Overview

Cold sores — also called fever blisters — are a common viral infection. They are tiny, fluid-filled blisters on and around your lips. These blisters are often grouped together in patches. After the blisters break, a scab forms that can last several days. Cold sores usually heal in two to three weeks without leaving a scar.

Cold sores spread from person to person by close contact, such as kissing. They're usually caused by herpes simplex virus type 1 (HSV-1), and less commonly herpes simplex virus type 2 (HSV-2). Both of these viruses can affect your mouth or genitals and can be spread by oral sex. Cold sores are contagious even if you don't see the sores.

There's no cure for cold sores, but treatment can help manage outbreaks. Prescription antiviral pills or creams can help sores heal more quickly. And they may reduce the frequency, length and severity of future outbreaks.

Symptoms

A cold sore usually passes through several stages:

  • Tingling and itching. Many people feel itching, burning or tingling around the lips for a day or so before a small, hard, painful spot appears and blisters erupt.
  • Blisters. Small fluid-filled blisters typically erupt along the border of your lips. Sometimes they appear around the nose or cheeks or inside the mouth.
  • Oozing and crusting. The small blisters may merge and then burst, leaving shallow open sores that ooze and crust over.

Signs and symptoms vary, depending on whether this is your first outbreak or a recurrence. The first time you have a cold sore, symptoms may not start for up to 20 days after you were first exposed to the virus. The sores can last several days, and the blisters can take two to three weeks to heal completely. Recurrences typically appear at the same spot each time and tend to be less severe than the first outbreak.

In a first-time outbreak, you also might experience:

  • Fever
  • Painful gums
  • Sore throat
  • Headache
  • Muscle aches
  • Swollen lymph nodes

Children under 5 years old may have cold sores inside their mouths and the lesions are commonly mistaken for canker sores. Canker sores involve only the mucous membrane and aren't caused by the herpes simplex virus.

When to see a doctor

Cold sores generally clear up without treatment. See your doctor if:

  • You have a weakened immune system
  • The cold sores don't heal within two weeks
  • Symptoms are severe
  • You have frequent recurrences of cold sores
  • You experience irritation in your eyes

Causes

Cold sores are caused by certain strains of the herpes simplex virus (HSV). HSV-1 usually causes cold sores. HSV-2 is usually responsible for genital herpes. But either type can spread to the face or genitals through close contact, such as kissing or oral sex. Shared eating utensils, razors and towels might also spread HSV-1.

Cold sores are most contagious when you have oozing blisters because the virus easily spreads through contact with infected body fluids. But you can spread the virus even if you don't have blisters. Many people who are infected with the virus that causes cold sores never develop signs and symptoms.

Once you've had an episode of herpes infection, the virus lies dormant in nerve cells in your skin and may emerge as another cold sore at the same place as before. Recurrence may be triggered by:

  • Viral infection or fever
  • Hormonal changes, such as those related to menstruation
  • Stress
  • Fatigue
  • Exposure to sunlight and wind
  • Changes in the immune system
  • Injury to the skin

Risk factors

Almost everyone is at risk of cold sores. Most adults carry the virus that causes cold sores, even if they've never had symptoms.

You're most at risk of complications from the virus if you have a weakened immune system from conditions and treatments such as:

  • HIV/AIDS
  • Atopic dermatitis (eczema)
  • Cancer chemotherapy
  • Anti-rejection drugs for organ transplants

Complications

In some people, the virus that causes cold sores can cause problems in other areas of the body, including:

  • Fingertips. Both HSV-1 and HSV-2 can be spread to the fingers. This type of infection is often referred to as herpes whitlow. Children who suck their thumbs may transfer the infection from their mouths to their thumbs.
  • Eyes. The virus can sometimes cause eye infection. Repeated infections can cause scarring and injury, which may lead to vision problems or loss of vision.
  • Widespread areas of skin. People who have a skin condition called atopic dermatitis (eczema) are at higher risk of cold sores spreading all across their bodies. This can become a medical emergency.

Prevention

Your doctor may prescribe an antiviral medication for you to take on a regular basis if you develop cold sores more than nine times a year or if you're at high risk of serious complications. If sunlight seems to trigger your recurrences, apply sunblock to the spot where the cold sore tends to erupt. Or talk with your doctor about using an oral antiviral drug as a preventive if you expect to be doing an activity that tends to trigger your condition, such as intense sunlight exposure.

To help avoid spreading cold sores to other people or to other parts of your body, you might try some of the following precautions:

  • Avoid kissing and skin contact with people while blisters are present. The virus spreads most easily when the blisters leak fluid.
  • Avoid sharing items. Utensils, towels, lip balm and other personal items can spread the virus when blisters are present.
  • Keep your hands clean. When you have a cold sore, wash your hands carefully before touching yourself and other people, especially babies.