Acyclovir has been used to treat herpes infections since 1982. It's the original, extremely well-studied antiviral for HSV-2. For suppressive therapy, the standard dose is 400mg twice daily. It's often the cheapest option available, typically $10-20 per month for generic. The main practical difference from valacyclovir is that it requires twice-daily rather than once-daily dosing. Clinically, both drugs work extremely well.
History and why it matters
Acyclovir was approved by the FDA in 1982. That means it has over 40 years of clinical use behind it. In medicine, that's genuinely a good thing. We have more long-term safety data on acyclovir than on almost any other antiviral drug in existence. We know what it does, what it doesn't do, and what to expect from four decades of use across millions of patients worldwide.
Its development won Gertrude Elion a share of the 1988 Nobel Prize in Physiology or Medicine, alongside George Hitchings and James Black. Elion and Hitchings pioneered the approach of designing drugs that target specific enzymes in pathogens rather than relying on general toxicity. Acyclovir was one of the landmark examples of that approach working.
Acyclovir is on the World Health Organization's List of Essential Medicines. It has been generic for a long time, which is the main reason it's so affordable. There's no reason to pay for brand-name Zovirax when the generic is identical in composition and effect.
How acyclovir works
Acyclovir is a nucleoside analog. That means it's designed to look like a building block the herpes virus needs to copy its DNA. When acyclovir gets into a virus-infected cell, viral enzymes (specifically thymidine kinase) phosphorylate it. This activated form then gets incorporated into the growing viral DNA chain, where it acts as a chain terminator. The viral DNA can't continue growing, replication stops, and the virus can't produce new copies of itself.
This mechanism is highly selective. The viral thymidine kinase that activates acyclovir is very different from human cellular enzymes, which is why acyclovir has minimal effect on human cells and such a clean safety profile. The drug essentially works inside virus-infected cells while largely leaving healthy cells alone.
The one limitation: acyclovir doesn't reach the latent virus hiding in nerve ganglia. When HSV-2 is dormant between outbreaks, it's not replicating and the viral thymidine kinase isn't active. Acyclovir has nothing to block in that state. This is why antivirals manage HSV-2 rather than curing it. For more on that distinction, see our cure research article.
Why twice-daily dosing is needed
One of the most common questions about acyclovir is why it needs to be taken twice daily for suppressive therapy, when valacyclovir can be taken once daily.
The answer is bioavailability. When you take oral acyclovir, only about 15-20% of the dose is absorbed into the bloodstream. The rest passes through the gut without being absorbed. This is a known limitation of the drug's chemistry.
Valacyclovir is a prodrug, meaning it's a modified form of acyclovir designed to be better absorbed. After swallowing valacyclovir, the intestines and liver convert it to acyclovir with much higher efficiency, producing blood levels roughly 3-5 times higher than an equivalent oral acyclovir dose. That higher bioavailability is what allows once-daily dosing for suppression.
At the end of the day, the active compound in your blood is acyclovir either way. Valacyclovir is essentially a delivery system that produces higher levels of that same compound. The twice-daily acyclovir schedule achieves very similar therapeutic outcomes. The practical difference is just convenience.
Dosing
Acyclovir Dosing for HSV-2
- Suppressive therapy: 400mg twice daily (morning and evening)
- Episodic treatment (standard 5-day): 400mg three times daily for 5 days
- Episodic treatment (alternative): 800mg twice daily for 5 days
- Starting episodic treatment: Begin at the very first sign of prodrome or outbreak
- Kidney disease: Dose adjustment needed, your provider will guide this
The twice-daily suppressive schedule is easy to work into a routine. Many people take it with breakfast and dinner, or pair it with morning and evening teeth brushing. Consistency matters more than exact timing. Find a schedule that makes it easy to remember and stick to it.
For episodic treatment, the most important factor is timing. Starting acyclovir during the prodrome (the tingling, itching, or burning that often precedes visible sores) produces significantly better results than starting after sores have formed. If you experience prodrome symptoms, that's your signal to start medication immediately.
Ask your provider about having an episodic supply on hand rather than waiting to call during an outbreak. Being able to start treatment at the first sign of prodrome, rather than waiting for a pharmacy appointment, can meaningfully reduce outbreak severity and duration.
How effective is acyclovir?
Acyclovir has been proven effective in numerous clinical trials spanning four decades. For suppressive therapy, it reduces outbreak frequency by approximately 70-75%. This is very comparable to valacyclovir outcomes. For episodic treatment, starting acyclovir early in the prodrome phase reduces both the duration and severity of the outbreak.
Acyclovir also reduces asymptomatic viral shedding, which is the main driver of transmission between outbreaks. When equivalent active drug levels are maintained (which is the case with proper twice-daily dosing), the shedding reduction with acyclovir is similar to valacyclovir.
The main caveat with acyclovir vs. valacyclovir when it comes to transmission data: the large transmission reduction trial (Corey et al., NEJM 2004) used valacyclovir specifically. Acyclovir doesn't have equivalent large-scale transmission data, though the mechanism of shedding reduction is the same. Most providers assume the transmission benefit is similar when adequate acyclovir blood levels are maintained.
Which approach is right for you?
| Factor | Suppressive Therapy | Episodic Treatment |
|---|---|---|
| When you take it | Every day, regardless of symptoms | Only when an outbreak starts |
| Best for | Frequent outbreaks (6+ per year), active relationships, reducing transmission risk | Infrequent outbreaks, people who prefer minimal daily medication |
| Transmission protection | Yes, significant | Minimal (not taken between outbreaks) |
| Outbreak prevention | ~70-75% reduction in frequency | Reduces severity and duration once started |
| Monthly cost (generic) | ~$10-20 | Lower overall if outbreaks are infrequent |
If you have a partner who is HSV-negative, suppressive therapy is generally the right choice regardless of outbreak frequency. Daily suppressive therapy reduces both symptomatic and asymptomatic shedding, providing transmission protection throughout the month rather than only during visible outbreaks.
Side effects and safety
Acyclovir is generally very well tolerated. This isn't just marketing language. It's based on 40+ years of data from millions of patients.
The most commonly reported side effects at HSV-2 suppressive doses are mild nausea and headache. Both tend to be transient, resolving within the first week or two as the body adjusts. The majority of people experience no side effects at all.
A few things worth knowing:
- Kidney effects: At very high IV doses used for severe infections (like HSV encephalitis or neonatal herpes), acyclovir can crystallize in the kidney tubules if patients are dehydrated. At the oral doses used for genital herpes, this isn't a practical concern for people with normal kidney function. Staying reasonably hydrated is good practice regardless.
- Neurological effects: Reported at high doses in patients with kidney impairment (since acyclovir clears through the kidneys). Not relevant at standard HSV-2 oral doses in people with normal kidney function.
- Drug interactions: Acyclovir has very few significant drug interactions. Some medications that affect kidney function (like NSAIDs or certain antibiotics) may theoretically interact, but this is rarely clinically significant at standard doses. Tell your provider about all medications you take.
- Pregnancy: Acyclovir has a long track record in pregnancy and is generally considered safe, especially in the third trimester for preventing neonatal transmission. Your OB/GYN will guide this specifically.
Long-term safety is very well established. Acyclovir has been used for continuous suppressive therapy for decades with no pattern of emerging problems. For people who do well on it and have reasons to continue suppressive therapy, there's no reason to stop based on duration concerns alone.
Cost
This is where acyclovir clearly shines. It's consistently the most affordable antiviral option for HSV-2. Generic acyclovir 400mg tablets for twice-daily suppressive therapy typically costs $10-20 per month without insurance, and often less with GoodRx or similar discount programs.
For comparison, generic valacyclovir runs $15-30 per month, and famciclovir is typically $20-40. If cost is the primary consideration in choosing between antivirals, acyclovir is the answer.
Always request generic acyclovir. Brand-name Zovirax is substantially more expensive and chemically identical to the generic. There is no clinical reason to pay the brand-name premium.
How to get a prescription
Acyclovir requires a prescription in the United States. Options for getting one include:
- Your primary care physician or gynecologist
- A sexual health clinic or Planned Parenthood location
- A telehealth service (often the most convenient option for ongoing refills)
- An urgent care clinic if you need treatment during an active outbreak
Telehealth services have made accessing HSV-2 medication considerably easier. A brief online consultation, typically completed same-day, results in a prescription sent to your pharmacy. Services like Wisp and Nurx handle this routinely. For ongoing refills, a telehealth relationship is often more practical than scheduling repeat in-person appointments.
Get an acyclovir prescription online
Telehealth services can prescribe acyclovir or valacyclovir after a brief online consultation. Fast, private, and convenient.
Affiliate links. We may earn a small commission.
Frequently asked questions
What is the dose of acyclovir for HSV-2 suppressive therapy?
The standard suppressive dose is 400mg twice daily. This is taken every day regardless of symptoms. For people with kidney issues, the dose may be adjusted by their provider.
Is acyclovir as effective as valacyclovir for HSV-2?
At equivalent doses, yes. Valacyclovir achieves higher blood levels of active drug per dose, which is why it can be taken once daily vs. twice. But the underlying active compound is the same, and both drugs are highly effective for suppressive and episodic therapy.
Why does acyclovir need twice-daily dosing?
Oral acyclovir has poor bioavailability, around 15-20%. To maintain adequate blood levels throughout the day, twice-daily dosing is needed. Valacyclovir converts to acyclovir with much better absorption, allowing once-daily dosing for the same effect.
How much does acyclovir cost?
Generic acyclovir is typically the cheapest antiviral option for HSV-2, around $10-20 per month for suppressive dosing. It's been off-patent for decades, which keeps costs very low compared to brand-name alternatives.
Is acyclovir safe to take long-term?
Yes. Acyclovir has been in continuous clinical use since 1982 and has an excellent long-term safety record. Studies of people on long-term suppressive therapy show no emerging safety concerns. It's one of the most well-studied antivirals available.
Can I switch from acyclovir to valacyclovir?
Yes, switching is straightforward since both drugs work via the same active compound. The main reason people switch is convenience, going from twice-daily to once-daily dosing. Talk to your prescriber about adjusting your prescription.
Related: Valacyclovir guide | Compare all antivirals | Treatment overview | Transmission risk data