Cold sores are caused by herpes simplex virus, usually type 1, which stays dormant in nerve cells after the first infection and can wake up again. Triggers such as sun, fever, stress or illness bring back painful clusters of blisters on or around the lips.
Key symptoms
- Tingling, burning or itching at the lip a day before sores appear
- Clusters of small painful blisters on or near the edge of the lip
- Blisters break, crust over and heal in about 7 to 10 days
- First infection in children: fever, sore mouth and gums, trouble eating
- Repeat outbreaks at the same spot after sun, fever or stress
What you find on exam
- Grouped blisters on a red base at the lip border
- Shallow ulcers or yellow crusts in later stages
- First episode: swollen, bleeding gums and mouth ulcers
- Tender lymph nodes under the jaw or in the neck
- Check the eye and look for spreading lesions on eczema skin
Tests
- Usually diagnosed by appearance alone
- PCR swab from an opened blister if the diagnosis is unclear
- Viral culture or antigen test where PCR is not available
- Urgent eye exam if sores are near the eye or the eye is red
Treatment basics
- Oral valacyclovir started at the first tingle, as a 1-day course (2 g twice)
- Topical docosanol or acyclovir cream, with modest benefit
- Pain relief with acetaminophen or ibuprofen; fluids for children with mouth sores
- Daily suppressive valacyclovir for frequent or severe outbreaks
- Lip balm with sunscreen; no kissing or sharing cups while sores are active
Red flags
- Eye pain, redness or blurry vision with nearby sores
- Widespread blisters on eczema skin (eczema herpeticum)
- Fever with confusion, or an ill newborn or immunocompromised patient
Practice it in Making Synapses
Making Synapses cases
Related guides
Sources: NCCPA PANCE Content Blueprint; NBOME COMLEX-USA clinical presentations; AAMC Core Entrustable Professional Activities (Core EPAs). Written by Making Synapses. Making Synapses is not affiliated with these organizations.