Prophylatic use of Zovirax (acyclovir)
Bottom line is that it can't hurt, other than the usual standard warnings re: possible complications from side effects and other low percentage yet possible negative drug interactions.
However, other than for peace of mind, the can't hurt outcome is really all you're going to get. There's no real medical or microbiological mechanism to support taking a drug such as acyclovir in prophylaxis to avoid or deter 'the morning after' potential for infection with a viral infection such as herpes (HSV-1 or HSV-2)
Prophylatic or immediate post-contact use of antibiotics against a biotic organism is a whole different story - but for a suspected viral contact, there's no real solution or mechanism to lower your chances of infection. With a virus, especially HSV-1 or 2, there's simply managment of the infection, not avoidance.
I did a combined MD / PhD in MCB so while it's fascinating to me, I'm sure it would bore most to tears here, so I'll just stick to the short layman's version on the microbiology - acyclovir is an SPN or synthetic purine nuceloside analogue - meaning it has good inhibitory effect, repeat inhibitory, against viruses such as herpes. If you'll recall some basic biology regarding viral replication - viruses have no method to replicate on their own, they need to infect and take over your own cellular machinery to replicate. The long and short of which is that there are a number of pre and post chain reaction steps that a virus could be controlled because of the specific signature of attack each virus has.
The reason acyclovir is effective in controlling and managing the outbreak of herpes symptoms once you are infected (e. G. The living with it sort of management, not a cure) is because all viruses have two major weak points to be contained by - either try to hold back the horde by preventing the virus from 'recognizing' the appropriate target cells they go after by altering or masking the specific binding site markers they look for, or simply go after the viral ability to replicate. It would be wonderful if mankind had yet made a drug that 100% stopped a virus from being able to replicate, but unfortunately we haven't. Hence why subtle yet very important words of distinction are used when describing anti-viral drug compounds - "inhibit", "suppress", "control", etc. Not erradicate or cure.
So, acyclovir is a pretty smart little package - it, as well as various other drugs, has a high affinity for binding with and masking the TK enzyme, or thymidine kinase, that the herpes virus encodes and uses as part of it's replication sequence. Any MCB majors out there might at this point be wondering what happened to the mono, di, and triphosphate stages for the halting and conversion of HSV replication, but like I said before, this is the quickie version. The bottom line is acyclovir inhibits and terminates where it can the replication sequence of herpes and other similar viruses by masking the enzyme herpes uses to encode replication of itself.
The problem is that when we talk about viruses in general and especially with something like herpes, stopping 99% of replication sounds good, which acyclovir does not get anywhere near to, but even if it did, the remaining 1% is still a fairly overwhelming load to the immune system and you still end up infected, and stay infected. So the sad reality is that perhaps in the future we'll have a more effective means of eradication, but at the moment we're simply talking about supression and management of the disease.
So, if you take acyclovir prohylatically, the drug will do nothing to prevent or help prevent infection - it has no impact on the virus entering your body and getting a foothold. Once infected and the virus starts the initial phase of invasion by hijacking your cellular machinery to RNA and DNA encode more of the nasty buggers, that's when acyclovir or other similar drug compounds will help manage just how much you are affected.
There are some controversial studies on the alternative approach - stopping the virus at the gates before it gets a foothold in your system via immuno-enhancing mechanisms. E. G. Prophylatically boosting your immune system to very high levels. But most conventional science is in the approach of finally producing a stable and reliable vaccination.