Smile Cold sores vs mouth ulcers: how to tell the difference
Cold sores are caused by a virus and usually appear on or around the lips. Mouth ulcers are not infectious and appear inside the mouth. Most heal on their own within about two weeks.
Cold sores
- Caused by the herpes simplex virus and contagious.
- Usually on the lips or around the mouth, starting with tingling or burning.
- Blisters form, then burst and crust over, healing in about 7 to 10 days.
- Triggers can include illness, stress, sunlight or a weak immune system.
Avoid kissing and sharing cups, towels or lip products while you have one. Antiviral creams from a pharmacy may help if used early.
Mouth ulcers
- Round or oval sores inside the mouth, on the cheeks, tongue or gums.
- Not contagious.
- Can be caused by an injury, braces or dentures rubbing, stress or some foods.
Most mouth ulcers heal in 10 to 14 days. See our guide on mouth ulcers and when to see a dentist or doctor.
When to get checked
See a dentist or doctor if a sore lasts more than three weeks, keeps coming back, is very painful, or comes with fever. A sore or lump in the mouth that does not heal should always be checked.
Quick answers
Are cold sores contagious?
Yes. They spread most easily when blisters are present, so avoid close contact and sharing items.
How long do mouth ulcers last?
Most heal in about 10 to 14 days. See a professional if one lasts longer than three weeks.
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