Acyclovir Dosage

Acyclovir dosing varies by indication, route, age, and renal function. Below is a practical summary drawn from clinical pharmacology and pediatric/emergency medicine references.

Adults (immunocompetent, normal renal function)

Genital herpes (HSV)

First episode: 400 mg PO 3x/day or 200 mg PO 5x/day for 7-10 days
Recurrent episodes: 800 mg PO 2x/day for 5 days, or 400 mg PO 3x/day for 5 days, or 800 mg PO 3x/day for 2 days
Chronic suppression: 400 mg PO 2x/day for up to 12 months, then reassess
Severe/hospitalized disease: 5 mg/kg IV every 8 hours for 5-7 days

Herpes zoster (shingles) - immunocompetent

800 mg PO 5x/day (every 4 hours while awake) for 7-10 days, started ideally within 48-72 hours of rash onset - "Adults older than 50 years showed the greatest benefit" (Fitzpatrick's Dermatology, p. 1655)

Herpes zoster/VZV, immunocompromised host

10 mg/kg IV every 8 hours for 7 days

Oral-labial (herpes labialis)

Topical 5% cream to lesions; oral regimens used off-label for outbreaks/suppression

HSV encephalitis (any age, immunocompetent or not)

10 mg/kg IV every 8 hours - "The antiviral of choice for HSV encephalitis is high-dose acyclovir at 10 mg/kg per dose IV every 8 hours" (Tintinalli's Emergency Medicine, p. 1052-1059)
Duration: 14-21 days (manufacturer lists 10 days; CDC and most current practice favor 14-21 days given relapse risk)

Pediatrics (Harriet Lane Handbook, 23rd ed., p. 998-999)

Neonatal HSV / HSV encephalitis (birth-3 months)

<34 weeks postmenstrual age: 40 mg/kg/24 hr divided every 12 hr IV
≥34 weeks postmenstrual age: 60 mg/kg/24 hr divided every 8 hr IV
Duration: 14 days (cutaneous/mucous membrane) or 21 days (CNS/disseminated)
Oral suppression after IV course: 300 mg/m²/dose every 8 hr PO for 6 months (improves neurodevelopmental outcomes)

HSV encephalitis by age

Birth-3 mo: as above
3 mo-12 yr: 60 mg/kg/24 hr divided every 8 hr IV
≥12 yr: 30 mg/kg/24 hr divided every 8 hr IV
Duration: 14-21 days

Mucocutaneous HSV, ≥12 yr, initial infection

IV: 15 mg/kg/24 hr (or 750 mg/m²/24 hr) divided every 8 hr for 5-7 days
PO: 1000-1200 mg/24 hr divided into 3-5 doses for 7-10 days
Younger children (pediatric dosing): 40-80 mg/kg/24 hr divided every 6-8 hr for 5-10 days (max 1000 mg/24 hr)

Chickenpox (varicella), children ≥2 yr, immunocompetent, >40 kg

20 mg/kg/dose (max 800 mg) PO 4x/day for 5 days

Renal impairment

Dose reduction is required in renal insufficiency because acyclovir is cleared mainly by glomerular filtration and tubular secretion; half-life extends from 2.5-3 hours (normal renal function) to about 20 hours in anuric patients. IV acyclovir carries a risk of crystalline nephropathy/interstitial nephritis with rapid bolus dosing - prevention relies on slow infusion and adequate hydration/urine output. Hemodialysis removes significant amounts of the drug (National Kidney Foundation Primer on Kidney Diseases, 8e, p. 2004-2007). Renal dosing tables (by creatinine clearance) should be consulted for exact adjustments in each indication.

Pharmacologic notes relevant to dosing

Oral bioavailability is low (about 10-20%) and does not improve with higher doses, which is why valacyclovir (prodrug, ~55% bioavailability) is often preferred when oral dosing convenience matters (Goodman & Gilman's Pharmacological Basis of Therapeutics; Katzung's Basic and Clinical Pharmacology, p. 1350).
CSF concentrations reach only 20-50% of serum levels, which is why CNS infections require the highest weight-based doses.
Common adverse effects: nausea, diarrhea, headache; with IV use, reversible nephrotoxicity and neurotoxicity (tremor, delirium, seizures), and dose-dependent neutropenia, generally avoidable with proper hydration and infusion rate.

A quick check of recent systematic reviews/meta-analyses on acyclovir dosing did not surface new randomized evidence changing these standard regimens; current guidance remains consistent with the above.

Always verify exact mg/kg or fixed doses and duration against the specific indication, patient's actual renal function, and current product labeling before prescribing, since some regimens (e.g., encephalitis duration, dose in obesity) vary between sources and by clinical judgment.

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