Valacyclovir or Valtrex is actually a "prodrug," meaning it is not active itself, until is converted to acyclovir in the body, and it is the acyclovir that is active against the viruses.
The difference between Valtrex and Acyclovir is duration of meds.
Valtrex has a longer duration of action than acyclovir, and, therefore, can be taken fewer times each day.
Usually, Valterex is used once a day, but, acyclovir needs to be taken twice a day.
Valtrex is more expensive and costs about two hundred dollars a month.