A sore sitting right on the gum is often not an ordinary canker sore. Gum is tough tissue bound down onto bone, and canker sores prefer lining that moves — so one there usually has something holding it there. Find the edge that touches it and the sore follows.
Most writing about canker sores describes the inside of the lip and then assumes the mouth is the same everywhere. It is not, and the gum is where that stops being a technicality.
Yes, and it is worth knowing that it is the less usual place for one. Put a fingertip on the inside of your lower lip and slide it: the lining moves, because it is not attached to anything underneath. Now do the same on the gum in front of your lower front teeth. It barely moves at all. That is keratinised tissue bound tightly down onto the bone, and it is built for being chewed against.
Ordinary aphthous ulcers — canker sores — favour the loose, movable lining: the inner lip, the cheek, the floor of the mouth, the underside of the tongue. They turn up on gum far less often. So a sore that has appeared on the gum, and especially one that keeps appearing in the same place on the gum, is more likely than a sore elsewhere to have a cause sitting next to it.
That is good news, because a cause sitting next to it is something a person can find.
Narrow it down before the appointment, not during it.
Two things you can see in a mirror.
Feel along the two central incisors, the narrower laterals either side of them, and the pointed canines behind those, on the lower arch.
Run a clean fingertip along their biting edges and then along their necks, where the tooth meets the gum, on the side the sore is on. Anything that catches a fingernail is catching your gum in exactly the same way several thousand times a day.
Usually it is one of these: A retainer wire bonded behind the lower front teeth, which is the single most common object in this position. Otherwise a chipped incisor, or brushing hard at a gum margin that is thin to begin with.
The sentence worth arriving with: “I keep getting a sore on the lower gum by these teeth — can you feel whether anything there is sharp?” It takes a minute and it is the question that ends the cycle, if something sharp is what is causing it.
Mark exactly where it is on the mapBecause tissue does not break down in one fixed place unless something keeps happening in that place. A sore that lands on the gum beside the same tooth every few weeks is not describing your health — it is describing an object.
The objects are boring and the list is short: a cusp that has chipped to an edge, the margin of a filling or a crown that a fingernail can catch, an orthodontic bracket or the cut end of a wire, the clasp of a partial denture, the rim of a night guard, a retainer wire bonded behind the lower front teeth. Add to those the ones that are a technique rather than an object: a hard brush driven along the gum margin, and floss snapped down onto the gum instead of slid against the tooth.
None of that is diagnosable from a web page, and none of it needs to be. It needs one person to feel the spot for thirty seconds. What this page can do is make sure you know which spot to point at, which is why the map stores a position rather than a name — "lower gum, by the second tooth from the front" is a place someone can check, and "my gums" is not.
If the sores turn up on the gum in a different place every time, that is the other story entirely, and it is about the days before rather than the place.
This is the distinction on this page that actually matters, and it is mostly about crater versus bump.
An ulcer is a crater: a shallow break in the surface with a pale yellow or white floor and a red rim, and gum around it that otherwise looks like gum. An abscess is swelling: a bump on or above the gum, with the surface intact over it. It often comes with throbbing that keeps time with your pulse, a bad taste, a tooth that has gone tender to bite on, or a face that looks different in the mirror.
Swelling, throbbing, a bad taste or a tender tooth is not a fortnight-of-waiting problem. That is a dentist this week, and sooner if there is any swelling spreading into the face or difficulty swallowing.
Left alone, the same 7 to 14 days as anywhere else in the mouth, and the same sequence — worst pain in the first three days, widest around the middle of the week, closing from the edges inward.
Held against something sharp, it heals on that object's schedule instead. This is the whole reason a gum sore can feel like it has lasted for months: it has not been one sore for months, it has been the same three millimetres of tissue breaking down, half-healing, and breaking down again. The width tells you which of the two is happening. A sore that is genuinely healing gets narrower every couple of days; a sore being re-injured hovers at the same size, and hovering is the finding, not a failure of the measurement.
Several at once, spread along the margin, is a different picture again, and two things are worth separating. Gum that is red, puffy and bleeding when brushed along its whole length is inflamed rather than ulcerated, and that belongs with a dentist and a hygienist. Multiple genuine ulcers appearing across the gums at once — particularly alongside a fever or feeling generally unwell — is also not this page, and is worth being seen for the same week rather than measured.
Three things, in order, and none of them take long.
Canker Core is a log, not a diagnosis. Nothing here is medical advice. A sore that has not started shrinking after two weeks, keeps coming back in the same spot, or comes with a fever or a rash is worth showing to a dentist or doctor.
Mark the sore, log its width and pain, and compare tomorrow. About fifteen seconds.