Acyclovir 400mg Dosing for Chickenpox
For chickenpox, 400mg is an underdose—the correct dosing is weight-based at 20 mg/kg per dose (maximum 800 mg/dose) orally four times daily for 5-7 days, which means a 400mg dose would only be appropriate for a child weighing approximately 20kg. 1, 2
Weight-Based Dosing Algorithm
The standard approach is to calculate the dose based on the child's weight:
- Calculate 20 mg/kg per dose 1, 2
- Maximum single dose is 800 mg 3, 1, 2
- Administer four times daily 1, 2
- Continue for 5-7 days 1, 2
For example:
- A 20kg child receives 400mg per dose (20kg × 20mg/kg = 400mg) 4
- A 30kg child receives 600mg per dose (30kg × 20mg/kg = 600mg) 1
- A child >40kg receives the adult dose of 800mg per dose 2
Timing of Initiation
Treatment must be initiated within 24 hours of rash onset for maximum benefit 1, 5:
- Starting therapy on day 1 of rash provides the greatest clinical benefit, with significantly shortened time to maximum lesion formation, faster healing, and reduced viral shedding 5
- Therapy initiated on day 2 (24-48 hours after rash) still provides some benefit but is less effective than day 1 initiation 5
- There is no information about efficacy when initiated more than 24 hours after onset 2
Duration: 5 vs 7 Days
Five days of therapy is sufficient—a 7-day course provides no additional benefit in immunocompetent children 5:
- The FDA label states 5 days for standard treatment 2
- CDC guidelines recommend 5-7 days for immunocompetent children 1
- For children with mild to moderate immunosuppression, extend to 7-10 days 3, 1
Special Populations Requiring Different Dosing
Immunocompromised children require IV acyclovir instead:
- Use IV acyclovir 10 mg/kg every 8 hours (or 500 mg/m² every 8 hours) for severe immunosuppression 1
- Continue for 7-10 days or until no new lesions for 48 hours 1
Critical Safety Considerations
Ensure adequate hydration throughout treatment to prevent acyclovir crystalluria 3:
- Monitor for clinical improvement within 48-72 hours of initiating therapy 3
- Watch for neutropenia with prolonged use, though uncommon with standard 5-day courses 3
Common Pitfall to Avoid
The most critical error is using a fixed 400mg dose without weight-based calculation—this results in underdosing for most children and fails to achieve therapeutic drug levels needed to reduce lesion formation and symptom duration 1, 2. Always calculate the dose as 20 mg/kg per dose, up to the 800mg maximum 1, 2.