What is the recommended dosing of acyclovir (antiviral medication) for a 13-year-old patient weighing 46 kg with herpes zoster (shingles)?

Acyclovir Dosing for Shingles in a 13-Year-Old, 46 kg Patient

For a 13-year-old weighing 46 kg with shingles, give acyclovir 800 mg orally five times daily for 7-10 days. 1, 2

Oral Dosing Regimen

• The standard dose is 800 mg orally five times daily (every 4 hours while awake) for 7-10 days 1, 2
• This patient weighs 46 kg, which exceeds the 40 kg threshold where adult dosing applies 2
• The FDA-approved adult dose for herpes zoster is 800 mg every 4 hours, 5 times daily for 7-10 days 2
• Treatment should be initiated within 72 hours of rash onset for maximum efficacy 1

Alternative Weight-Based Calculation

• If using pediatric weight-based dosing: 20 mg/kg per dose (maximum 800 mg) orally 4 times daily 3, 1
• For this 46 kg patient: 20 mg/kg × 46 kg = 920 mg per dose, but the maximum is capped at 800 mg per dose 3, 1
• However, the five-times-daily adult regimen (800 mg × 5 = 4,000 mg/day) provides higher total daily exposure than the four-times-daily pediatric regimen (800 mg × 4 = 3,200 mg/day) and is preferred for herpes zoster 2, 4

When to Use Intravenous Therapy

• Switch to IV acyclovir 10 mg/kg every 8 hours if the patient has severe disease, disseminated infection, or complications requiring hospitalization 3
• Some experts use body surface area dosing: 500 mg/m² IV every 8 hours for children >1 year 3
• Continue IV therapy for 5-7 days or until clinical resolution, then may transition to oral therapy 3

Critical Monitoring and Safety

• Ensure adequate hydration throughout treatment to prevent acyclovir crystalluria 1, 5
• Monitor for clinical improvement within 48-72 hours of starting therapy 1, 5
• Continue treatment for the full 7-10 days even if symptoms improve earlier 1
• No dose adjustment needed with normal renal function 2

Renal Impairment Dosing

• If creatinine clearance 10-25 mL/min/1.73 m²: reduce to 800 mg every 8 hours 2
• If creatinine clearance 0-10 mL/min/1.73 m²: reduce to 800 mg every 12 hours 2

Common Pitfalls to Avoid

• Do not use the lower 400 mg five-times-daily dose for herpes zoster—studies show it is inferior to 800 mg five times daily 4
• Do not delay treatment beyond 72 hours of rash onset, as efficacy decreases significantly 1
• Do not use topical acyclovir, which is substantially less effective than oral therapy 1
• Acyclovir does not eradicate latent virus or prevent future recurrences 1

Alternative Agents for Adolescents

• Famciclovir 500 mg orally three times daily for 7-10 days is an alternative for adolescents who can receive adult dosing 3, 1
• Valacyclovir offers simpler dosing with equivalent efficacy but pediatric data are limited 6