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Antiviral — HSV Encephalitis / Neonatal Herpes / VZV in Immunocompromised Pregnancy: Safe — aciclovir widely used for HSV/VZV in pregnancy; international registry shows no increased birth defects

Aciclovir (Paediatric)

Brand names: Zovirax

Adult dose

Dose: HSV encephalitis: 10 mg/kg IV every 8 hours × 14–21 days; VZV: 10–15 mg/kg IV every 8 hours
Route: IV infusion over 1 hour
Frequency: Every 8 hours
Max: 30 mg/kg/day (HSV); 45 mg/kg/day (VZV)
Adult reference — see paediatric dose section

Paediatric dose

Dose: HSV encephalitis (children ≥3 months): 500 mg/m² or 10 mg/kg every 8 hours × 14–21 days; Neonatal herpes: 20 mg/kg every 8 hours × 14–21 days (CNS/disseminated) or × 14 days (skin/eyes/mouth); VZV in immunocompromised: 250 mg/m² every 8 hours mg/kg
Route: IV infusion over 1 hour
Frequency: Every 8 hours
Max: Neonatal: 20 mg/kg per dose; Children: 10 mg/kg per dose
BNFc: NEONATAL HERPES — 20 mg/kg every 8 hours is a HIGH dose (not 10 mg/kg) — critical distinction; inadequate treatment of neonatal HSV causes death or severe neurodevelopmental impairment. IV must be given over 1 hour (nephrotoxic if given rapidly — crystalluria in renal tubules). Ensure adequate hydration throughout. Oral aciclovir bioavailability is poor (15–30%) — not suitable for serious infections. (or mg/m²)

Dose adjustments

Renal

CrCl 25–50: extend interval to every 12 hours; CrCl 10–25: every 24 hours; CrCl <10: 50% dose every 24 hours; ensure adequate hydration in all cases

Hepatic

No dose adjustment required

Paediatric weight-based calculator

BNFc: NEONATAL HERPES — 20 mg/kg every 8 hours is a HIGH dose (not 10 mg/kg) — critical distinction; inadequate treatment of neonatal HSV causes death or severe neurodevelopmental impairment. IV must be given over 1 hour (nephrotoxic if given rapidly — crystalluria in renal tubules). Ensure adequate hydration throughout. Oral aciclovir bioavailability is poor (15–30%) — not suitable for serious infections. (or mg/m²)

Clinical pearls

  • Neonatal herpes dosing: 20 mg/kg (not 10 mg/kg) every 8 hours — higher dose required because of immature thymidine kinase activity and different pharmacokinetics; confusion between adult and neonatal doses is a patient safety issue
  • HSV encephalitis: empirical IV aciclovir must not be delayed pending PCR results — start immediately on clinical suspicion (fever + encephalopathy); CSF HSV PCR can be false-negative in first 24–48 hours
  • Hydration critical: aciclovir crystallises in renal tubules when concentrated — give 150–200 mL/hour IV fluids alongside aciclovir infusion in neonates/children; reduce rate in those at risk of fluid overload
  • VZV in immunocompromised: chickenpox in immunocompromised children (leukaemia, transplant) — life-threatening; start IV aciclovir immediately, do not wait for hospitalisation

Contraindications

  • Hypersensitivity to aciclovir or valaciclovir

Side effects

  • Nephrotoxicity (crystalluria — ensure hydration; give over 1 hour)
  • Neurotoxicity (agitation, confusion, hallucinations — especially in renal failure)
  • Phlebitis (IV)
  • Nausea
  • Elevated LFTs
  • Rash

Interactions

  • Probenecid — increases aciclovir levels
  • Nephrotoxic drugs — additive nephrotoxicity
  • Ciclosporin — additive nephrotoxicity
  • Mycophenolate — competitive tubular secretion (increased levels of both)

Monitoring

  • Renal function and urine output (daily)
  • Hydration status
  • Neurotoxicity (agitation, tremor)
  • CSF HSV PCR (repeat at day 14–21 for CNS herpes — guide duration)
  • Clinical response

Reference: BNF for Children; NICE NG41 (Meningitis); PHE Neonatal Herpes Guidelines; BPSU/BPNG Neonatal Herpes Simplex Guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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