What is the recommended dosing of acyclovir for pediatric and adult patients with chickenpox, considering factors such as renal impairment and immunocompromised status?

Acyclovir Dosing for Chickenpox

For immunocompetent children with chickenpox, administer oral acyclovir 20 mg/kg per dose (maximum 800 mg/dose) four times daily for 5-7 days, initiated within 24 hours of rash onset for maximum benefit. 1, 2

Pediatric Dosing (Immunocompetent)

Oral acyclovir remains the preferred first-line agent for chickenpox treatment in children:

• Standard dose: 20 mg/kg per dose orally, four times daily (maximum 800 mg/dose) 3, 1
• Duration: 5 days is sufficient; 7-day courses provide no additional benefit 4
• Maximum daily dose: 80 mg/kg/day or 3,200 mg/day, whichever is lower 1
• Children >40 kg: Use adult dosing of 800 mg four times daily 1

Timing is critical for efficacy: Treatment initiated within 24 hours of rash onset produces the most significant clinical benefit, including fewer total lesions (294 vs 347 with placebo), accelerated healing, reduced fever duration, and decreased constitutional symptoms 2. Starting therapy between 24-48 hours after rash onset still provides some benefit, though less pronounced, particularly for reducing time to maximum lesion formation and achieving 50% healing 4.

Immunocompromised Pediatric Patients

For children with severe immunosuppression (HIV CDC category 3 or equivalent), intravenous acyclovir is required:

• IV dose: 10 mg/kg every 8 hours (or 500 mg/m² every 8 hours as alternative) 3, 5
• Duration: 7-10 days or until no new lesions for 48 hours 3
• Infusion: Administer over 1-2 hours with adequate hydration to prevent crystalluria 5

For children with mild to moderate immunosuppression (HIV CDC categories 1-2):

• Oral acyclovir 20 mg/kg four times daily (maximum 800 mg/dose) for 7-10 days may be used 3

Acyclovir-resistant cases: Switch to foscarnet 40-60 mg/kg IV every 8 hours for 7-10 days 3

Adult Dosing (Immunocompetent)

Standard adult dose: 800 mg orally four times daily for 5 days 1

• Treatment should begin within 24 hours of rash onset 1
• Intravenous acyclovir is indicated for immunocompromised adults 1, 6

Renal Impairment Adjustments

Dose modifications are mandatory for patients with reduced renal function 1:

For 800 mg every 4 hours regimen:

• CrCl >25 mL/min/1.73m²: 800 mg every 4 hours (5 times daily) - no adjustment
• 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 an additional dose after each dialysis session, as hemodialysis removes approximately 60% of plasma acyclovir over 6 hours 1

Augmented renal clearance (eGFR >250 mL/min/1.73m²): Consider increasing IV dose to 15-20 mg/kg every 6 hours to maintain therapeutic concentrations 7

Alternative Agent: Valacyclovir

Valacyclovir offers less frequent dosing but has limited pediatric data:

• Older children/adolescents able to swallow tablets: May use adult dosing, though specific pediatric formulations are lacking 3, 8, 9
• For 20 kg child: 400 mg three times daily (total 1,200 mg/day) based on weight-based calculations 8
• The CDC emphasizes that acyclovir remains preferred for pediatric chickenpox due to established efficacy and available liquid formulations 3

Critical Clinical Considerations

Treatment efficacy decreases significantly after 24 hours: While some benefit persists when initiated between 24-48 hours, the reduction in lesion count, fever duration, and symptom severity is most pronounced with early treatment 4, 2. After 48 hours, efficacy data are limited 1.

Five days of therapy is adequate: A controlled trial demonstrated that 7-day courses provide no additional clinical benefit compared to 5-day regimens when treatment begins within 24 hours of rash onset 4.

Viral resistance concerns are unfounded: Viruses shed during standard 5-day acyclovir treatment retain normal thymidine kinase function and acyclovir susceptibility 4. Resistance primarily emerges in immunocompromised patients receiving prolonged therapy.

Hydration is essential: Maintain adequate fluid intake and urine output throughout treatment to prevent acyclovir crystalluria, particularly with IV administration or high-dose oral therapy 5, 6.

Bioavailability considerations: Oral acyclovir bioavailability is only 10-20% and decreases with increasing doses, which is why the 800 mg dose is used despite lower absorption 1. Food does not affect absorption 1.