Canker sore or cold sore? How to tell them apart, and which one is contagious
Canker sores and cold sores are both painful, both come back, and both get called mouth sores, so they are often confused. They are completely different things: one is a virus and contagious, the other is not an infection at all. Where the sore is gives most of the answer.
Canker sores are small, round ulcers inside the mouth, on the soft lining of the cheeks, inner lips, under the tongue or gums, with a white or yellow centre and a red edge. They are not caused by a virus and not contagious. Cold sores are clusters of blisters, usually outside the mouth on or around the lip, caused by the herpes simplex virus, and they are contagious until healed. Both usually heal within one to two weeks.
The differences side by side
| Canker sore | Cold sore | |
|---|---|---|
| Where | Inside the mouth on soft, loose lining: inner cheeks, inner lips, under the tongue | Outside the mouth on or around the lip; less often on the gums or hard palate |
| Look | Single round or oval ulcer, white or yellow centre, red border | Cluster of small fluid-filled blisters that weep and crust |
| Warning signs | Sometimes a burning spot before | Often tingling, itching or burning beforehand |
| Cause | Not fully understood; not an infection | Herpes simplex virus, usually type 1 |
| Contagious | No | Yes, until fully healed |
| Common triggers | Minor injury, stress, acidic foods, some toothpastes, low iron, B12 or folate | Sun, illness, stress, tiredness, hormonal changes |
| Heals in | About one to two weeks | About 7 to 10 days |
The location rule
Most of the time location settles it. A sore inside the mouth on the soft, moveable lining is almost certainly a canker sore. A sore outside, on the lip or the skin around it, is much more likely to be a cold sore. The exception is that cold sores can occasionally appear inside the mouth, usually on the firm tissue of the gums or the roof of the mouth rather than the soft lining, often as a cluster of tiny sores.
What triggers each
Canker sores often follow a minor injury, such as biting your cheek, a sharp food or a toothbrush slip. Stress, acidic or spicy foods, and toothpastes containing sodium lauryl sulphate are commonly reported triggers. Frequent canker sores can be linked to low iron, vitamin B12 or folate, and sometimes to conditions such as coeliac disease or inflammatory bowel disease, which is why repeated ones are worth mentioning to a doctor.
Cold sores are the virus reactivating. Sun, a cold or other illness, stress, tiredness and hormonal changes are the classic triggers, and they tend to come back in the same spot. More in why you keep getting cold sores.
Treatment is different
- Canker sores: rinse with salt water or an antiseptic mouthwash, avoid acidic and spicy foods while they heal, use a soft toothbrush, and ask a pharmacist about numbing gels or protective pastes
- Cold sores: start antiviral cream or ask about antiviral tablets at the first tingle, protect the sore with a patch, and do not pick it. See how to get rid of a cold sore fast
Can I kiss someone with a canker sore?
Yes. Canker sores are not contagious, so kissing, sharing a drink or cutlery will not pass them on. Cold sores are different: avoid kissing and oral sex, and do not share cups, towels or lip products until the sore has completely healed.
When to see a doctor or dentist
If a mouth sore lasts more than two to three weeks, is unusually large, keeps coming back, comes with fever, makes it hard to eat or drink, or appears alongside weight loss or other symptoms, get it checked. Urgently for any sore near the eye or in a baby. If you are not sure whether a sore on the lip is a cold sore or a spot, see cold sore vs pimple.
Frequently asked questions
How can I tell if it is a canker sore or a cold sore? +
Look at where it is. Canker sores are single round ulcers inside the mouth on the soft lining, with a white or yellow centre and a red edge. Cold sores are clusters of blisters outside the mouth, on or around the lip, often after a tingle.
Are canker sores contagious? +
No. Canker sores are not caused by a virus and cannot be passed on by kissing or sharing drinks. Cold sores are contagious until fully healed.
Are canker sores a form of herpes? +
No. Canker sores are not caused by the herpes virus. Cold sores are.
Can you get a cold sore inside your mouth? +
Occasionally, usually on the firm tissue of the gums or the roof of the mouth, often as a cluster of small sores. Sores on the soft lining of the cheeks and inner lips are much more likely to be canker sores.
Can I kiss someone with a canker sore? +
Yes. Canker sores are not contagious. Avoid kissing if you have a cold sore until it has completely healed.
What gets rid of canker sores fast? +
Most heal on their own within one to two weeks. Salt water or antiseptic rinses, avoiding acidic and spicy food, and numbing or protective gels from a pharmacist can ease them. See a doctor or dentist if one lasts longer than two to three weeks.
This article is general information from Velora Health, not medical advice, and it is not a substitute for care from a clinician. It does not tell anyone when or whether to take a medication; only your own prescriber can do that. Speak to a healthcare professional about diagnosis, treatment options and transmission risk.