Acyclovir Dosing for Pediatric Varicella (Chickenpox)
For pediatric patients with chickenpox, administer acyclovir 20 mg/kg orally per dose (maximum 800 mg/dose) 4 times daily for 5-7 days, starting within 24 hours of rash onset for maximum benefit. 1, 2
Standard Oral Dosing Regimen
- Weight-based dosing: 20 mg/kg orally 4 times daily (every 4-6 hours while awake) 1, 3, 2
- Maximum single dose: 800 mg per dose 1, 3, 2
- Duration: 5-7 days, or continue until no new lesions appear for 48 hours 1, 3
- Age requirement: FDA-approved for children ≥2 years of age 2
- Alternative for children >40 kg: May use adult dosing of 800 mg 4 times daily 2
Timing and Efficacy Considerations
- Initiate therapy within 24 hours of rash onset for optimal efficacy—treatment started after 24 hours has uncertain benefit 2, 4
- In controlled trials, acyclovir reduced total lesion count (294 vs 347 with placebo), accelerated healing, and limited fever duration to 3-4 days in >80% of treated children 4
- Over 95% of acyclovir-treated children stopped forming new lesions by day 3, compared to 20% of placebo recipients still forming lesions on day 6 or later 4
Immunocompromised Patients
For children with moderate to severe immunosuppression (HIV CDC category 3 or equivalent):
- Intravenous acyclovir is preferred: 10 mg/kg IV every 8 hours 1, 2
- Alternative body surface area dosing: 500 mg/m² IV every 8 hours for children >1 year 1
- Duration: 7-10 days IV therapy 1
- Oral acyclovir 800 mg 5 times daily for 7 days may be considered for mild immunocompromise with close physician monitoring, though 2 of 25 patients in one study required conversion to IV therapy 5
Renal Impairment Adjustments
For patients with renal dysfunction, adjust the 800 mg every 4 hours regimen as follows 2:
- CrCl >25 mL/min/1.73m²: 800 mg every 4 hours (standard dosing)
- CrCl 10-25 mL/min/1.73m²: 800 mg every 8 hours
- CrCl 0-10 mL/min/1.73m²: 800 mg every 12 hours
- Hemodialysis patients: Administer additional dose after each dialysis session 2
Special Populations and Pharmacokinetic Considerations
- Neonates and infants <3 months: Acyclovir elimination half-life is markedly prolonged (10-15 hours vs 2.5 hours in older infants), requiring dose adjustment and close monitoring 6, 7
- Bioavailability: Oral acyclovir has only 12% bioavailability in young children, necessitating higher mg/kg dosing compared to IV route 6
- Children with augmented renal clearance (eGFR >250 mL/min/1.73m²): May require higher or more frequent dosing to maintain therapeutic concentrations 8
Critical Safety and Monitoring Points
- Ensure adequate hydration throughout treatment to prevent acyclovir crystalluria, particularly with IV administration 3
- Acyclovir does not eradicate latent varicella-zoster virus or prevent future herpes zoster (shingles) 3
- Monitor for clinical improvement within 48-72 hours; lack of response should prompt evaluation for complications or resistance 3
- Topical acyclovir is ineffective for systemic varicella infection and should not be used 3
Common Clinical Pitfalls
- Do not delay treatment waiting for laboratory confirmation—initiate based on clinical diagnosis within the first 24 hours 2, 4
- The FDA label states efficacy is uncertain when treatment begins >24 hours after rash onset, though guidelines support treatment up to 72 hours in high-risk patients 3, 2
- For immunocompromised children, oral therapy may be inadequate—maintain low threshold for IV conversion if new lesions continue after 3-4 days of oral treatment 5
- Mild self-limiting diarrhea occurs in approximately 36% of patients (9 of 25 in one study) but rarely requires discontinuation 5