Acyclovir 400mg Dosing for Pediatric Chickenpox
For a child with chickenpox when only 400mg tablets are available, administer the 400mg tablet based on weight-based dosing of 20 mg/kg per dose, four times daily for 5-7 days, which means most children will receive 1-2 tablets per dose depending on their weight.
Weight-Based Dosing Algorithm
The CDC-recommended dose is 20 mg/kg per dose (maximum 800 mg/dose) four times daily, initiated within 24 hours of rash onset for maximum benefit 1.
Practical Dosing with 400mg Tablets:
Children weighing 10-20 kg: Give one 400mg tablet four times daily (this provides 20-40 mg/kg/dose, which is appropriate) 1
Children weighing 20-40 kg: Give one 400mg tablet four times daily (provides 10-20 mg/kg/dose, at the lower end but acceptable) 1
Children weighing >40 kg or adolescents: Give two 400mg tablets (800mg) four times daily, as this reaches the maximum dose 1, 2
Treatment Duration and Timing
Continue treatment for 5-7 days total 1. The research demonstrates that 5 days of therapy is equivalent to 7 days when started early 3.
Treatment must be initiated within 24 hours of rash onset for maximum efficacy—this timing produces the greatest reduction in lesion count, fever duration, and time to healing 1, 3.
Treatment started on day 2 of rash still provides some benefit, though less pronounced than day 1 initiation 3.
Expected Clinical Response
New lesion formation should cease by day 3 of treatment in >95% of patients 4.
Fever and constitutional symptoms should resolve within 3-4 days 4.
Clinical improvement should be evident within 48-72 hours; if not, reassess for complications 5.
Critical Safety Considerations
Ensure adequate hydration throughout treatment to prevent acyclovir crystalluria 2.
The 400mg tablet formulation may be challenging for younger children to swallow; if the child cannot swallow tablets, liquid formulation should be obtained instead 1.
Monitor for bacterial superinfection of skin lesions, which occurs in approximately 10-13% of cases regardless of acyclovir use 4.
Common Pitfall to Avoid
Do not underdose based solely on tablet strength—if a child weighing >40 kg receives only one 400mg tablet per dose, they are receiving subtherapeutic dosing (10 mg/kg instead of the recommended 20 mg/kg) 1. In this scenario, two tablets per dose are required to reach the maximum 800mg dose 1, 2.