Cold sores — also called fever blisters — are a common viral infection. They are tiny, fluid-filled blisters on and around your lips. These blisters are often grouped together in patches. After the blisters break, a crust forms over the resulting sore. Cold sores usually heal in two to four weeks without leaving a scar.

Cold sores spread from person to person by close contact, such as kissing. They’re caused by a herpes simplex virus (HSV-1) closely related to the one that causes genital herpes (HSV-2). Both of these viruses can affect your mouth or genitals and can be spread by oral sex. Cold sores are contagious even if you don’t see the sores.

There’s no cure for HSV infection, and the blisters may return. Antiviral medications can help cold sores heal more quickly and may reduce how often they return.

A cold sore usually passes through several stages:

Tingling and itching. Many people feel an itching, burning or tingling sensation around their lips for a day or so before a small, hard, painful spot appears and blisters erupt.
Blisters. Small fluid-filled blisters typically break out along the border where the outside edge of the lips meets the skin of the face. Cold sores can also occur around the nose or on the cheeks.
Oozing and crusting. The small blisters may merge and then burst, leaving shallow open sores that will ooze fluid and then crust over.
Signs and symptoms vary, depending on whether this is your first outbreak or a recurrence. They can last several days, and the blisters can take two to four weeks to heal completely. Recurrences typically appear at the same spot each time and tend to be less severe than the first outbreak.

During first-time outbreaks, some people also experience:

Fever
Painful eroded gums
Sore throat
Headache
Muscle aches
Swollen lymph nodes
Children under 5 years old may have cold sores inside their mouths and the lesions are commonly mistaken for canker sores. Canker sores involve only the mucous membrane and aren’t caused by the herpes simplex virus.

When to see a doctor

Cold sores generally clear up without treatment. See your doctor if:

You have a weakened immune system
The cold sores don’t heal within two weeks
Symptoms are severe
You have frequent recurrences of cold sores
You experience irritation in your eyes

Cold sores are caused by certain strains of the herpes simplex virus (HSV). HSV-1 usually causes cold sores. HSV-2 is usually responsible for genital herpes. However, either type can cause sores in the facial area or on the genitals. Most people who are infected with the virus that causes cold sores never develop signs and symptoms.

Cold sores are most contagious when oozing blisters are present. But you can transmit the virus to others even if you don’t have blisters. Shared eating utensils, razors and towels, as well as kissing, may spread HSV-1. Oral sex can spread HSV-1 to the genitals and HSV-2 to the lips.

Once you’ve had an episode of herpes infection, the virus lies dormant in nerve cells in your skin and may emerge as another cold sore at the same place as before. Recurrence may be triggered by:

Viral infection or fever
Hormonal changes, such as those related to menstruation
Stress
Fatigue
Exposure to sunlight and wind
Changes in the immune system

Dr. Lawrence Jaeger is a board certified dermatologist who has a practice in New York. Dr Lawrence Jaeger specializes in the treatment of all skin, hair and nail disorders including all skin growths.