Valacyclovir and acyclovir are the two antivirals you will be offered for herpes. People assume they are different drugs that work differently. They are not. Valacyclovir turns into acyclovir inside your body, so the active medication doing the work is identical. What differs is how much of it your gut absorbs, how many pills you swallow a day, and what it costs. This post explains those differences so you can pick on facts rather than on which name you have heard before.
They become the same drug
Acyclovir is the active antiviral. Valacyclovir is a prodrug, which means it is acyclovir with a small chemical group attached that makes it absorb better. Once valacyclovir is absorbed, your body strips that group off and you are left with acyclovir circulating in your blood. The mechanism from that point is identical: the viral enzyme thymidine kinase activates the drug inside infected cells, and the activated form blocks the viral DNA polymerase and terminates the viral DNA chain.
Because the selectivity comes from a viral enzyme, both drugs act mainly inside infected cells and leave healthy cells largely alone. That is why both have a clean safety record in healthy adults.
So the question is never which drug is stronger. It is the same drug at the business end. The question is how efficiently you get it there.
The pharmacokinetic difference that matters
Oral acyclovir is poorly absorbed. Only about 10 to 20 percent of an oral dose makes it into your bloodstream, and that fraction drops further as the dose goes up. To keep enough drug in circulation, you have to dose acyclovir frequently, which historically meant up to five times a day for some indications.
Valacyclovir was engineered to fix exactly this. Its oral bioavailability as acyclovir is three to five times higher. That single improvement is the entire practical reason valacyclovir exists. Better absorption means you reach effective blood levels with fewer, larger, less frequent doses.
This is not a small convenience. The difference between a pill twice a day and a pill five times a day is the difference between a regimen people actually complete and one they abandon.
Dosing convenience, side by side
The absorption gap shows up directly in the dosing schedules.
Genital herpes suppression. Valacyclovir for daily suppression is 500 mg once daily, stepped to 1,000 mg once daily for people with 10 or more recurrences a year. Acyclovir for the same job is 400 mg twice daily. One pill versus two.
Episodic genital herpes. Valacyclovir is 1,000 mg once daily for five days, or 500 mg twice daily for three days. Acyclovir is 400 mg three times daily for five days. Fewer daily doses with valacyclovir, which matters when timing the start of treatment fast is the whole game.
Cold sores. Valacyclovir compresses to 2,000 mg twice daily for a single day. The acyclovir cold-sore course runs longer and more frequent.
The pattern holds across every indication: valacyclovir buys you fewer doses per day. For a once-daily suppressive regimen the gap is one pill versus two. For acute treatment the gap is wider and more useful, because a simpler schedule started promptly is more likely to be taken on time.
Efficacy is a wash
Because the active drug is identical, head-to-head efficacy between the two is essentially equivalent when each is dosed properly. Suppressive therapy with either reduces recurrence frequency by roughly 70 to 80 percent, though how often you need either drug depends on your type and site. The CDC STI Treatment Guidelines list both as first-line options for suppression and episodic treatment (CDC, 2021). One notable point: the landmark transmission-reduction data come specifically from once-daily valacyclovir 500 mg (Corey et al., NEJM 2004), so if lowering transmission to a partner is the goal, valacyclovir is the one with the direct evidence behind it.
Neither drug is more potent. Adherence is where the real-world difference lives, and the simpler schedule wins on adherence.
Cost
Both drugs are generic and inexpensive. Acyclovir has been generic longer and is often the cheaper of the two per pill. For someone paying cash, acyclovir can come out lower on raw drug cost, particularly for suppression where you are buying 365 doses a year.
That said, the gap at generic pricing is usually modest, and the higher pill count of acyclovir can offset its lower per-pill price in ways that matter to people who value taking one pill instead of two or three. The cost calculation is real but rarely decisive. For most people it comes down to a small dollar difference against a clear convenience difference.
Safety and cautions, shared
Since both reduce to acyclovir, they share the same safety profile. Common effects are headache, nausea and abdominal pain, generally mild. The serious risks are rare and concentrate in specific situations: acute kidney injury from crystal formation, especially with dehydration or very high doses, and neurotoxicity (confusion, tremor, hallucinations) mostly in people with reduced kidney function. Both drugs need dose reduction in renal impairment, and both warrant staying well hydrated. Anyone with kidney disease or who is older should have that flagged before starting either one.
How to choose
The decision is short.
- If you want the simplest schedule, especially for acute treatment, choose valacyclovir.
- If lowering transmission to a partner is the goal, valacyclovir has the direct trial evidence.
- If you are paying cash and want the lowest drug cost and do not mind dosing more often, acyclovir is reasonable and equally effective.
There is no wrong answer between two forms of the same medication. Most people land on valacyclovir for the convenience, and that is a sound default.
Medically reviewed by: [Reviewer name, credentials] — [Date]
If you are deciding between valacyclovir and acyclovir, a licensed clinician can match the regimen to your outbreak pattern and your goals. Start an online visit.
This article is for general education and is not a substitute for personalized medical advice from a licensed clinician.