"The big difference between the three antiviral drugs for herpes (Zovirax - which is acyclovir, Famvir - with is famciclovir, and Valtrex - which is valacycovir) is how often you need to take them every day.
Zovirax was the first antiviral available for herpes, and you needed to take it something like five times per day. It was effective, but the drug didn't stay in your system very long, so you needed to take it often.
Famvir was next, and you needed to take it less often than Zovirax, but still more than once a day.
Valtrex is advertised as "once-daily" Valtrex because they finally got the time-release stuff good enough that you only need to take it once per day."
Though my specialist said Valtrex was better than Acyclovir for ME/CFS. I hate codeine. "but no means a cure."... your right... in our lifetime NO cure is expected... we are just dreaming for help, relief, quality of life with a little space for happiness... with ME/CFS. I use LDN for my pain and that fixed it just fine...
Francesca
I apologize for being so blunt, but everything you pasted here is completely incorrect.
Valtrex (valaciclovir or valacyclovir) is just a
prodrug for Zovirax (aciclovir), it rapidly gets converted to Zovirax (aciclovir) in the body and was developed because Zovirax has poor bioavailability (15-20%) i.e. it doesn't get absorbed very well. Valtrex has a bioavailability of 55% and therefore for the same dosage as Zovirax yields much higher levels of active aciclovir in the body. Otherwise Valtrex and Zovirax are identical.
Famvir (Famciclovir) is a prodrug for penciclovir and was also developed because penciclovir has terrible bioavailablity. Famvir gets rapidly converted to penciclovir in vivo.
Both active drugs, aciclovir and penciclovir have essentially the same extracellular half-life in vivo, something around 2-3 hours and in general the extracellular half-life of a drug governs how often you need to take it. The somewhat special case with penciclovir is that it has a much longer intracellular half-life than aciclovir and therefore reaches higher intracellular concentrations for the same dosage but this is totally offset by the fact that the penciclovir molecule is much poorer at getting initially phophorylated by viral thymidine kinase compared to aciclovir. So clinically they essentially have the same efficacy
Unless a person cannot take Valtrex there is no reason to take Zovirax, Valtrex is superior because it's the same drug and gives you much higher concentrations for the same dosage. Usually if you cannot take Valtrex due to allergic reaction or similar then you also won't be able to take Zovirax.
Both Famvir and Valtrex have to be taken 3-4 times/per day and at fairly high dosages to maintain effective steady state concentrations to reduce viral replication and they must be taken for a very long time. Killing herpes viruses is a long-term battle and you must maintain constant high concentrations of the antiviral for many months to years. But they do work, I know this personally I've seen my antibody titers go steadily down by taking such drugs.
Famvir has one advantage over Valtrex in that is does not cause the crystalluria at high dosages. You have to drink a lot of water when you take high dosages of Valtrex.
pet peeve of mine, people just putting out misinformation. I know you put "as far as I know" but that isn't enough disclaimer and people tend to just read and believe what you say as fact. 
