Cold sores, sometimes called fever blisters, are clusters of small blisters on the lip and outer edge of the mouth. Cold sores are caused by the herpes simplex virus.

Once a person is infected with the herpes simplex virus, cold sores can occur repeatedly. The sores often weep a clear fluid and form scabs after a few days. A cold sore typically lasts 8 to 10 days and is contagious until the blister is completely crusted over.

Cold sores usually heal on their own. Medications are available for more rapid healing, reducing pain, and reducing the severity of future outbreaks.