A canker sore starts as a small, round, yellowish bump surrounded by a red halo on the inside of your mouth, usually on the inner cheek, inner lip, or tongue. Before you can see anything, though, you’ll likely feel it: a tingling, burning, or stinging sensation that lasts one to three days before a visible sore appears.
What the Earliest Stage Feels Like
The first sign of a canker sore isn’t something you see. It’s a localized burning or prickling feeling on a specific spot inside your mouth. This is called the prodromal stage, and it typically lasts one to three days. During this window, the area might feel tender when your tongue brushes over it or when you eat something acidic or spicy. You may notice the spot feels slightly swollen or irritated, but if you look in the mirror, you might not see much of anything yet.
Common triggers for this initial irritation include biting the inside of your cheek, getting poked by a sharp chip or crusty bread, stress, and certain foods like chocolate, peanuts, and eggs.
What the First Visible Signs Look Like
Over the next one to three days, that tender spot develops into something you can actually see. The earliest visible stage is a small, round, raised yellowish spot with a distinct red ring around it. The red ring is inflamed tissue, and it often looks slightly swollen.
That raised spot then breaks open into a shallow ulcer. At this point, the sore has a white, yellow, or grayish center covered by a thin membrane, still ringed by the red halo. Most canker sores are small, about 2 to 4 millimeters across, roughly the size of a pencil eraser or smaller. The shape is usually round or oval with clean, well-defined edges, almost like a tiny crater punched into the tissue.
As the sore matures, the floor of the ulcer shifts from yellowish to a slightly gray tone. This color change is normal and signals that healing has begun, even though the sore may still hurt.
Where Canker Sores Appear
Canker sores only form inside the mouth. The most common locations are the inner cheeks, the inner surface of the lips, the sides and underside of the tongue, and the soft tissue at the base of the gums. They tend to show up on soft, movable tissue rather than on the hard palate or the firm gum tissue directly around your teeth. If you notice a sore on the outside of your lips or on your skin, it’s not a canker sore.
Canker Sore vs. Cold Sore
The easiest way to tell these apart is location. Canker sores appear inside the mouth. Cold sores (fever blisters) appear outside the mouth, typically along the border of the lips or on the surrounding skin. Cold sores also start as a cluster of tiny fluid-filled blisters that eventually burst and crust over. Canker sores never blister and never form a scab. They’re open, shallow ulcers from the start.
Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not caused by a virus and cannot be passed to someone else.
Size Variations
Most canker sores are the minor type: 2 to 4 millimeters wide, round, and gone within a week or two without scarring. Occasionally, a canker sore grows larger than a centimeter (bigger than a pea). These major sores are deeper, more painful, and can take weeks to heal. A third, less common type called herpetiform canker sores appears as clusters of very small ulcers that can merge together. Despite the name, they have nothing to do with the herpes virus.
When a Mouth Sore Needs Attention
A single small canker sore that heals within two weeks is rarely a concern. But certain patterns deserve a call to your doctor or dentist: sores that last longer than two weeks, sores larger than a centimeter, sores that come back two or three times a year, sores accompanied by fever or flu-like symptoms, or sores painful enough to interfere with eating and drinking. Persistent or unusually large mouth sores can sometimes signal nutritional deficiencies, immune system issues, or other conditions that look similar but require different treatment.