Canker Sore or Cold Sore? How to Tell the Difference
Clinically reviewed by Dr. Abinaash Kaur, DDS, RCDSO, on 2026-09-14. Dr. Kaur is a general dentist in Toronto (Bloor West Village) registered with the Royal College of Dental Surgeons of Ontario.
Three things separate them: whether it started with a tingle, whether there were blisters, and which kind of tissue it's sitting on. A canker sore appears on the soft movable parts of your mouth with no warning tingle and no blister stage. A cold sore starts with a tingle or burn, blisters, then crusts.
Most people go by a simpler rule. Inside the mouth means canker sore, on the lip means cold sore. Right?
That rule is wrong, and it's wrong in a way that matters.
Herpes shows up inside the mouth too. In a study of 52 patients with laboratory-confirmed intraoral herpes, 47 had ulcers on the firm tissue inside the mouth, and of the 27 who kept getting them, 22 had been told they were canker sores (Oral Surgery Oral Medicine Oral Pathology via PubMed). Four out of five, given the wrong answer.
Wait? Is there a better way to tell than where it is? There is, and I've practised in Bloor West Village for about 25 years watching people get this one wrong.
What does a canker sore look like?
Round or oval, with a grey or yellow floor and a red ring around it. Usually small, often more than one.
Canker sores turn up on the parts of your mouth that move. Inside your lips, inside your cheeks, on your tongue, on the floor of your mouth (Journal of the Formosan Medical Association via PubMed). The clinical name is recurrent aphthous stomatitis. A long-running review describes them as small round or oval ulcers with a clean edge, a red ring, and a yellow or grey centre (Oral Diseases via PubMed).
No tingle beforehand. No blister that pops. It's an ulcer from the start.
Can you give one to your partner or your kids? No. Researchers have spent decades hunting for a bug behind canker sores and haven't found one, and the same review concludes they don't appear to be infectious (Oral Diseases via PubMed).
What does a cold sore look like?
It announces itself. A tingle, an itch or a burning spot comes first, usually hours before anything is visible.
Then small blisters cluster together, break, and crust over. They sit at the edge of your lip most of the time. Canada's public health agency says recurrent cold sores turn up mainly on one particular spot on the lip (Government of Canada).
When they do appear inside the mouth, they favour the firm tissue that doesn't move, meaning your gums and the roof of your mouth (OOOO via PubMed).
So: soft and movable, no tingle, no blisters points to a canker sore. Firm tissue or lip edge, tingle first, blisters points to a cold sore. The warning tingle, the location and the look of the sore are the three things clinicians use to tell them apart (Journal of Contemporary Dental Practice via PubMed).
Why does it come back in the same spot?
Because the virus never leaves. It settles into a nerve and stays there quietly, reactivating from time to time (Virulence via PubMed).
That's also why the tingle is such a reliable warning. The nerve wakes up before your skin shows anything.
Recurrent cold sores heal in roughly 8 to 10 days (Government of Canada). Canker sores generally clear on their own within a week or two.
Ask your pharmacist about antiviral creams and tablets if you get them often. That's their department rather than mine.
Should you keep your cleaning appointment?
Call us first, and expect us to suggest moving a routine cleaning.
That isn't only my preference. The American Academy of Oral Medicine, in its clinical practice statement on treating patients with an oral herpetic lesion, says that "delaying care until an HSV lesion is scabbed over or completely healed is prudent for minimizing recurrences and spread of the infection, and that the presence of an infectious HSV lesion orally or periorally can be a reason for deferral of care" (American Academy of Oral Medicine).
Read the order of their reasons, because it isn't the one people expect. Minimizing recurrences comes first. Rebooking protects you from a dental appointment setting off another outbreak, before it protects anyone else from catching it.
There's a good reason it sits in that order. Most of us carry this virus quietly. One analysis of shedding studies put it at seven people in ten releasing the virus at least once a month without a sore or a symptom (Oral Surgery Oral Medicine Oral Pathology via PubMed). An office that only worried about contagion would have to worry about nearly everybody.
The same academy asks us to keep aerosol-producing instruments away from an active sore, and names the ultrasonic scaler specifically (American Academy of Oral Medicine). A scaler is the main instrument in a cleaning, which is most of the answer on its own.
Note their endpoint: scabbed over or completely healed. Britain's health service draws the window wider still, calling cold sores contagious from that first tingle right through to fully healed (NHS). Erring toward healed rather than scabbed costs you a few days at most.
If you're in pain, something's broken, or your face is swelling, that's different. Come in, and we'll work around it. Our guide to what counts as a dental emergency covers where that line sits.
Keep your hands off it in the meantime, and away from your eyes. The same virus can infect fingers and eyes (Government of Canada), and that's the one piece of prevention entirely in your control.
Why do canker sores keep turning up?
Have no fear, there's usually nothing sinister behind them. Nobody has a single answer though, which is genuinely frustrating if you get them often.
The research points to a mix: a genetic tendency, with about a third of patients having a family history, and nutritional shortfalls showing up in as many as one in five (Oral Diseases via PubMed). Iron, vitamin B12 and folate come up repeatedly in studies of people with recurrent ulcers (Frontiers in Oral Health via PubMed).
Mechanical injury is on the list too. A sharp filling edge, a rough denture, a tooth you keep catching your cheek on.
Getting them in runs? Ask your physician about bloodwork, and ask us whether something in your mouth is doing the nicking. Those are two different conversations and both are worth having.
When is a sore worth showing someone?
Two weeks is my rule of thumb, and Australia's national health service tells people the same: see a doctor if a mouth ulcer outlasts two weeks or keeps returning (healthdirect Australia).
Worth knowing that national services don't agree on the number. Britain's referral guidance uses three weeks for an unexplained mouth ulcer before it triggers a suspected cancer referral (NICE). I'd rather look a week early than a week late.
Both of these sores heal on their own on a fairly predictable clock. If something is still sitting there after a fortnight, it isn't behaving like either one, and the reason to look is that it's stopped following the pattern.
Usually it's dull. A sharp edge on a tooth rubbing the same spot, a denture that needs adjusting, a cheek that keeps getting bitten. Those are the common answers.
An oral cancer check is part of every routine examination, and the Canadian Dental Association lists sores or patches that don't heal among the things worth reporting (Canadian Dental Association). Show it to any dentist, or to your family doctor. It doesn't need to be us.
If it's been years since your last visit and that's part of why you're hesitating, our post on coming back after a long gap was written for you.
Frequently Asked Questions
Q: Are canker sores contagious? No. Researchers have looked hard for an infectious cause and haven't found one, and a major review concludes canker sores don't appear to be infectious (Oral Diseases via PubMed). You can't pass one to a partner or a child, and you didn't catch it from anyone.
Q: Can you get a cold sore inside your mouth? Yes, and this is the most common mix-up. In 52 patients with confirmed intraoral herpes, 47 had ulcers on the firm tissue inside the mouth, and 22 of the 27 with recurrences had previously been diagnosed with canker sores instead (OOOO via PubMed). Inside the mouth, herpes favours the gums and the roof of the mouth, while canker sores favour the soft movable tissue.
Q: How long does a cold sore last? Recurrent cold sores heal in approximately 8 to 10 days (Government of Canada). A first-ever infection is a different and longer event, and it usually happens in childhood. Anything still present at two weeks is worth showing to a dentist or physician.
Q: Will my dentist cancel my appointment if I have a cold sore? Call and ask rather than guessing, and expect a routine cleaning to be moved. The American Academy of Oral Medicine advises that "delaying care until an HSV lesion is scabbed over or completely healed is prudent for minimizing recurrences and spread of the infection" (American Academy of Oral Medicine). Note that preventing another outbreak for you is the first reason listed, ahead of contagion. Urgent problems are handled differently, so say what's going on when you phone.
Q: Why do I keep getting canker sores? There's rarely one cause. A family history is present in around a third of people who get them, nutritional deficiencies turn up in up to one in five, and mechanical irritation from a sharp tooth or appliance is a recognised trigger (Oral Diseases via PubMed). Iron, B12 and folate are the deficiencies that come up most often in the research (Frontiers in Oral Health via PubMed), which makes bloodwork a reasonable conversation with your physician.
Q: Does a sore that won't heal mean something serious? Usually not. The common explanations are mechanical, like a sharp edge rubbing the same spot. That said, the Canadian Dental Association does list sores or patches that don't heal among the signs of oral cancer worth reporting (Canadian Dental Association), and a check for it is part of a routine dental examination. Two weeks is a sensible point to have it looked at by any dentist or your physician.
Reviewed by Dr. Abinaash Kaur, B.Sc., DDS (University of Toronto Faculty of Dentistry), who has practised general and family dentistry in Bloor West Village for about 25 years. This article is general oral health information, not a diagnosis or a substitute for an examination.
Got a sore you can't place, or one that's outstayed its welcome? Give us a ring and we'll tell you honestly whether it needs looking at and when to come in. We're at 750 Annette Street in Bloor West Village, serving the Junction, High Park, and Baby Point.
