Written by:

Overview

Cold Sores Overview

Cold sores are recurring, contagious blisters usually caused by herpes simplex virus type 1 (HSV-1). They often appear on or around the lips, although anyone can develop them, including people who have few or no symptoms after infection. A cold sore on the lip is sometimes called a fever blister. Most outbreaks heal on their own without lasting problems.

Symptoms

Cold Sore Symptoms

Cold sore symptoms often develop in stages. Tingling or burning may begin before a visible sore appears, followed by blisters that can break open, crust, and heal. These mouth sores usually occur on or around the lips rather than inside the mouth.

  • Tingling or burning at the site
  • Small fluid-filled blisters
  • A painful open sore
  • Crusting as the sore heals
  • Fever during a first outbreak
  • Swollen glands during a first outbreak

Cold sore

Usually appears on or around the lips and can spread through close contact.

Canker sore

Usually appears inside the mouth and is not caused by herpes simplex or spread by contact.

How It Spreads

How Cold Sores Spread

Herpes simplex spreads through contact with an active sore, saliva, or infected skin. It can spread through kissing, oral contact, or shared items that touch the mouth. Transmission can sometimes occur even when no sore is visible, although the risk is generally higher during an outbreak.

  • Kissing or oral contact with someone who has the virus can spread herpes simplex.
  • Sharing items that touch the mouth, such as drinks, utensils, or lip products, can spread the virus during an outbreak.
  • Touching a sore and then touching another person’s skin or eyes can transfer the infection.
  • Avoid kissing and oral sex while a sore is present to reduce transmission.
  • Wash your hands after touching the sore or applying medicine.

Risk Factors

Cold Sore Risk Factors and Triggers

  • Previous infection with herpes simplex virus
  • Close contact with someone who carries herpes simplex virus
  • A weakened immune system
  • Fever or another illness
  • Emotional or physical stress
  • Tiredness or lack of sleep
  • Sun or wind exposure
  • Injury or irritation to the lips
  • Hormonal changes

Complications

Cold Sore Complications

Most cold sores heal without lasting problems. Rarely, the infection can spread to the eye, severely affected skin, or other areas, especially in people with weakened immunity.

  • Herpes infection of the eye
  • Widespread infection in a person with a weakened immune system
  • A severe infection of eczema-affected skin
  • Serious infection in a newborn exposed around birth

Get prompt medical help

Eye and high-risk warning

Seek prompt medical care for eye pain, light sensitivity, vision changes, rapidly spreading sores, or sores affecting a newborn or someone with a weakened immune system.

Diagnosis

How Cold Sores Are Diagnosed

A clinician usually diagnoses a cold sore by examining its symptoms, characteristic appearance, and location. The timing and pattern of previous outbreaks can also help distinguish it from other mouth sores.

When testing may be needed

A clinician may take a swab or order laboratory testing when the sore looks unusual, is severe, recurs in an unexpected pattern, affects the eye, or needs to be distinguished from another condition. Testing is not needed for every typical outbreak.

Treatment Options

Cold Sore Treatment Options

Cold sore treatment may include antiviral medicine and supportive care. The choice depends on how early the outbreak is treated, how severe or frequent it is, and whether the person has health risks that require medical advice.

Prescription antiviral medicines

A clinician may prescribe oral antivirals such as acyclovir or related medicines for frequent, severe, prolonged, or high-risk outbreaks. These medicines work best when started during tingling or early blistering, but they do not eradicate herpes simplex from the body. A clinician can explain the dose, timing, and expected course of treatment.

Over-the-counter care and home measures

Prevention

How to Prevent Cold Sores From Spreading

  • Avoid kissing and oral sex while a sore is present.
  • Do not share lip products, utensils, towels, or drinks during an outbreak.
  • Wash your hands after touching the sore.
  • Avoid touching your eyes after touching your lips.
  • Use lip balm with sun protection if sunlight triggers outbreaks.
  • Ask a clinician about preventive antiviral treatment if outbreaks are frequent.

Outlook and Prognosis

Cold Sore Outlook and Prognosis

Most cold sores heal within about 1–2 weeks. The virus remains in the body and may reactivate, causing recurring cold sores, but outbreaks often become less frequent or less severe over time. Treatment can help shorten some outbreaks and manage discomfort.

When Should You See a Doctor

When Should You See a Doctor for a Cold Sore?

  • The sore is very painful or widespread.
  • The sore has not started healing within about 2 weeks.
  • Outbreaks happen frequently.
  • There is eye pain, light sensitivity, or vision change.
  • A newborn has a blister or sore.
  • The person has a weakened immune system.