Please note: This information was current at the time of publication but now may be out of date. This handout provides a general overview and may not apply to everyone. 

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Am Fam Physician. 2010;82(9):1084

What are cold sores?

Cold sores usually show up on the lips or mouth as a cluster of blisters with a red base. They are also called “fever blisters.” They may be painful and feel itchy, dry, and crusty. If the blisters come back another time, they may cause a tingling feeling before they appear on the skin.

What causes them?

Cold sores are caused by a type of virus called herpes, which is contagious. It is usually spread by kissing or sharing eating utensils or towels. Washing your hands a lot may help keep it from spreading. The first cold sores show up two to 20 days after being in contact with an infected person. People are most contagious when they have open, blister-like sores.

How do I know if I have herpes?

Your doctor usually can tell if you have the type of herpes that causes cold sores by looking at the sores on your lips or mouth. If the sores look unusual, your doctor may order a culture test to make the diagnosis. This test uses a swab to get a sample of cells from the sore.

How are cold sores treated?

If treated as soon as your sores appear, your doctor may prescribe a type of medicine called an antiviral. This is usually taken as a pill. Common side effects include headache, nausea, and diarrhea. Your doctor also may prescribe a medicated cream that you put on the sores many times a day. However, creams are less effective than antiviral pills. Aspirin, acetaminophen (one brand: Tylenol), or ibuprofen (one brand: Advil) may be used for pain relief.

How long do cold sores last?

A cold sore usually lasts two to three weeks with the first herpes infection. It will heal completely, but more sores may return later on. It is called a recurrence when cold sores come back. These sores usually heal faster, though—in about seven to 10 days if no medicines are used. The herpes virus stays in your body even when you don't have cold sores. If the virus becomes active again, it causes cold sores to return. Things that may cause this include:

  • Fever, flu, or cold

  • Hot or cold weather extremes

  • Stress or trauma

  • Too much sun exposure

  • Weakened immune system

It may help to avoid getting a lot of sun and to use sunscreen on your lips and face.

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