How long is too long for a canker sore?

Three weeks is the line worth treating as firm. Before that, the distinction that matters is not how long it has been there but whether it is measurably narrowing: a wide sore can take the full fortnight and a few days more while genuinely closing, and a sore that reads the same width on two dates a week apart is not on that course at all.

Why is my canker sore not healing?

The commonest reason is that it is healing and you cannot see it. A crater fills in from the edges by a millimetre or so a day, which is invisible to memory and obvious against a ruler. The next commonest is that something keeps reopening it — a tooth edge, a bracket, a denture rim, or a tongue that will not leave it alone — so the wound restarts rather than continues.

Why do canker sores take so long to heal?

Because a mouth ulcer is an open wound in tissue that is used several thousand times a day, kept wet, and disturbed by every meal. Healing is the crater filling in from its edges, so the time it takes is set mostly by how wide it got: filling a 10mm crater is more work than filling a 3mm one.

Should I see a dentist or a doctor about a mouth ulcer?

Either can look at it, and a dentist is usually quicker to get and better placed to find a sharp edge holding it open. A doctor is the better first call when there are ulcers elsewhere, several at once, a fever, a rash, or a change in how often you get them, because those are questions about the rest of the body rather than about the mouth.

What should I bring to the appointment?

The date you first felt it, the widest width you measured and the date you measured it, the width now, and where exactly it is. That is a thirty-second history that answers most of what will be asked, and it is the part nobody can reconstruct in the chair.

Two weeks, and it is still there

Most canker sores close within 7 to 14 days. Past two weeks at the same width, or past three weeks at all, the useful move is an examination rather than another week of waiting — and two dated measurements are what make that appointment short.

September 17, 2026

Most pages about mouth ulcers are written for day three. This one is for the fortnight mark, when the sore has outlasted every timeline you have read and the question has quietly changed from how long does this take to is something wrong.

The first thing to do with that question is split it, because "still there" and "not healing" feel identical from the inside and are not the same situation at all.

Still there is not the same as not healing

A canker sore closes by filling in from its edges, at something like half a millimetre to a millimetre a day. That is real progress and it is completely invisible to you. A sore you looked at on Tuesday and again on Thursday looks the same both times, because the change is smaller than your memory of a crater in a mirror at an awkward angle.

So the honest position on day fourteen, for nearly everyone who arrives here, is: you do not currently know whether yours is healing slowly or not moving at all, and those two have different answers. One is waiting. The other is an appointment.

Is it lasting, or is it stuck?

The two feel the same and they are not the same.

Days since you first felt it14

Compared with a week ago, the width is

Measurably narrower than a week ago

The same width as a week ago

Wider than a week ago

I have not measured it, so I could not say

This is the question you cannot answer from memory

A sore you looked at on Tuesday and again on Thursday looks the same to you both times, because two millimetres is invisible to recollection and obvious against a ruler. Measure the widest point today, write the number down with the date, and measure again in two days. At day 14, that comparison is the whole difference between waiting sensibly and waiting pointlessly.

Keep the dates and the numbers

Why canker sores take so long to heal

It is an open wound in the one place in the body that is never given a rest. The mouth is wet, it is used several thousand times a day, and every meal disturbs the surface that is trying to close. A cut on your forearm gets to scab and sit still; this one does not.

Within that, four things set the pace:

How wide it got. The best single predictor. A crater fills from its rim, so the area to close grows faster than the width does — a 10mm ulcer is not twice the work of a 5mm one.
Where it is. The inside of the lip, the edge of the tongue and the line the teeth close on all get re-irritated daily. A wound reopened every day is a wound that restarts rather than continues.
Whether something is holding it open. A sharp cusp, a rough filling margin, a bracket, a denture clasp or a retainer wire will keep a sore in business indefinitely, and this is the reason a sore can seem to last for months when what is really happening is that the same few millimetres keep reopening. That is a question about which tooth, and it has a same-week fix at the end of it.
Whether you keep prodding it. Almost involuntary, and it genuinely extends the course.

None of that makes a slow sore abnormal. It makes a slow sore explicable, which is different from a sore that has stopped changing.

The three lines worth acting on

Day fourteen at the same width. Not still there — still the same size, measured twice. That is the distinction memory cannot make and the one that actually means something.
Still widening after day four. Ordinary sores stop growing early, so late growth is a course that has departed from the usual one. If the worry is early growth instead, growing in the first three days is what they all do.
Three weeks, full stop. Regardless of what it looks like, whether it hurts, or whether it seems to be slowly improving. Any ulcer in the mouth still present at three weeks should be looked at.

Add to those: wider than 10mm, several at once with a fever or feeling unwell, ulcers appearing elsewhere in the body, or a sore that leaves the tissue looking different from how it looked before.

What is worth bringing

The reason to keep measuring a sore you are about to have looked at is that it turns a vague complaint into a short appointment. The history that gets asked for is nearly always the same four things, and they are the four things nobody can reconstruct in the chair:

When you first felt it, not when you first looked at it.
The widest it got, with the date you measured it.
What it measures now.
Exactly where it is — the specific spot, not the region. "Left cheek" and a point on the left cheek are different pieces of information, which is why the map is a map rather than a dropdown.

If it has happened before in the same place, bring that too: "this exact spot, four times since June" is a different conversation from "I get a lot of canker sores", and it points at something physical to find.

What this page will not tell you

What a long-lasting ulcer means. Anything on the internet that offers you a list of what it might be is asking you to diagnose yourself from a description, and a description is exactly what cannot settle it — a sore is examined, and if there is real doubt, sampled. That takes minutes and it is the entire reason the three-week line exists.

What a log gives you is not a diagnosis. It is the difference between arriving with a worry and arriving with a record: two dates, two numbers, and a place. That is a question someone can actually answer, and the ordinary course is what it gets compared against.

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.

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