Canker sores are inside the mouth, on tissue that moves — inner lip, cheek, tongue. Cold sores are on the lip border or the skin outside it, and blister before they crust. Canker sores are not contagious and not viral; cold sores are both.
Three questions settle this, and you can answer all three in front of a mirror in about ten seconds.
Drawn rather than photographed, and drawn against the lip border on the right, because where the sore sits relative to that line is most of the answer.
| Canker sore | Cold sore | |
|---|---|---|
| Where | Inside the mouth, on movable lining | Lip border, or skin just outside it |
| Starts as | An open crater | A cluster of blisters |
| Looks like | White or yellow floor, red ring | Blisters, then weeping, then a scab |
| How many | Usually one | Several together in one patch |
| Cause | Not a virus | Herpes simplex, usually type 1 |
| Contagious | No | Yes, most while blistered or weeping |
| Comes back | Anywhere, or the same spot | Nearly always the same spot |
| Typical course | 7 to 14 days | 7 to 10 days |
Two rows do all the work: where it is and whether it is contagious. Those are facts about two different conditions rather than judgements about your sore, which is what makes them worth having. Nearly every other difference — how much it hurts, how big it gets, whether it woke you up — overlaps enough between the two to be useless.
This is the case that sends people looking, because the lip is the one place both are plausible, and the answer is a line rather than a feature.
The vermilion border is the edge where the coloured part of your lip meets the skin of your face. Run a fingertip over it: the lip side is smooth and slightly damp, the outside is ordinary dry skin.
If it is genuinely sitting on the line and you cannot call it, the sequence decides instead: wait a day. Blisters mean cold sore. A crater that was never a blister means canker sore.
Fever blister, cold sore and herpes labialis are three names for one thing. The comparison people usually search for — canker sore, cold sore and fever blister — is really a comparison of two conditions with three names, and the canker sore is the only one that is a separate thing.
No. Canker sores are not caused by a virus, are not contagious, and have nothing to do with herpes simplex. That question deserves more than a paragraph, so it has its own page — including the part most comparisons skip, which is that a first herpes infection genuinely can cause ulcers inside the mouth, and what tells that apart from an ordinary canker sore.
A cold sore runs about 7 to 10 days: tingle, blisters within a day, weeping, crust, then skin. A canker sore takes 7 to 14 days, and a big one longer.
Both share the trap that makes people think theirs is going wrong: the pain peaks early and eases while the sore is still at its widest, so by the middle of the week it hurts less and looks worse. That is the normal shape of both, not a sign that something has gone wrong.
Then you know two useful things: it is nobody else's business — there is nothing to catch or pass on — and it has a course, which means today's appearance is not information on its own. One look tells you nothing; the same sore measured twice tells you everything, and the gap between those two facts is the whole reason people spend a fortnight wondering whether theirs is stuck.
The five stages are what to expect. Whether yours is healing is a question about millimetres, and it takes two measurements two days apart to answer.
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.