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Antiviral (purine nucleoside analogue) Pregnancy: No evidence of teratogenicity. IV aciclovir recommended for serious HSV in pregnancy.

Aciclovir

Brand names: Zovirax

Adult dose

Dose: HSV encephalitis: 10 mg/kg every 8 hours IV; Genital herpes: 200 mg 5 times daily oral; VZV: 800 mg 5 times daily oral
Route: IV (serious infections) or oral
Frequency: Every 8 hours (IV) or 5 times daily (oral)
Max: 30 mg/kg/day IV
HSV encephalitis: 10 mg/kg every 8 hours for 14–21 days IV. Neonatal HSV: 20 mg/kg every 8 hours IV for 14–21 days. Severe VZV (immunocompromised): 10 mg/kg every 8 hours IV. Oral herpes simplex: 200 mg 5×/day or 400 mg TDS for 5 days.

Paediatric dose

Dose: 20 mg/kg
Route: IV
Frequency: Every 8 hours
Max: 500 mg/m²/dose
Concentration: 25 mg/ml
Neonatal HSV (≤3 months): 20 mg/kg every 8 hours IV for 14–21 days. HSV encephalitis (3 months–12 years): 500 mg/m² every 8 hours. VZV (immunocompromised): 500 mg/m² every 8 hours. Oral (chickenpox ≥2 years): 20 mg/kg (max 800 mg) 4 times daily for 5 days.

Dose adjustments

Renal

eGFR 25–50: increase interval to every 12h; eGFR 10–25: every 24h; eGFR <10: reduce dose and increase interval — specialist guidance.

Hepatic

No dose adjustment required.

Paediatric weight-based calculator

Neonatal HSV (≤3 months): 20 mg/kg every 8 hours IV for 14–21 days. HSV encephalitis (3 months–12 years): 500 mg/m² every 8 hours. VZV (immunocompromised): 500 mg/m² every 8 hours. Oral (chickenpox ≥2 years): 20 mg/kg (max 800 mg) 4 times daily for 5 days.

Clinical pearls

  • Adequate hydration essential during IV administration (crystal nephropathy prevention)
  • Infuse over 1 hour minimum (phlebitis and nephrotoxicity prevention)
  • For HSV encephalitis — do not wait for CSF PCR — start empirically
  • Resistance rare in immunocompetent patients; in HIV — use foscarnet
  • Valaciclovir (prodrug) better oral bioavailability — preferred for genital herpes and shingles (BD/TDS dosing)

Contraindications

  • Hypersensitivity to aciclovir or valaciclovir

Side effects

  • Nausea and vomiting
  • Neurological side effects (confusion, agitation, seizures) — especially in renal impairment
  • Nephrotoxicity (IV — crystal nephropathy if dehydrated)
  • Phlebitis at IV site
  • Rash
  • Elevated LFTs

Interactions

  • Probenecid — increases aciclovir levels
  • Mycophenolate — increased levels of both drugs
  • Ciclosporin, tacrolimus — increased nephrotoxicity

Monitoring

  • Renal function (U&E, creatinine)
  • Neurological status (IV high-dose)
  • Fluid balance (maintain hydration during IV)

Reference: BNFc; BNF; BNFc; BASHH guidelines; NICE CG105. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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