10 Causes and Treatments for Cold Sores

Treatment: Protection from the Sun

If overexposure to the sun can trigger the dormant herpes simplex virus into an active outbreak, then being mindful of how much sun your mouth or lips are getting is important.

A person's sensitivity to the sun is relative to that person; only you know how much ultraviolet light it would take to set off another cold sore.

Apply sunscreen to your face (and particularly the sensitive skin around the mouth) before going out into the sun for prolonged periods [source: WebMD].