Canker sores typically appear as small, round or oval ulcers inside the mouth with a white or yellowish center and a distinct red border. They form on soft tissue that isn’t tightly bound to bone: the inner cheeks, inner lips, the underside of the tongue, or the soft palate. Most are less than 5 mm across, roughly the size of a pencil eraser, and they develop over a predictable timeline of a few days before healing on their own.
What Canker Sores Look Like
A canker sore starts as a raised, reddened spot on the inside of your mouth. Within a day or two, that spot breaks down into a shallow, “punched-out” ulcer covered by a loosely attached membrane that looks white, yellow, or grayish. The tissue immediately surrounding the ulcer stays red and inflamed, creating a halo effect. The overall shape is usually round or oval with clean edges, which distinguishes it from more irregular mouth injuries.
The sore itself is an open wound in the mucous membrane, so it’s soft and sensitive to the touch. Eating acidic or salty foods, brushing your teeth, or even talking can irritate it. Despite how painful they feel, most canker sores are small and shallow enough to heal without leaving any scar.
How They Develop Stage by Stage
Canker sores follow three distinct stages, and recognizing the first one can help you know what’s coming.
The prodromal stage lasts one to three days before any visible sore appears. During this phase, you’ll feel a burning, tingling, or prickling sensation at a specific spot inside your mouth. The area may look slightly red or swollen, but there’s no ulcer yet. This is the body’s early inflammatory response beginning beneath the surface.
The ulcer stage begins around day three, when the tissue breaks open and the characteristic white or yellow crater forms. This is when pain peaks. The ulcer is fully formed and surrounded by its red halo. This stage typically lasts three to six days, though it can stretch longer for larger sores.
The healing stage follows as healthy tissue gradually closes over the sore from the edges inward. Pain fades as the ulcer shrinks. For minor canker sores, healing wraps up within 10 to 14 days total from the first tingling sensation. No medication is needed for them to resolve.
Three Types and How They Differ
Minor canker sores account for about 80% of cases. They’re under 5 mm in diameter, appear one or two at a time, and heal within one to two weeks without scarring. If you’ve had a canker sore, this is almost certainly the type.
Major canker sores exceed 1 cm in diameter and are noticeably deeper. They can take up to six weeks to heal and often leave a scar. These are less common and significantly more painful, sometimes making it difficult to eat normally.
Herpetiform canker sores look completely different from the other two types. Instead of one or two distinct ulcers, they appear as clusters of many tiny pinpoint sores, sometimes as many as 100 at once. These small ulcers can merge together into larger, irregularly shaped wounds. Despite the name, they have nothing to do with the herpes virus. They generally heal within a month.
What Triggers Them
The exact cause of canker sores isn’t fully understood, but the immune system plays a central role. White blood cells attack the mouth’s own mucosal lining, causing localized tissue breakdown. Why this happens varies from person to person, but several consistent triggers have been identified.
Physical trauma to the mouth is one of the most common. Biting your cheek, poking your gum with a tortilla chip, or irritation from braces or a hard toothbrush can all set off a sore. The ulcer doesn’t appear at the moment of injury. It develops over the following one to three days as the inflammatory process ramps up.
Nutritional deficiencies are another well-documented factor. One study found that about half of people with recurrent canker sores were deficient in vitamin B12, compared to none in the control group. Low levels of folate and iron have also been linked to frequent outbreaks. If you get canker sores repeatedly, these deficiencies are worth investigating.
Stress, hormonal changes, and certain foods (particularly acidic fruits, chocolate, and coffee) are frequently reported triggers, though the connection is harder to measure. Some people notice a clear pattern tied to stressful periods or their menstrual cycle. Toothpastes containing sodium lauryl sulfate, a foaming agent, have also been associated with more frequent sores in some individuals.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re different conditions. The simplest way to tell them apart is location. Canker sores only form inside the mouth. Cold sores (fever blisters) form outside the mouth, typically around the border of the lips.
Their appearance is also distinct. A canker sore is a single round or oval ulcer with a white or yellow center. A cold sore is a cluster of small, fluid-filled blisters that eventually burst and crust over. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not caused by a virus and cannot be spread to another person.
What Affects Healing Time
Most minor canker sores heal completely within 10 to 14 days without any treatment. The pain is usually worst during the first few days of the ulcer stage and gradually fades from there. Over-the-counter topical gels or rinses can reduce discomfort during that peak pain window but don’t dramatically speed up healing.
Major canker sores are a different situation. Because they’re deeper and larger, they can persist for up to six weeks. Sores that last longer than three weeks, that are unusually large, that spread rapidly, or that come with a fever warrant professional evaluation. Recurrent outbreaks, defined as sores that keep returning multiple times a year, can sometimes signal an underlying condition like celiac disease, an immune disorder, or a nutritional deficiency that needs to be addressed directly.