Infection: Cold Sore

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Infection: Cold Sore


Cold sores are blisters on the lips or around the mouth caused by herpes simplex virus 1 (HSV-1) and sometimes by herpes simplex virus 2 (HSV-2). Unlike canker sores, which occur inside the mouth, cold sores are contagious.


Cold sores are painful blisters that develop on the lips or around the mouth after a person has become infected with the herpes simplex virus. The herpes simplex virus 1 (HSV-1) that generally causes cold sores is related to the herpes simplex virus (HSV-2), that typically causes genital herpes but can also cause cold sores.

People who become infected with HSV-1 develop cold sores within twenty days of infection, although some may develop symptoms sooner. The first episode of infection (called the primary infection) may not have any symptoms or may cause two to three weeks of fever and blisters/sores both in and around the mouth. In most cases, the recurrence of symptoms is preceded by a prodrome, or period of warning symptoms before the main phase of the illness. The prodrome of cold sores usually consists of a tingling, itching, or burning sensation that starts one or two days before the blisters appear. The area of

skin where the blisters will erupt may swell up, turn red, and be sore to the touch.

The sores themselves last for about a week after they erupt. They appear most commonly on the lips or the area of skin between the upper lip and the nose.

The blisters are small and thin-walled, filled with a clear fluid, and become sores after several days. The HSV-1 virus is shed in the fluid from the sores and can be transmitted to other people if they come in contact with the blisters. This is the stage in the development of cold sores when the infection is most contagious. After a few days, the ulcers form a yellow crust that eventually drops off, leaving an area of pinkish skin underneath. There is no permanent scar from a cold sore. People who get cold sores may have one or two recurrences per year, although some have an outbreak every month and some never have relapses.


According to the National Institutes of Health (NIH), about 80 percent of people in the United States are infected with HSV-1. Most acquire the infection as children from contact with oral fluids from an infected person. Infection with the virus is thought to be equally common in both sexes and all races and ethnic groups.

People with weakened immune systems, such as patients being treated for cancer or HIV infection, are at increased risk of getting cold sores if they are exposed to HSV-1.

Causes and Symptoms

The cause of cold sores is usually herpes simplex virus 1 or HSV-1, although occasionally type 2 may cause the process. The virus enters the body through tiny breaks in the tissues lining the mouth, which is one reason it can easily be spread by kissing or by sharing drinking glasses and other food utensils. HSV-1 then lies dormant in the cells of the nervous system until it is activated by stress, an upper respiratory infection, or some other trigger. It then travels back down the nerves to the skin surface, usually in the same area of skin each time. HSV-2 oral infection usually does not have recurrences.

The symptoms of cold sores have already been described. In most cases, people do not need to see the doctor for ordinary cold sores. They should, however, make an appointment if they have any of the following symptoms or conditions:

  • They have HIV or any other illness that weakens the immune system.
  • The cold sores do not heal on their own by the end of two weeks.
  • The patient's eyes feel sore or irritated.
  • The cold sores recur frequently.
  • The blisters are unusually large or painful.


Most people can tell whether they have cold sores by the way they feel and where they appear. If necessary, the patient's doctor can run a blood test to tell whether the person is infected with HSV-1. The virus can also be cultured from the blister fluid or the sore.


There is no permanent cure for HSV-1 infection. After a person is infected with the virus, it hides within nerve cells, making it difficult for the immune system to find and destroy it. HSV-1 remains in the body, so that cold sores can reappear at any time. Recurrences of oral

herpes can be triggered by a number of factors, including getting the flu or a cold, not getting enough sleep, having dental work or oral surgery, getting traveler's diarrhea, menstruation, emotional stress, an injury to the mouth or lips, or exposure to the sun for long periods of time. The connection between colds and flu in reactivating HSV-1 is the reason why oral herpes is commonly known as cold sore or fever blister.

The best time to start treating cold sores is during the prodromal stage before the blisters appear. The doctor can prescribe an antiviral medication to shorten the length of the outbreak and reduce discomfort. Other treatments that can be used are topical anesthetics applied directly to the sores, and aspirin, Advil, or Tylenol to bring down fever. Some people also find that ice applied to the blisters helps to relieve discomfort.


Most cases of cold sores heal without long-term problems; however, HSV-1 can cause an eye infection that may lead to permanent blindness if fluid from the sores gets into the eyes. For this reason it is important for people with cold sores to avoid scratching or squeezing the blisters.


The NIH recommends the following measures to lower the risk of spreading HSV-1 to other parts of the body or to other people, and to lower the frequency of recurrences.

  • Avoid kissing or close contact with others while the blisters are present.
  • Avoid sharing items that touch the mouth. These include towels, washcloths, lipsticks, lip balms, razors, and toothbrushes as well as drinking glasses and food utensils.
  • Keep the hands clean. Wash them frequently and avoid touching the eyes or genital area during an outbreak.
  • Try to avoid such common triggers as colds or flu, high stress levels, or being short on sleep.
  • Use sunblock on the lips and face when outdoors for long periods of time.
  • Some people with frequent recurrences of cold sores benefit from an over-the counter dietary supplement called lysine, which is an amino acid (one of the chemical building blocks of proteins). Patients should check with their doctor to see whether lysine might be helpful for them.

The Future

Cold sores are likely to continue to be a common health problem in the general population. Although researchers are working on vaccines against both HSV-1 and HSV-2, an effective vaccine against either herpes virus is at least several years away.

SEE ALSO Canker sores; Genital herpes


Prodrome: A period before the acute phase of a disease when the patient has some characteristic warning symptoms.

Topical: Referring to any medication applied directly to the skin or the surface of the body.

For more information


Blenkinsopp, Alison, Paul Paxton, and John Blenkinsopp. Symptoms in the Pharmacy: A Guide to the Management of Common Illness. Malden, MA: Blackwell Publishing, 2005.

Smith, Rebecca W. Columbia University School of Dental and Oral Surgery's Guide to Family Dental Care. New York: W. W. Norton, Inc., 1997.


American Dental Association (ADA). Common Mouth Sores. Available online at (accessed May 13, 2008).

Mayo Clinic. Cold Sore. Available online at (updated March 13, 2008; accessed June 27, 2008).

National Library of Medicine (NLM). Cold Sores (Herpes). Available online at (accessed June 27, 2008). This is an online tutorial with voiceover; viewers have the option of a self-playing version, a text version, or an interactive version with questions.

TeensHealth. Cold Sores (HSV-1). Available online at (updated March 2008; accessed June 27, 2008).