If you've been prescribed treatment for genital herpes, you've probably come across aciclovir and valaciclovir. Both are effective antivirals, both can be prescribed through Pharmica if they're suitable for you, and they do essentially the same job. So when it comes to aciclovir vs valaciclovir, what's the actual difference, and which one suits you better?
This guide puts the two side by side. We'll cover how they work, how you take them, what the research says and what to weigh up when choosing between them. That applies whether you're treating an outbreak or taking a daily tablet to keep outbreaks away.
Neither antiviral is stronger than the other. The real difference is how many tablets you need to take each day.
How do aciclovir and valaciclovir treat genital herpes?
Both are antiviral medicines. They stop the herpes simplex virus (HSV) from copying itself, which helps outbreaks heal faster and feel less severe.
Here's the interesting bit. Valaciclovir is a prodrug of aciclovir, which means your body converts it into aciclovir after you swallow it. Once it's in your bloodstream, it works in exactly the same way, because it's become the same active ingredient.1
The key difference is absorption. Your gut only absorbs around 10 to 20% of an aciclovir dose.2 Valaciclovir is absorbed much better. After a 1,000mg dose, about 54% reaches your blood as aciclovir.1
That better absorption is why valaciclovir can reach similar antiviral levels in your blood with fewer doses a day.
Think of valaciclovir as aciclovir in a better-absorbed form. It's the same medicine doing the work, with fewer tablets needed to get it there.
Aciclovir vs valaciclovir at a glance
Here's how the two compare on the standard UK regimens for adults.3 These example schedules are for adults with a healthy immune system and kidneys that work normally.
If you have kidney problems, a weakened immune system or you're pregnant, you'll need a dosing plan tailored to you.1 Your prescriber will confirm the exact dose and course length that's right for you.
| Feature |
Aciclovir |
Valaciclovir |
| First episode |
400mg three times daily for 5 days (may be extended) |
500mg twice daily for 5 days (may be extended) |
| Recurrent outbreak (episodic treatment) |
800mg three times daily for 2 days, or 400mg three times daily for 3 to 5 days |
500mg twice daily for 3 to 5 days |
| Suppressive dose |
400mg twice daily |
500mg once daily |
| Bioavailability |
10 to 20% |
About 54% |
| Doses per day (episodic) |
3 |
2 |
| Doses per day (suppressive) |
2 |
1 |
| Available from Pharmica |
Yes, after prescriber assessment |
Yes, after prescriber assessment |
Dosing and convenience
This is where the practical difference between aciclovir and valaciclovir really shows.
With either medicine, timing matters. Treatment works best when you start it as soon as you notice the first signs of an outbreak, before blisters have formed.3
For a recurrent outbreak, UK guidelines recommend aciclovir 800mg three times a day for two days, or 400mg three times a day for three to five days. A first episode is usually treated with 400mg three times a day for five days.3
Three doses a day means spacing them roughly eight hours apart. Easy enough on a quiet day. Harder when you're at work, travelling or just busy.
With either medicine, a first episode needs follow-up. If new sores keep appearing or symptoms are still significant, your prescriber may extend the course. Follow the review and course plan you're given.1
If you get treatment through Pharmica, our consultation asks whether a GP or sexual health clinic has already diagnosed you with genital herpes. If you think this is your first outbreak, get checked by a GP or sexual health clinic.
If your symptoms do not improve after 7 days of treatment, you must see your doctor. Do not wait that long if your symptoms are severe or you cannot pee.
Valaciclovir keeps things simpler. The standard dose is 500mg twice a day, for three to five days for a recurrence or five days for a first episode.3 Two doses a day fits much more easily around everything else going on in your life.
Why does that matter? Fewer doses means fewer chances to miss one. UK guidelines point to valaciclovir's less frequent dosing as a way to help people stick with their treatment.3
And a medicine can only work as intended if you take it on schedule. If you know a mid-afternoon dose is likely to slip, valaciclovir's morning-and-evening timing may be much easier to keep up.
Whichever antiviral you're prescribed, the first tingle is your cue. The earlier you take that first dose, the better your chance of a shorter, milder outbreak.
How effective is aciclovir compared to valaciclovir?
When it comes to results, the two perform the same.
A randomised controlled trial by the International Valaciclovir HSV Study Group, published in 1997, compared them for treating recurrent genital herpes in 739 people. Valaciclovir 500mg twice daily was tested against aciclovir 200mg five times daily for five days. There was no significant difference in how long symptoms lasted, how quickly sores healed or how quickly the virus stopped shedding.4
A separate trial by Fife and colleagues, also published in 1997, looked at 643 adults with a first episode of genital herpes. Over ten days, valaciclovir 1000mg twice daily and aciclovir 200mg five times daily showed no significant difference in healing time, pain or viral shedding.5
These trials used older aciclovir regimens than the ones in the table above. But the 2024 UK guidelines reach the same conclusion: head-to-head studies show no advantage of one antiviral over the other.3
So whether you're treating a first episode or a recurrence, you're not giving up effectiveness by choosing one over the other. The difference is convenience, not strength.
Two large trials and the latest UK guidelines agree. Pick either one and you can expect the same healing, just on a different timetable.
Want the bigger picture? Pharmica's guide to treating and suppressing herpes explains how these treatments fit into a wider management plan.
Suppressive therapy for recurring outbreaks
Getting outbreaks often, typically six or more a year? You may be offered suppressive therapy. Rather than treating each outbreak as it happens, you take an antiviral every day to reduce how often outbreaks come back and how severe they are.3
It can also help if your outbreaks are less frequent but painful, or if herpes is causing you a lot of worry.
Both antivirals are used for suppressive therapy, but the daily schedules differ.3
- Aciclovir: 400mg twice daily
- Valaciclovir: 500mg once daily
When you're taking something every single day, a once-daily tablet can make a real difference to staying consistent. It's also worth knowing that the full suppressive effect usually kicks in around five days after you start.3
Treatment is normally reviewed after 6 to 12 months to check whether you still need it.1 If you're taking it to cut down outbreaks, UK guidelines suggest a break after a year at most to see how things go without it. If outbreaks come back too often, you can restart.3
Protecting a partner
All the antivirals reduce how often the virus is shed from your skin, including when you have no symptoms.3 But the direct evidence on protecting a partner comes from one medicine: daily valaciclovir.
A landmark trial published in the New England Journal of Medicine in 2004 followed 1,484 couples where one partner had HSV-2 and the other didn't. The couples were heterosexual, in monogamous relationships and HIV-negative. Over eight months, daily valaciclovir cut the overall rate of HSV-2 transmission by 48%, and cut symptomatic genital herpes in partners by 75%.6
Other antivirals, including aciclovir, may help too. But that hasn't been proven in clinical trials, and the results may not apply to every type of relationship.3
Even on daily treatment, some risk remains. Condoms lower it further, and it's best to avoid sex during an outbreak or when you feel one starting.1,3
If protecting a partner is part of your reason for considering suppressive therapy, that's a conversation worth having with your prescriber. In that case, guidelines advise against taking breaks, because the risk of transmission rises again during a gap.3
Do not pause daily antiviral treatment without speaking to your prescriber first, especially if you take it to protect a partner.
Side effects of aciclovir and valaciclovir
Because valaciclovir turns into aciclovir in your body, the two have very similar side effects. Both are generally well tolerated.
The most commonly reported side effects are listed below, with how often each is classed in the official product information. "Common" means up to 1 in 10 people, and "very common" means more than 1 in 10.1,2
- Headache: common with aciclovir, very common with valaciclovir
- Nausea and vomiting: common with both
- Diarrhoea: common with both
- Stomach pain: common with aciclovir, uncommon with valaciclovir
- Dizziness: common with both
- Itching or rash: common with both, including skin that's more sensitive to sunlight
These tend to be mild and settle on their own.
More serious side effects can happen, and how often depends on the medicine. With valaciclovir, confusion and agitation are classed as uncommon (up to 1 in 100 people), and kidney problems as uncommon to rare. With aciclovir, these are classed as rare to very rare.1,2
Confusion and other effects on the nervous system are more likely in older people and anyone with kidney problems. Your prescriber may adjust your dose if either applies to you.1
Get medical advice promptly if you feel confused or agitated, pass little or no urine, or get pain in your lower back. These can be signs of effects on your brain or kidneys.1
Most side effects are mild. Changes in how clearly you're thinking, or in how much you pee, are the ones to get checked rather than wait out.
If you notice swelling of your face, lips or tongue, or have trouble breathing, stop taking the medicine and get medical help straight away. For anything else unusual, speak to our pharmacy team or your GP.
Staying well hydrated helps your kidneys clear the medicine and lowers the risk of kidney-related side effects.1 This matters most for older people and anyone on higher doses of either medicine.
Which treatment should you choose?
When you weigh up aciclovir vs valaciclovir, neither is clinically superior. The decision comes down to your routine and what matters most to you.
“Neither aciclovir nor valaciclovir is clinically superior, because head-to-head studies show no advantage of one over the other. The real difference is convenience, as valaciclovir needs fewer doses a day and that can make it easier to stay on track. The best choice is the schedule you're most likely to keep up.
Ana Carolina GoncalvesSuperintendent Pharmacist
Choose aciclovir if:
- you don't mind a three-times-a-day schedule
- you'd like the option of a short two-day course for an outbreak
- you've used it before and it's worked well for you
Choose valaciclovir if:
- you want fewer doses a day so it's easier to stay on track
- you're starting suppressive therapy and would rather take one tablet a day
- you've struggled to stick to a three-times-daily schedule in the past
Either way, you're getting the same proven antiviral. The best choice is simply the schedule you're most likely to keep up.
Still unsure? Pharmica's prescribers can help you decide when they review your consultation. You can also read more in Pharmica's guide to genital herpes symptoms, causes and treatments or browse expert answers to common questions about genital herpes.
Frequently asked questions
Can you switch between aciclovir and valaciclovir?
Yes. Valaciclovir turns into aciclovir in your body, so switching between them is straightforward. Plenty of people start on one and move to the other because the dosing suits them better. If your current treatment isn't fitting around your routine, your prescriber can change it for you. Do not take both at the same time unless a prescriber has told you to.
Is valaciclovir stronger than aciclovir?
No. Valaciclovir isn't more potent. Your body absorbs more of each dose, which is why you need fewer tablets a day, but the active antiviral doing the work is exactly the same. Clinical trials comparing the two found no significant difference in healing time or symptom relief. Valaciclovir is easier to take, not stronger.
How quickly do aciclovir and valaciclovir work for genital herpes?
Both reach their peak level in your blood within a couple of hours of a dose. They work best when you start them at the very first sign of an outbreak, such as tingling or itching. Treatment usually shortens an outbreak by a day or two, and starting early can sometimes stop blisters forming at all.
Can you buy aciclovir or valaciclovir over the counter?
Not for genital herpes. In the UK, aciclovir and valaciclovir tablets are prescription-only medicines, but Pharmica's prescribers can assess whether they're suitable for you through an online consultation. Aciclovir cream for cold sores is available over the counter, but it isn't the right fit for genital herpes. UK guidelines say tablets work better than creams, and using both adds nothing extra.
Does suppressive therapy reduce the risk of passing herpes to a partner?
Daily valaciclovir has been shown to reduce transmission. In a large trial of HIV-negative, monogamous heterosexual couples where one partner had HSV-2, it cut overall transmission by 48%. Other antivirals may help too, but that hasn't been proven in trials. Either way, some risk remains. Use condoms, and avoid sex during an outbreak or when you feel one starting.
Can you take aciclovir or valaciclovir during pregnancy?
Antivirals can be used in pregnancy, but your GP, midwife or obstetric team should make the call. There's more pregnancy safety data for aciclovir, so it's usually the first choice if treatment is needed during pregnancy or close to delivery, with valaciclovir sometimes used as an alternative. Do not start or stop antiviral treatment during pregnancy without medical advice.
References
- GlaxoSmithKline UK. Valtrex 500mg Tablets: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Milpharm Ltd. Aciclovir 400 mg Tablets: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Patel R, Green J, Moran B, et al. British Association of Sexual Health and HIV UK national guideline for the management of anogenital herpes, 2024. International Journal of STD & AIDS. 2025;36(2):90-105. doi:10.1177/09564624241282396
- Bodsworth NJ, Crooks RJ, Borelli S, et al. Valaciclovir versus aciclovir in patient initiated treatment of recurrent genital herpes: a randomised, double blind clinical trial. International Valaciclovir HSV Study Group. Genitourinary Medicine. 1997;73(2):110-116. doi:10.1136/sti.73.2.110
- Fife KH, Barbarash RA, Rudolph T, et al. Valaciclovir versus acyclovir in the treatment of first-episode genital herpes infection. Results of an international, multicenter, double-blind, randomized clinical trial. The Valaciclovir International Herpes Simplex Virus Study Group. Sexually Transmitted Diseases. 1997;24(8):481-486. doi:10.1097/00007435-199709000-00007
- Corey L, Wald A, Patel R, et al. Once-daily valacyclovir to reduce the risk of transmission of genital herpes. New England Journal of Medicine. 2004;350(1):11-20. doi:10.1056/NEJMoa035144
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.