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
Common questions
What's the difference between a cold sore and a canker sore?
The fastest tell is location. Cold sores sit ON or around the lips and outer mouth - they are caused by herpes simplex virus, start as tingling or burning, then form fluid-filled blisters that weep and crust, and they are contagious from first tingle until fully healed: no kissing, no shared lip balm or towels during an outbreak. Canker sores live INSIDE the mouth - inner cheeks, gums, tongue - as small round white or yellow ulcers with a red halo; they are not caused by a virus, not contagious, and you cannot catch one from anyone. Their triggers are different too: cold sores flare with illness, sun, and stress; canker sores follow mouth injuries (cheek bites, braces), acidic foods, SLS toothpaste in prone people, stress, and sometimes low iron, B12, or folate. Treatment differs: cold sores respond to early antivirals; canker sores to salt rinses, numbing gels, and steroid pastes for stubborn ones. Recurrent mouth ulcers with other symptoms deserve a workup. Pymander is a free AI doctor, 24/7 by text, that can help you identify which one you are looking at.