Some people get canker sores at the same point in every cycle and most do not. It is one of the few trigger questions with a clean test, because cycle day is knowable for every sore — three or four cycles of dates settle what memory cannot.
Most trigger questions are hard because the candidate is vague and its timing is unrecorded. This one is the exception, and it is worth saying plainly: your cycle is the only trigger on the list that comes with dates already attached.
That makes it the one question in this whole subject that a few months of logging can genuinely settle.
For a minority of people who get recurrent canker sores, they arrive in a consistent relationship to the cycle — most often in the week before a period. For most, they do not, and the ulcers turn up scattered across the month with no relation to it at all.
Both of those are ordinary. What no article can tell you is which one you are, and the general finding is a poor guide: hearing that hormones are a recognised trigger will make you notice the sores that fit the story and forget the ones that did not.
Where a pattern exists, it tends to sit in the luteal phase — the stretch between ovulation and the start of bleeding — with the sore appearing in the few days before the period rather than during it.
But the phase matters much less than the consistency. A sore in the week before your period three cycles running is a pattern. One in that week, one mid-cycle and one during a period is not a pattern with an exception in it; it is an absence of pattern, and reading it as the former is how a coincidence becomes a belief.
Two dates per event, and arithmetic:
Three cycles is roughly three months, which is the honest cost. It is also the shortest version of this that exists, because a cycle is the unit and you cannot get four of them any faster.
The premenstrual week is not only a hormonal event. For a lot of people it also carries worse sleep, more stress, different eating and sometimes a bitten cheek from a clenched jaw — which are four other entries on the same trigger list, all arriving in the same window.
So a sore that lands reliably in that week has genuinely told you when, and has told you nothing about which. Separating them needs the other triggers logged as well, so that the premenstrual weeks where you slept fine can be compared against the ones where you did not. That is a subtler question than most people need answered — but it is the difference between "my cycle" and "the week my cycle makes hard", and only one of those has anything to do with hormones.
The same base-rate trap applies here as everywhere else, and the stress page is where that arithmetic is worked through: a trigger that is present most of the time explains nothing, however often it precedes a sore.
Reports of hormonal contraception changing canker sores go in both directions and there is no reliable general answer, which is exactly what makes a start or stop date one of the most informative things a log can contain.
It is the closest this subject comes to an experiment: a clear date, with months of ordinary record either side of it. If your rate changes noticeably a couple of months after that date, that is a finding about you. Without the record, it is a feeling about a year you will not remember clearly.
If they do track your cycle, you gain two real things: you stop hunting for a cause in the food you ate, and you know roughly when the next one is due, which is the only form of warning this condition offers.
If they do not — which is the likelier result — you have removed the most plausible-sounding candidate from your list, and what remains is the ordinary pair of questions: whether yours are reporting a time or a place, and how the one you have now is getting on.
Either way, the cycle is on the trigger list in the app for this reason. It is the entry that needs no judgement call to record and no memory to interpret — just two dates and a subtraction, kept for three months.
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.