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Cold sores are a fairly common, small red blister or lesion on your lip. They are, according to MayoClinic.com, a fluid-filled lesion caused by the herpes simplex virus type 1. They usually go away between 10 and 14 days. They can be painful, but unfortunately, there is no way to cure them. The scarring left behind from a cold sore may seem minor, but if you get more than one, it can start to leave a mark.