What causes rid of a cold sore overnight?

Prevent it: The best way to get rid of a cold sore is to prevent it from breaking out. When the first sign of itching, tingling, or burning occurs, take Valtrex (valacyclovir) 2 gms twice per day for 1 day. Valtrex (valacyclovir) is a prescription anti-viral medication. It works well as long as you start it soon enough. Once the cold sore breaks out, you can use warm water compresses followed by aquaphor healing ointment.