Valacyclovir Dosage for Chickenpox
For immunocompetent pediatric patients aged 2 to less than 18 years with chickenpox, valacyclovir should be dosed at 20 mg/kg orally three times daily for 5 days, not to exceed 1 gram three times daily, initiated at the earliest sign of rash. 1
Pediatric Dosing (Ages 2-17 Years)
- Standard dose: 20 mg/kg administered orally three times daily for 5 days 1
- Maximum dose: Do not exceed 1 gram three times daily regardless of weight 1
- Timing: Therapy should be initiated at the earliest sign or symptom of chickenpox 1
- Formulation: An oral suspension (25 mg/mL or 50 mg/mL) can be prepared extemporaneously from 500-mg tablets for children who cannot swallow solid dosage forms 1
Important Limitations
- Age restriction: Valacyclovir is not recommended for children under 2 years of age with chickenpox, as efficacy and safety have not been established in this age group 1
- Herpes zoster: Valacyclovir is also not recommended for treatment of herpes zoster (shingles) in children, as safety data beyond 7 days are not available 1
Evidence Basis
The FDA approval for valacyclovir in pediatric chickenpox is based on pharmacokinetic bridging studies rather than direct efficacy trials 1. A single-dose pharmacokinetic and multiple-dose safety trial in 27 pediatric subjects aged 1 to less than 12 years demonstrated that acyclovir exposures from valacyclovir 20 mg/kg three times daily were approximately 100% greater than those achieved with acyclovir 20 mg/kg four times daily 1. The efficacy is extrapolated from three randomized, double-blind, placebo-controlled trials of oral acyclovir in pediatric chickenpox 1.
Alternative: Acyclovir Dosing
If valacyclovir is unavailable or not appropriate, acyclovir remains an effective alternative:
- Immunocompetent children: Acyclovir 20 mg/kg orally four times daily (maximum 800 mg/dose) for 5 days 2
- HIV-infected children with mild-moderate immunosuppression: Acyclovir 20 mg/kg orally four times daily (maximum 800 mg/dose) for 7-10 days or until no new lesions for 48 hours 3
- Severely immunosuppressed children: IV acyclovir 10 mg/kg (or 500 mg/m² body surface area) every 8 hours for 7-10 days 3
Timing Considerations
Critical pitfall: The effectiveness of antiviral therapy decreases significantly with delayed initiation 4, 2. Treatment begun within the first 24 hours of rash onset provides the greatest clinical benefit, including:
- Fewer total lesions (294 vs 347 with placebo) 2
- Faster cessation of new lesion formation 4, 2
- Reduced duration of fever and constitutional symptoms (3-4 days vs >4 days in 20% of placebo recipients) 2
- Less itching and fewer residual lesions 2
Treatment initiated on the second day of rash (24-48 hours after onset) still provides some benefit but is less effective than first-day treatment 4. Starting therapy beyond 48 hours offers minimal benefit in immunocompetent patients 4.
Duration of Therapy
Five days of treatment is sufficient for immunocompetent patients, as a 7-day course provides no additional benefit 4. However, immunocompromised children may require 7-10 days of therapy or treatment until no new lesions appear for 48 hours 3.
Renal Impairment Adjustments
Important caveat: Data are not available for valacyclovir use in pediatric patients with creatinine clearance less than 50 mL/min/1.73 m² 1. In such cases, consider using IV acyclovir with appropriate dose adjustments or consulting a pediatric nephrologist.
Immunocompromised Patients
For immunocompromised children with chickenpox, a sequential approach may be appropriate 5:
- Start with IV acyclovir 1500 mg/m²/day (or 10 mg/kg every 8 hours) in three divided doses 3
- Switch to oral therapy after minimum 48 hours of IV treatment if the patient is afebrile, has no new lesions for 24 hours, has no internal organ involvement, and can tolerate oral medications 5
- Continue oral therapy to complete 7-10 days total treatment 5
This approach reduces hospitalization duration while maintaining safety and efficacy 5.