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Symptoms and Signs of Cold Sores

Doctor's Notes on Cold Sores

Cold sores are infections with the herpes simplex virus (HSV) in or around the mouth. Of the two herpes simplex viruses (HSV-1 and HSV-2), cold sores are more commonly caused by HSV-1. Once the HSV has infected a person, the virus remains dormant in the nerve roots and typically reactivates in response to triggers, causing the same symptoms at the same location. Triggers that can cause the virus to reactivate include emotional or physical stress, illness, a weakened immune system, and exposure to UV radiation.

Signs and symptoms of a cold sore are painful, fluid-filled blisters around the upper lip, mouth, or chin. Associated symptoms can include a burning or tingling sensation that occurs before the outbreak of the blister and itching of the skin. The blisters dry and leave a crust or scab at the involved area.

Medical Author:
Medically Reviewed on 3/11/2019

Cold Sores Symptoms

  • Some patients have a "prodrome," which is when certain symptoms occur before the actual sores appear. The prodrome to herpes infections typically involves a burning or tingling sensation that precedes the appearance of blisters by a few hours or a day or two. As the cold sore forms, the area may become reddened and develop small fluid-filled blisters. Several of these small blisters may even come together and form one large blister. Cold sores are mild to moderately painful.
  • When cold sores recur, the blisters stage is usually short. Blisters dry up rapidly and leave scabs that last anywhere from a day to several days, depending on the severity of the infection.

Cold Sores Causes

The virus that causes cold sores is known as the herpes simplex virus (HSV). There are two types of HSV, type I and type II. Cold sores are usually caused by type I.

Herpes simplex is a contagious oral virus. The virus is spread from person to person by kissing or another close contact with sores or even from contact with apparently normal skin that is shedding the virus. Infected saliva is also a means of spreading the virus. The most contagious period is when a person has active blister-like sores. Once the blisters have dried and crusted over (within a few days), the risk of contagion is significantly lessened. However, a person infected with HSV can pass it on to another person even when a cold sore is not present. This is because the virus is sometimes shed in saliva even when sores are not present. Despite popular myth, it is almost impossible to catch herpes (cold sores) from contaminated surfaces, towels, or washcloths.

After the first infection, the virus enters the nerve cells and travels up the nerve until it comes to a place called a ganglion, which is a collection of nerve cells. There, it resides quietly in a stage that is called "dormant" or "latent." In more active stages, the virus starts multiplying again and travels down the nerve to the skin, causing blisters on the lips known as cold sores. The exact way this happens is not clear, but it is known that some conditions seem to be associated with recurrences, including

Sometimes there is no apparent cause of the recurrence.

Cold sores have a tendency to recur in more or less the same place each time. Such recurrences may happen often (for example, once a month) or only occasionally (for example, once or twice a year).

Cold Sores Causes, Remedies, & Diagnosis Slideshow

Cold Sores Causes, Remedies, & Diagnosis Slideshow

Cold sores, also known as fever blisters, can make you feel uncomfortable and self-conscious. What’s worse, it seems like some people get them while others are apparently spared. Even though there is no cure, treatments are available to shorten the duration of the symptoms or bring some relief. This guide will show you how to recognize and manage cold sores.


Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.