What's the difference between a cold sore and a canker sore?
Last updated September 3, 2026.
Cold sores sit on or around the lips - a contagious herpes virus, tingling then blisters that crust. Canker sores live inside the mouth - small white or yellow ulcers with a red halo, not viral, and not contagious. Location and contagion settle most cases. Triggers differ: cold sores flare with illness, sun, and stress; canker sores follow cheek bites, braces, acidic foods, SLS toothpaste, stress, and sometimes low iron or B12. Treatment differs too: early antivirals for cold sores; salt rinses, numbing gels, and steroid pastes for canker sores. During a cold sore outbreak: no kissing and no shared lip products or towels. Recurrent mouth ulcers with other symptoms deserve a workup. Pymander is a free AI doctor, 24/7 by text, and can help you identify yours.
What to do
- Check the location first: lip edge or outside equals cold sore; inside the mouth equals canker.
- Recall the start: tingling before a blister points to cold sore.
- Treat accordingly: antivirals early for cold sores; rinses and pastes for cankers.
- Respect cold sore contagion: tingle to fully healed, no kissing or shared items.
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