Cause of fever blisters?

Originally Published: October 20, 1995 - Last Updated / Reviewed On: March 18, 2011
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What causes fever blisters on lips and how should they be treated? Also, what is the difference between fever blisters and shingles?


Dear Sores,

Many different types of fever blisters can occur around a person's mouth, or on one's lips and face. The most common types are canker sores and colds sores caused by the herpes simplex virus type I (HSV-1). Other rarer forms of mouth sores are caused by tuberculosis, syphilis, Vincent's disease, Behcet's syndrome, leukemia, anemia, or drug allergies.

As canker sores and cold sores are the most common fever blisters, here's some information on both. Canker sores are tiny, crater-like lesions inside of the mouth that can appear on or under the tongue or inside the cheeks, alone, or in a group. Though painful and irritating, cankers have not been proven to have a viral origin and they are not contagious or a sign of any other disease.

It is not clear what causes canker sores to appear. They seem to be stress-related for some people, but stress can also be a side effect of the sores. Heredity may play a role, and some women find that they recur at the same time each month during their menstrual cycle. Some people have claimed that food allergies instigate the sores, and others blame a lack of Vitamin C in the diet. One last suspect is trauma, the kind that comes from biting your tongue or the inside of your cheek.

Cold sores caused by HSV-1 are different than canker sores in that they are very, very contagious. HSV-1 is the virus that affects the mouth and facial areas, although it can be transmitted to the genital area through oral-genital sex. The virus can be transmitted through direct contact with a lesion, through contact with fluid from a lesion, and through contact with the virus even when no symptoms are present in the infected person.

The first outbreak is usually the worst and lasts the longest. After this outbreak, the virus lives in the nerve pathways around the area of the initial infection. The virus symptoms can and probably will recur, always at the same site, but they will be much less serious than the first outbreak. Often HSV-1 is reactivated because of circumstances that can be controlled, such as extensive sun exposure, elevated stress, fever, and for some people, possibly from foods, such as nuts, seeds, and chocolate.

There is no cure for HSV-1 (as there isn't for any virus), so treatment is prescribed to relieve pain, avoid infections, and prevent spreading of the virus. Various over-the-counter medications alleviate pain, burning, and itching, and can shorten the life of the outbreak. You can read more about the difference between canker sores, cold sores, and other mouth lesions in the related questions below.

Lastly, the difference between fever blisters and shingles is the type of virus that causes them. Herpes zoster is the medical term for shingles, and it is an infection of the nerves in certain areas of the skin. It causes a painful rash of small, crusting blisters, most often on a strip of skin over the ribs on one side of the body. It is caused by the varicella-zoster virus, which also causes chicken pox. It is a common disease, affecting a few hundred people per 100,000 each year in the United States. It mainly affects people over the age of 50 years, and the incidence rises with age. Herpes zoster is also common in people with compromised immune systems.

For more information about any of these types of blisters, you can contact the American Social Health Association Herpes Resource Center and the National Herpes Hotline. If you are a Columbia student who needs help diagnosing the type of fever blisters you have, you can make an appointment with a health care provider at Medical Services at Columbia by using Open Communicator or calling x4-2284; outside of Columbia, you can contact your primary care provider.