Cold sore

Overview

Cold sores are small painful blisters on or around the lips caused by herpes simplex virus, usually HSV-1. After the first infection, the virus stays in the body and can flare up again, often after illness, sunlight, or stress.

How common is it?

Cold sores are very common. Many people catch HSV-1 in childhood, sometimes without noticing symptoms at the time.

Causes and risk factors

HSV-1 spreads through close skin contact, saliva, and shared items such as lip products or utensils while symptoms are active. Once you have caught it, the virus can become active again later.

Common risk factors

• Close contact with someone who has an active cold sore
• Strong sunlight on the lips
• Fever or another illness
• Stress or not getting enough sleep
• Hormonal changes
• A weakened immune system

Symptoms

• Tingling, burning, or itching before the blister appears
• Small fluid-filled blisters on or around the lips
• Sores that crust over and usually heal within about 7 to 10 days
• Pain or tenderness around the sore
• The first infection can sometimes cause a sore mouth, swollen gums, or fever

When to see a doctor

See a GP if the sore has not started to heal within 10 days, is very large or painful, keeps coming back, or you have a weakened immune system. Get urgent medical help if the sore seems to spread to the eye or you develop eye pain, redness, or changes in vision.

Diagnosis

A cold sore is usually diagnosed from how it looks and where it is. Tests are only sometimes needed if the diagnosis is unclear or the infection is severe.

Treatments

Self-care and lifestyle

• Use SPF lip balm outdoors if sunlight triggers outbreaks
• Do not kiss anyone or share lip balm, towels, or cutlery while the sore is active
• Wash your hands after touching the sore
• Avoid oral sex while you have a cold sore

Prevention

Avoid close contact with an active cold sore, do not share lip products or utensils, and protect your lips from strong sunlight if this tends to trigger outbreaks.