Cold blister
A cold sore is a fluid-filled blister usually on the outer lip, while an ulcer inside the mouth has different causes and care.
The short answer
A cold sore is a fluid-filled blister usually on the outer lip, while an ulcer inside the mouth has different causes and care. Cold sores can crust after breaking and may recur in a similar area.
What causes cold sores on the lip or lip border?
Cold sores affect skin or the lip border and are associated with herpes simplex virus. Canker sores affect non-keratinised mucosa such as the inner lip and do not begin as the same type of external blister.
How should you care for a cold sore at home?
Keep the area clean, avoid touching or peeling it, and wash hands after contact. Avoid kissing and sharing lip products, cups, or utensils while a blister is present; a clinician or pharmacist can advise about antiviral treatment early in an outbreak.
When does a cold sore need professional assessment?
Seek assessment if the lesion is near the eye, lasts unusually long, is extensive, keeps recurring, or occurs with marked illness or reduced immunity. Dental review is useful when it is unclear whether the lesion is a cold sore, ulcer, or another growth.
Could this lesion be a cold sore or an intraoral ulcer?
Ask whether the location and appearance fit a cold sore or an intraoral ulcer, and whether a medicine could shorten the episode. Ask what symptoms should prompt medical rather than dental care.
Frequently asked questions
Is a cold sore the same as a mouth ulcer?
No. A cold sore usually affects the outer lip, while a mouth ulcer is commonly found on movable inner-mouth lining.
Can you pick a cold sore?
No. Picking can irritate the lesion and transfer virus to the fingers or another site.
Keep reading
Who wrote this page
This library is published by Dental Nation, a licensed dental group practising in the United Arab Emirates. Pages are written and overseen by the same clinical team that treats patients in our clinics, and are held to a fixed editorial standard for safety and accuracy.
This page is patient education, not medical advice, and cannot replace an examination. Anything urgent — facial swelling, trauma, uncontrolled bleeding or severe pain — needs a dentist or emergency care now.