Question: how long does it take for acyclovir to work on cold sores?
Answer: If started at the onset of symptoms (during the prodrome stage), acyclovir (400 mg 5 times daily for 5 days) decreases pain and healing time to loss of crust and valacyclovir (2 g twice daily for 1 day) reduces the lesion duration and time to healing and may prevent lesion development (SOR: A, based on RCTs).