What are cold sores?

Quick Answer
An infectious disease characterized by thin-walled vesicles around the mouth caused by the herpes simplex virus.
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Causes and Symptoms

Cold sores are an infectious disease caused by the herpes simplex virus. Cold sores and fever blisters are two terms used for sores that develop around the mouth. They are among the most common disorders around the mouth area, causing pain and annoyance. An estimated 45 to 80 percent of adults and children in the United States have had at least one cold sore. There are two types of herpes simplex. Type 1 usually causes oral herpes, or fever blisters, while type 2 usually causes genital herpes. About 95 percent of the cold sores located on the mouth are caused by herpes simplex type 1.

Cold sores are highly contagious, more so in the first day or two of an outbreak. Once the blister has formed a dry scab, transmission is low and the herpes simplex virus usually cannot be recovered from the site. The virus can spread to others through touch; frequently, infection spreads through kissing. The chance of infection is higher if the body’s defenses are weakened by stress, illness, or injury.

Once a person is infected with oral herpes, the virus remains in the nerve located near the cold sore. It may stay dormant at this site for years. People who have had fever blisters in the past can sometimes predict when an outbreak is going to occur. The appearance of cold sores may be preceded by a few hours of a tingling, burning, or itching sensation, a phenomenon called a prodrome. Typically, the sores erupt in a small cluster, with each blister about the size of a large pimple. The blisters quickly dry to form a scab. Generally, no scarring or loss in sensation occurs. Outbreaks usually last from three to ten days.

Treatment and Therapy

Treatment includes keeping the area clean and dry to prevent bacterial infection. The patient should avoid irritating the sores, as touching them may spread the virus. For example, if a person rubs the sore and then rubs an eyelid, a new sore may appear on the eyelid in a few days. When the fever blister is contagious, kissing should be avoided. People whose cold sores appear in response to stress should try to avoid stressful situations. Some investigators have suggested that adding L-lysine to the diet or eliminating certain foods (such as nuts, chocolate, and seeds) may help, although no research studies have validated these suggestions. Even sunlight has been linked as a trigger for cold sores. The National Institute of Dental and Craniofacial Research recommends the use of sunscreen on the lips to prevent sun-induced recurrences of herpes.

Antiviral therapy of mucocutaneous herpes simplex virus infection is rarely curative, but is given for treatment of severe cases and to prevent or ameliorate recurrences. Treatment can also decrease viral shedding, making transmission to other people less likely. Available antiviral compounds include nucleoside analogues, which selectively interfere with viral deoxyribonucleic acid (DNA) replication. Oral acyclovir (Avirax or Zovirax) and valacyclovir (Valtrex), antiviral drugs that keep the virus from multiplying, can be taken to prevent recurrence. Acyclovir applied locally has also been found effective, and foscarnet (Foscavir) is useful in treating acyclovir-resistant infections. Two topical preparations, Zovirax and penciclovir (Vectavir), have been shown to influence the eruption and duration of cold sores when applied during the prodromal stage. Antibiotics may be used in treating secondary infections. An ophthalmologist should treat any eye lesions.

Some cold sore treatments that have been successful in selected people are solvents such as ether, alcohol, povidone iodine, and antiseptic mouthwash. Other patients have applied ice or toothpaste to the sores. While none of these treatments has scientific backing, eating an ice pop or applying an ice cube to the blister may relieve the discomfort.

Perspective and Prospects

Future research is aimed at determining the precise form and location of the inactive herpesvirus in nerve cells. This information might allow scientists to design antiviral drugs that can attack the virus while it lies dormant in nerves. Researchers are also trying to learn more about how sunlight, injury, and stress act as triggers so that the cycle of recurrences can be stopped.


Alan, Rick. "Cold Sores." Health Library, March 20, 2013.

"Cold Sore." Mayo Clinic, May 23, 2012.

Ignatavicius, Donna D., and M. Linda Workman, eds. Medical-Surgical Nursing: Critical Thinking for Collaborative Care. 7th ed. Philadelphia: Saunders/Elsevier, 2012.

Lewis, Sharon, et al., eds. Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 8th ed. 2 vols. St. Louis, Mo.: Mosby/Elsevier, 2010.

National Institutes of Health. National Institute of Dental Research. Fever Blisters and Canker Sores. Rev. ed. Bethesda, Md.: Author, 1992.

Smeltzer, Suzanne C., and Brenda G. Bare, eds. Brunner and Suddarth’s Textbook of Medical-Surgical Nursing. 12th ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins, 2010.

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