Where is it?
Canker sores favour the soft, mobile lining inside the mouth — the inner lip and the cheek above all. The further a sore sits from there, the more likely it is to be something else or to have a cause you can point at, which is why the site is worth writing down before anything else about it.
Pick the site below. Each one answers the same three questions in the same order, so that the sites can be compared: what the tissue there is, what keeps rubbing it, and what a sore there usually does.
Inside the lip
- What is there
- Thin lining that is not bound down to anything underneath, so it slides when you talk. This and the cheek are the two sites where ordinary canker sores turn up most, and it is the inside of the lower lip more often than the upper.
- What rubs it
- Your own front teeth, mostly. A lip caught while eating, the edge of a chipped or newly filled incisor, an orthodontic bracket, a toothbrush that runs over the same spot every morning.
- What a sore there usually does
- The textbook course, which is why most descriptions of a canker sore are really descriptions of this site: a day or two of tingling, then a round crater 2 to 5 mm across with a pale floor and a red rim, worst in the first three days, closed inside 7 to 14 days and leaving nothing behind.
Inside the cheek
- What is there
- The same soft, unattached lining as the lip, over a much larger area. Run your tongue along it and you can feel a faint horizontal ridge where the upper and lower teeth meet — the bite line, and a common place for a sore to sit.
- What rubs it
- The teeth on that side and anything next to them: a sharp cusp, a rough filling margin, a bracket or wire, the rim of a night guard, or a cheek caught between the teeth in your sleep.
- What a sore there usually does
- Like the lip, but often bigger, because there is more room for it to be. Anything over about 5 mm across is past the one-week version and can take three weeks — and a large one on the cheek is the case where the fortnight genuinely is not over yet.
Sores landing in the same few millimetres of cheek month after month are reporting a place rather than a time, and that is the branch with a one-appointment fix at the end of it.
Why they keep coming back in the same spotGums
- What is there
- Tough, keratinised tissue bound tightly down onto the bone — quite unlike the lip and the cheek. Ordinary canker sores prefer lining that moves, so a sore on the gum is less likely to be a plain aphthous ulcer and more likely to have a cause you can point at.
- What rubs it
- One specific tooth, usually. Brushing hard at the gum margin, floss snapping down onto it, a bracket or a denture clasp, or something sharp eaten the day before.
- What a sore there usually does
- A sore held against the gum by a tooth or an appliance keeps being re-injured, so it heals on the schedule of whatever is touching it rather than on its own. Take the touching away and it behaves like any other sore; leave it and the same 3 mm keeps reopening.
Gum that is swollen rather than ulcerated, a bad taste, throbbing that keeps time with your pulse, or a tooth that has become tender to bite on is not a canker sore question at all. That is a dentist this week.
A canker sore on the gum line, and which tooth to checkRoof of the mouth
- What is there
- Two different tissues in one view. The front two-thirds is hard palate: keratinised, ridged, stretched over bone. Behind it the soft palate is the mobile lining a canker sore actually favours.
- What rubs it
- Heat, more than anything — the first bite of pizza, coffee taken too early, a chip straight out of the fryer. Also the rim of a denture or a retainer, and hard food scraping across the ridges.
- What a sore there usually does
- The two things that happen here look different, which is the useful part. A burn scuffs a wide, irregular, shallow patch that stings for a few days and settles. A canker sore is round, punched-out and takes the full fortnight. Shape separates them more reliably than trying to remember what you ate.
Tongue
- What is there
- Muscle with a textured top surface. The sides and the underside are softer lining and are where sores turn up; the top, with its papillae, much less often.
- What rubs it
- The edges of the lower front teeth against the sides of the tongue, sharp food, and the tongue’s own habit of going back to anything sharp until the spot is sore.
- What a sore there usually does
- Out of all proportion to its size, and that is the honest thing to say about this site. A 3 mm sore on the side of the tongue can be the most painful one you get, because the tongue cannot be held still and every word moves it. It heals on the usual schedule regardless.
Under the tongue
- What is there
- Thin, very mobile lining under the tongue, with the frenulum running down the middle of it and a salivary duct opening either side.
- What rubs it
- The lower front teeth, and anything held under the tongue. Not much else reaches it.
- What a sore there usually does
- Normal enough, but hard to see, which is why a sore here tends to get logged late and measured badly. Lift the tongue in front of a mirror with a phone light on it rather than trying to work out by feel where the edges are.
Swelling under the tongue rather than an ulcer — particularly swelling that comes up at meals and goes down afterwards — is a different thing entirely, and belongs with a dentist rather than in a log.
Back of the mouth
- What is there
- The soft palate, the arches either side of it, and the tonsils behind them. Soft lining, but a long way from the teeth.
- What rubs it
- Very little, mechanically. Nothing back there is catching on a filling, and that absence is itself informative.
- What a sore there usually does
- This is the one site where the first thing to say is that it may well not be a canker sore. An ulcer far back — especially on a tonsil — is more often part of a throat infection, or one of the viral illnesses that put sores across the back of the mouth, and those arrive with symptoms a canker sore does not.
A sore throat with a fever, swollen glands, pain on swallowing, or pain on one side lasting more than a few days is worth showing someone rather than logging. Anything making it hard to swallow or breathe is today, not this week.
Why the place is worth recording at all
Because it is the one piece of information that splits recurring sores into two different problems. Sores that keep landing within a few millimetres of the same point are reporting a place: something in that place keeps injuring the tissue, and a dentist can often find it by running a fingernail along the teeth beside it. Sores that turn up anywhere are reporting a time: nothing at any one site is causing them, so the thing to look at is the few days before each one.
Nobody can tell those apart from memory. “Left cheek, usually” covers an area the size of a postage stamp folded twice, and a mechanical cause and a whole-mouth one look identical at that resolution. The recurrence article is the long version of this, and it is the reason the map stores a point rather than a name.
What the map records
A mark on one of the three views, which is a position rather than a label — the site name is worked out from where the mark lands, so it stays true if you nudge the mark later. Then, once a day, the sore’s width in millimetres and its pain out of ten. That is the whole record, and it is enough to answer the two questions a single look cannot: whether this one is shrinking and how much of the fortnight is left.
Before you use the map, make sure it is the right map
Everything here is about sores on the lining inside the mouth. A sore on the lip border itself, or on the skin outside it, is a different condition with a different course — which side of that border it sits on decides it, and it takes one look in a mirror.
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.
Find out instead of guessing
Mark the sore, log its width and pain, and compare tomorrow. About fifteen seconds.