Cold sores

Cold sores are painful red blisters that occur on or around the mouth. They are caused by herpes simplex virus (HSV) types 1 and 2. HSV type 1 is the most common cause of cold sores. HSV type 2 usually causes genital herpes, but it can also cause cold sores. Cold sores tend to appear in the same place every time because the virus lives in the nerves that lead to that spot on the skin.

At least half of all adults are infected with HSdV, which is easily spread from person to person. Once you are infected with HSV, you have the infection permanently. The virus lies "sleeping" inside the nerves, causing no symptoms most of the time. In some people, the virus periodically "wakes up" and causes cold sores.

Conditions that can trigger cold sores includef9401054-a813-405c-8b69-aa87b5d7f58d16207267125809

  • anxiety
  • stress
  • sunburn
  • hormonal changes
  • physical exhaustion.

Cold sores can also often develop with a fever, which is why they are sometimes called "fever blisters."

Symptoms of cold sores

A cold sore usually begins with a slight tenderness or tingling on the edge of the mouth. It develops into a painful, swollen, red lump. After a day or two, the area blisters, bursts, and crusts over. The yellow crust peels away and secretes a clear liquid. It can take up to two weeks for the sore to heal.

Treating cold sores

Applying an over-the-counter antiviral ointment like docosanol (Abreva) at the first sign of a cold sore can help shorten its duration. But doing this will not prevent cold sores from appearing again. Stronger prescription antiviral ointments include acyclovir (Zovirax) and penciclovir (Denavir).

Over-the-counter anesthetics can numb the skin and help relieve pain and discomfort. These include benzocaine (Orajel), lidocaine (Zilactin-L), and tetracaine (Viractin). Applying ice at the first sign of tenderness or tingling may help shorten the duration of the sore, or prevent it from becoming a full-blown cold sore.

Taking a prescription antiviral medicine may be an option for people who have frequent, painful cold sores.


As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles.

No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.

Free Healthbeat Signup

Get the latest in health news delivered to your inbox!

Sign Up
Harvard Health Publishing Logo

Thanks for visiting. Don't miss your FREE gift.

The Best Diets for Cognitive Fitness, is yours absolutely FREE when you sign up to receive Health Alerts from Harvard Medical School

Sign up to get tips for living a healthy lifestyle, with ways to fight inflammation and improve cognitive health, plus the latest advances in preventative medicine, diet and exercise, pain relief, blood pressure and cholesterol management, and more.

Harvard Health Publishing Logo

Health Alerts from Harvard Medical School

Get helpful tips and guidance for everything from fighting inflammation to finding the best diets for weight loss...from exercises to build a stronger core to advice on treating cataracts. PLUS, the latest news on medical advances and breakthroughs from Harvard Medical School experts.

BONUS! Sign up now and
get a FREE copy of the
Best Diets for Cognitive Fitness

Harvard Health Publishing Logo

Stay on top of latest health news from Harvard Medical School.

Plus, get a FREE copy of the Best Diets for Cognitive Fitness.