How the dose is calculated
Acyclovir dosing is conventionally expressed as a fixed mg/kg per dose given at a set interval, rather than a single mg/kg/day figure — the calculator reflects this by converting the selected per-dose amount and frequency into an equivalent total daily dose, then dividing it back across the same number of administrations. Severe IV indications (neonatal or CNS HSV, immunocompromised host disease) use higher per-dose amounts every 8 hours; oral indications for immunocompetent patients use lower per-dose amounts given more frequently, reflecting acyclovir's short (~2–3 hour) half-life and low oral bioavailability.
Typical dosing by indication
| Indication | Dose | Frequency | Typical duration |
|---|---|---|---|
| Neonatal / CNS HSV (IV) | 20 mg/kg/dose | every 8h | 14–21 days |
| Immunocompromised host, HSV/varicella (IV) | 10 mg/kg/dose | every 8h | 7–10 days |
| Chickenpox, oral (immunocompetent) | 20 mg/kg/dose (max 800 mg) | 4×/day | 5 days |
| Herpetic gingivostomatitis, oral | 15 mg/kg/dose (max 800 mg) | 5×/day | 7 days |
Fixed adult dosing (not weight-based)
Several common adult indications use a fixed dose rather than mg/kg — these aren't covered by the calculator above since they don't scale with weight.
| Indication | Dose | Duration |
|---|---|---|
| Herpes zoster (shingles), oral | 800 mg, 5×/day | 7–10 days |
| Genital herpes, first episode, oral | 400 mg 3×/day or 200 mg 5×/day | 7–10 days |
| Genital herpes, suppressive therapy, oral | 400 mg twice daily | ongoing, per response |
Clinical use
- IV acyclovir is the standard of care for neonatal HSV disease (localised, CNS, or disseminated) and HSV encephalitis at any age — early empirical treatment while awaiting confirmatory testing is standard practice given the high morbidity/mortality of untreated disease.
- Oral bioavailability is low (~15–20%), which is why severe or CNS disease always uses the IV route, while milder mucocutaneous disease in immunocompetent hosts can be treated orally at a higher, more frequent dose to compensate.
- Immunocompromised patients (transplant, chemotherapy, advanced HIV) generally warrant a lower threshold for IV therapy even for varicella or HSV that would be treated orally in an immunocompetent host, given the risk of dissemination.
- Valacyclovir (the oral prodrug of acyclovir) offers better oral bioavailability and less frequent dosing for adult outpatient indications, but is not typically used for weight-based paediatric dosing in the way acyclovir is.
Frequently asked questions
What is the acyclovir dose for neonatal herpes?
20 mg/kg/dose IV every 8 hours (60 mg/kg/day total) for 14–21 days depending on disease extent (skin/eye/mouth vs CNS vs disseminated) — always managed as an inpatient with close monitoring.
Why is acyclovir given so frequently?
Its elimination half-life is short (roughly 2–3 hours), so maintaining effective drug levels requires dosing every 8 hours for IV therapy or as often as 4–5 times daily for oral therapy — unlike longer half-life drugs such as fluconazole that can be dosed once daily.
Is acyclovir the same as aciclovir?
Yes — "aciclovir" is the International Nonproprietary Name (INN) used in the UK and much of the world, while "acyclovir" is the US Adopted Name. They are the identical drug.
Does acyclovir cure herpes?
No — acyclovir suppresses viral replication and shortens/reduces the severity of outbreaks, but it does not eliminate the latent virus from nerve tissue. Herpesviruses (HSV, VZV) remain latent for life once acquired; suppressive therapy reduces recurrence frequency but isn't a cure.
Why does IV acyclovir need to be infused slowly with good hydration?
Rapid infusion or dehydration increases the risk of acyclovir crystallising in the renal tubules, which can cause acute kidney injury — infusing over at least an hour and ensuring the patient is well-hydrated reduces this risk.
References
- Kimberlin DW, et al. Guidance on Management of Neonatal Herpes Simplex Virus Infection. Red Book: Report of the Committee on Infectious Diseases (AAP).
- British National Formulary for Children (BNFC) — Aciclovir.
- Workowski KA, et al. Sexually Transmitted Infections Treatment Guidelines (CDC) — Genital Herpes.
- Tripathi KD. Essentials of Medical Pharmacology, 8th edition — Antiviral agents.