Aciclovir vs Valaciclovir: Doses & Differences
The same active drug in two forms — what actually differs, the regimens used for first episodes, recurrences and prevention, and how to choose.
Part of the Complete Genital Herpes Guide.
Key fact: Valaciclovir is converted into aciclovir in the body, so the active drug is identical. It is absorbed better, which means one or two doses a day instead of up to five — but it costs more. Neither cures herpes; both shorten episodes, and taken daily both reduce how often they happen.
Two versions of the same medicine
Valaciclovir is a prodrug of aciclovir: the body converts it into aciclovir after absorption. So the active drug is identical, and the difference between them is pharmacokinetic rather than pharmacological. Valaciclovir is absorbed far more efficiently, which means fewer tablets a day for the same effect.
Neither cures herpes. Both reduce how long an episode lasts, how severe it is, and — when taken daily — how often episodes happen. Valaciclovir is sold as Valtrex; aciclovir is the older and cheaper generic.
| Aciclovir | Valaciclovir | |
|---|---|---|
| What it is | The active antiviral | A prodrug converted to aciclovir in the body |
| Absorption | Limited — roughly 10–20% of an oral dose | Substantially better, giving higher aciclovir levels |
| Doses per day | Two to five, depending on the regimen | One or two |
| Cost | Cheaper | More expensive |
| Effectiveness | Broadly equivalent when each is taken correctly | |
The regimens, by situation
The dose depends entirely on what you are treating. These are the regimens used in UK practice; your prescriber will confirm what applies to you.
| Situation | Aciclovir | Valaciclovir |
|---|---|---|
| First episode | 400mg three times daily for 5 days (sometimes longer if new lesions are still appearing) | 500mg twice daily for 5–10 days |
| Recurrence, short course | 800mg three times daily for 2 days, or 400mg three times daily for 3–5 days | 500mg twice daily for 3 days |
| Daily suppression | 400mg twice daily | 500mg once daily |
| In pregnancy | Aciclovir is the one with the longest track record — see herpes in pregnancy | Used less often in pregnancy in UK practice |
Short courses exist for a reason. For a recurrence, a two-day high-dose course of aciclovir works about as well as the older five-day one, because what matters most is starting quickly. Having a course at home to start at the first tingle is more useful than a longer course started three days in.
Which one to choose
- Cost matters most to you? Aciclovir, which is considerably cheaper
- You struggle to remember doses? Valaciclovir, at one or two a day rather than up to five
- You are taking it daily for prevention? Either works; valaciclovir’s once-daily dosing suits long-term use
- You are pregnant or planning pregnancy? Aciclovir has the longest safety record and is what UK guidance uses
- You have reduced kidney function? Both need dose adjustment — tell your prescriber
Side effects and safety
Both are well tolerated. The common side effects are nausea, headache, abdominal discomfort, dizziness and occasionally a rash, and they affect a minority of people. Serious effects are rare.
Two practical points matter. Both drugs are cleared by the kidneys, so drink plenty of fluids while taking them, and anyone with reduced kidney function needs a lower dose. And at very high doses, or in dehydration or kidney impairment, confusion and agitation have been reported — uncommon, but a reason to take the dose you were given rather than doubling up.
Tell your prescriber if you are pregnant or breastfeeding, have kidney problems, are immunocompromised, or take other medicines — particularly anything that affects the kidneys. Long-term suppressive treatment should be reviewed periodically rather than repeated indefinitely.
If it does not seem to be working
Three explanations are worth working through. Treatment started too late — antivirals need to begin within the first five days of a first episode, or at the first sign of a recurrence. The diagnosis is wrong — not every genital ulcer is herpes, and a swab is what settles it. Or the episodes are frequent enough that episodic treatment is the wrong strategy, and daily suppression would suit you better; our main herpes page compares the two approaches.
True antiviral resistance is rare and is seen mainly in people who are significantly immunocompromised. It needs specialist assessment rather than a higher dose bought online.
Frequently Asked Questions
Is valaciclovir better than aciclovir?
Not more effective — valaciclovir is converted into aciclovir in the body, so the active drug is the same. What differs is absorption: valaciclovir is absorbed more efficiently, so it needs one or two doses a day rather than up to five. Aciclovir is considerably cheaper. Convenience and cost decide the choice, not potency.
What is the dose of aciclovir for genital herpes?
For a first episode, 400mg three times daily for five days is the usual UK regimen, sometimes extended if new lesions are still appearing. For a recurrence, a short course of 800mg three times daily for two days works about as well as longer courses. For daily suppression the dose is 400mg twice daily. Your prescriber will confirm what applies to you.
How quickly does aciclovir work?
It starts acting immediately but the effect is on the course of the episode rather than instant relief. Started early, it typically shortens an episode by a couple of days and makes it less severe. Started late, once sores are crusting, it makes little difference — which is why having treatment available to start at the first tingle is so useful.
Can I take aciclovir every day?
Yes. Daily suppressive treatment — aciclovir 400mg twice daily or valaciclovir 500mg once daily — is a recognised approach for people with frequent recurrences, and it also reduces the risk of passing the virus to a partner. It should be prescribed and reviewed periodically rather than continued indefinitely without discussion.
Are there side effects?
Most people tolerate both drugs well. Nausea, headache, abdominal discomfort and dizziness are the common ones, and serious effects are rare. Both are cleared by the kidneys, so drink plenty of fluids, and anyone with reduced kidney function needs a lower dose. Never take more than the dose you were given.
Can I take aciclovir in pregnancy?
Aciclovir is used in pregnancy and has the longest safety record of the antivirals, which is why UK guidance relies on it — including suppressive treatment in the last weeks to reduce the chance of lesions at delivery. It should always be prescribed by a clinician who knows you are pregnant.
References
- NHS. Genital herpes. nhs.uk
- British Association for Sexual Health and HIV. UK national guideline for the management of anogenital herpes. bashh.org
- BASHH and Royal College of Obstetricians and Gynaecologists. National UK guideline for the management of herpes simplex virus (HSV) in pregnancy and the neonate (2024 update). journals.sagepub.com
- Royal College of Obstetricians and Gynaecologists. Management of genital herpes in pregnancy. rcog.org.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summary: Herpes simplex — genital. cks.nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Genital symptoms have several possible causes and herpes is only one of them; diagnosis needs a clinician and usually a swab. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.