no blistering, and no scab.
Location is the simplest tell. If the sore is on the inner gum tissue, inner cheek, or tongue, it’s almost certainly a canker sore. If it’s on the outer lip or the skin around the mouth, it’s more likely a cold sore. Cold sores are also caused by the herpes simplex virus and are contagious. Canker sores are not caused by a virus and cannot spread to other people.
When a Sore Looks Different Than Expected
A typical canker sore has a clean, smooth appearance and resolves within two weeks. Certain visual features suggest something else is going on. Be alert if you notice any of the following: red, white, or mottled patches that don’t fit the clean round shape of a canker sore; rough, cracked, or crusty texture on or around the sore; bleeding from the ulcer; a hard lump or bump underneath the sore; or a sore that lasts longer than two weeks without improving.
A canker sore that keeps growing, changes in texture, or refuses to heal on its own timeline is worth getting evaluated. The same applies if you notice unexplained swelling in your neck, cheek, or jaw, or if the tissue around the sore becomes permanently discolored. These features can signal oral conditions that need a professional exam to rule out.