Two patterns hide behind that question. Sores that keep returning to one spot usually point at something local — a sharp tooth, a bracket, a cheek you keep catching. Sores that turn up anywhere point at whole-mouth triggers. Telling them apart takes a map and a few dates, not memory.
Search this and you get a list of causes: stress, injury, acidic food, SLS toothpaste, hormones, a deficiency. The list is not wrong. It is just useless on its own, because most of it applies to most people most of the time, and nothing in it tells you which line is yours.
There is a cheaper question that narrows it enormously, and you can answer it from memory right now: do yours land in the same place, or anywhere?
Four sores, one spot. Look for something physical that keeps happening at that point.
Four sores, four places. Look for something whole-mouth in the days before each one.
Those two mouths had the same number of sores over the same three months. They are not the same problem, and looking for the same thing in both is why the cause list never resolves.
Sores in one fixed spot are reporting a place. Mouth lining does not break down in the same few millimetres over and over by coincidence — something is repeatedly happening there. That is a physical thing you can go and find.
Sores anywhere are reporting a time. There is nothing at any one site to look for, because the site keeps changing. What changes with them is the state you were in beforehand.
Because whatever caught the tissue the first time is still there. The usual suspects are dull and mechanical:
None of this is exotic, and that is the good news: it is the one branch of this question with a same-week fix at the end of it. A dentist can run a finger along the edge you cannot see and smooth it in a single appointment. Bring them the location, not the story — "this exact spot, four times since June" is a different conversation from "I get a lot of canker sores".
Which teeth to point at depends on where in the mouth it lands, and the mouth map, site by site, is the short version of that. The gum is the site where this branch is most often the answer, because the tissue there is bound down onto bone and something is usually holding the sore open: a sore on the gum line, and which tooth to check.
Two things are worth separating out from the ordinary version of this. A sore that comes back in the identical spot and never quite closes between rounds, or one that leaves the tissue looking different from how it looked before, is something to have examined rather than tracked.
Partly because that is what the condition does. Recurrence is the normal form of it rather than a sign something has gone wrong — the name for the ordinary kind is recurrent aphthous stomatitis, and it is recurrent in roughly one person in five. Getting another one is not evidence that you are handling it badly.
What varies between people is how often, and that is a more answerable question than why.
Two things you already know, without looking anything up.
Local pattern — look at what touches that point
Tissue rarely breaks down in one fixed place unless something keeps happening in that place. A sharp tooth edge, a rough filling margin, a bracket or wire, the rim of a night guard, a cheek you catch in your sleep. This is the branch with a one-appointment fix at the end of it, because a dentist can feel an edge you cannot see.
That is about one every 23 days.
Start marking where they landA crop of three or four at once argues against the local explanation. One sharp tooth does not produce five sores in five places, so a simultaneous batch points at whatever the whole mouth was exposed to in the days before they appeared.
Log them as separate sores even so. They rarely start on the same day and almost never finish together, and one of them being nearly closed tells you nothing about the one that opened yesterday. Each is its own place with its own course.
Here is the part that defeats memory. The thing that set a sore off happened one to three days before you noticed it, not on the day it started hurting. So the meal you are suspicious of — the one you ate the day the pain showed up — is close to the least likely candidate on the list, and the day that matters is one you have already stopped thinking about.
This is why Canker Core tallies what you logged in the three days before each sore was first marked, rather than on the day itself. It is also why one sore proves nothing. Citrus the day before a sore is a coincidence. Citrus before four out of five is a pattern, and there is no way to see it without several sores' worth of dates written down.
The list it counts against is fixed and short on purpose — stress, poor sleep, a bitten cheek, dental work, sharp food, spicy food, acidic food, citrus, nuts, chocolate, coffee, alcohol, SLS toothpaste, being ill, your period — because "spicy food" and "chilli" have to be the same word for a count to mean anything.
Two of those entries have their own page, because they are the two people arrive already believing. Stress, and what a log can and cannot prove is about why a tally of stressful days convinces everyone and establishes nothing. Whether yours track your cycle is the one trigger on the list that comes with dates already attached, and so the one a few months of logging can actually settle.
When there is barely a gap, trigger-hunting stops working: nothing stands out as unusual because nothing has a quiet week to stand out against. The question changes from what set this one off to what is constant.
That is also the threshold where it is worth someone else looking. Several a month, ulcers that stop you eating or sleeping, a pattern that changes sharply after years of being stable, or sores alongside anything elsewhere in the body — skin, eyes, gut, joints, a fever — are the cases where the mouth is not the whole story. A doctor's first move with frequent ulcers is usually to check things that have nothing to do with the mouth, iron and B12 and folate and coeliac disease among them. Which is a reason to get tested, not a reason to start taking something.
It takes months, not minutes, and it is four things:
Everything else about a canker sore — how long it will last, which stage it is in, whether it is healing — is about one sore. This is the one question that needs a year of them, which is precisely the question memory is worst at and a log is best at.
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.