What causes cold sores?
Last updated September 6, 2026.
Cold sores are caused by the herpes simplex virus, most often HSV-1, which stays in your nerve cells for life after the first infection and reactivates from time to time. You catch the virus through direct contact with saliva, skin, or an active sore, and many people are exposed in childhood without ever noticing. Once you have it, outbreaks tend to return in the same spot on or near the lip because the virus travels back down the same nerve. Sunlight, fever or another illness, stress, poor sleep, lip injury, and hormone changes are the usual triggers. Antiviral creams and tablets work best when you start them at the first tingle, before a blister forms. Daily sunscreen or SPF lip balm helps if sun sets off your outbreaks.
What to do
- Treat at the first tingle: Start an antiviral cream or tablet during the tingling or itching stage, before a blister appears.
- Protect your lips from sun: Use an SPF 30 or higher lip balm daily if sunlight reliably sets off your outbreaks.
- Avoid passing it on: Skip kissing and oral sex while you have an active sore, and do not share cups, utensils, razors, towels, or lip products.
- Keep hands clean: Wash your hands after touching a sore so you do not spread the virus to your eyes or fingers.
- Track your triggers: Note what came before each outbreak, such as illness, sun, stress, or your cycle, so you can prepare next time.
- Get medical care when: You have sores near your eye, sores lasting more than two weeks, frequent or severe outbreaks, a weakened immune system, eczema, or a baby under 3 months with mouth sores.
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