Antiviral
Pregnancy: Caution — use if benefit outweighs risk (herpes encephalitis in pregnancy is life-threatening)
Aciclovir
Brand names: Zovirax
Adult dose
Dose: Herpes simplex: 200–400 mg 5 times daily; Herpes zoster: 800 mg 5 times daily; IV herpes encephalitis: 10 mg/kg every 8 hours
Route: Oral / IV
Frequency: 5 times daily (oral); every 8 hours (IV)
Max: 4 g/day oral; 15 mg/kg/day IV (in severe immunosuppression)
Herpes simplex (oral): 5 days. Herpes zoster: 7 days (start within 72 hours of rash). IV: for encephalitis, neonatal HSV, immunocompromised. IV must be given over 1 hour — precipitation if too rapid.
Paediatric dose
Dose: 10–20 mg/kg
Route: IV
Frequency: Every 8 hours
Max: Based on ideal body weight
Neonatal HSV: 20 mg/kg IV every 8 hours for 14–21 days. Herpes zoster in immunocompromised child: 10 mg/kg IV every 8 hours. Oral doses depend on indication and age — BNFc guidance.
Dose adjustments
Renal
Dose reduction required: if eGFR 10–25: reduce to every 12 hours (IV); eGFR <10: every 24 hours. Ensure adequate hydration to prevent crystalline nephropathy.
Hepatic
No dose adjustment required.
Paediatric weight-based calculator
Neonatal HSV: 20 mg/kg IV every 8 hours for 14–21 days. Herpes zoster in immunocompromised child: 10 mg/kg IV every 8 hours. Oral doses depend on indication and age — BNFc guidance.
Clinical pearls
- IV aciclovir must be given with 1L of fluid per dose (IV bag) or good oral hydration — to prevent crystalline deposits in renal tubules
- Valaciclovir (prodrug) is better absorbed orally and preferred for outpatient treatment of herpes zoster
- Start within 72 hours of zoster rash — no benefit if started later unless new lesions still appearing
- Herpes encephalitis: start IV aciclovir empirically while awaiting PCR results — do not wait for confirmation
Contraindications
- Hypersensitivity to aciclovir or valaciclovir
Side effects
- Nausea
- Headache
- Rash
- Renal crystalline nephropathy (IV — if poorly hydrated)
- Neurological effects (tremor, encephalopathy — IV at high doses)
- Phlebitis at IV site
Interactions
- Probenecid (reduces renal excretion — increases aciclovir levels)
- Mycophenolate (both renally excreted — increased toxicity risk)
- Zidovudine (risk of neurotoxicity)
Monitoring
- Renal function (especially IV use)
- Urine output and hydration status
- Neurological status (high-dose IV)
Reference: BNFc; BNF 90; NICE CKS (Herpes simplex); BNFc; TOXBASE. Verify against your local formulary and the latest BNF before prescribing.
Related
Curated clinical cross-links plus same-class fallbacks.
Calculators