Aciclovir Dosing for Chickenpox in a 13-Year-Old, 47 kg Patient
For this 13-year-old patient weighing 47 kg with chickenpox, administer aciclovir 800 mg orally four times daily for 5 days, initiated as early as possible (ideally within 24 hours of rash onset). 1
Dosing Rationale
- The FDA-approved dose for chickenpox treatment in children over 40 kg and adults is 800 mg orally four times daily for 5 days 1
- Since this patient weighs 47 kg (exceeding the 40 kg threshold), they should receive the adult dosing regimen rather than weight-based pediatric dosing 1
- The weight-based pediatric dose of 20 mg/kg four times daily would calculate to 940 mg per dose for this patient, but the standardized adult dose of 800 mg four times daily (3,200 mg/day total) is the FDA-approved regimen for patients in this weight category 1
Timing of Initiation
- Therapy should be initiated at the earliest sign or symptom of chickenpox for maximum benefit 1
- Treatment begun within the first 24 hours of rash onset provides the greatest clinical benefit, including fewer total lesions, accelerated healing, reduced fever duration, and decreased constitutional symptoms 2, 3
- Initiation between 24-48 hours after rash onset still provides some benefit, though less pronounced than earlier treatment 2
- There is no information about efficacy when therapy is initiated more than 24 hours after onset of signs and symptoms 1
Duration of Treatment
- Five days of therapy is sufficient and recommended 1, 2
- Clinical trials demonstrate that 5 days of aciclovir treatment is equivalent to 7 days, with no additional benefit from extended therapy 2
- More than 95% of patients receiving aciclovir have no new lesion formation after day 3 of treatment 3
Special Considerations for Adolescents
- Adolescents are at higher risk for severe chickenpox and complications compared to younger children, including higher rates of pneumonia and disseminated disease 4
- This makes antiviral treatment particularly important in this age group, even in otherwise healthy individuals 5
Renal Function Assessment
- Before prescribing, assess renal function, as dose adjustment is required for renal impairment 1
- For normal renal function (creatinine clearance >25 mL/min/1.73 m²), the standard 800 mg every 4 hours (5 times daily) dose applies 1
- If creatinine clearance is 10-25 mL/min/1.73 m², reduce to 800 mg every 8 hours 1
- If creatinine clearance is 0-10 mL/min/1.73 m², reduce to 800 mg every 12 hours 1
Administration Instructions
- Administer 800 mg orally every 4 hours while awake (5 times daily), continuing for 5 consecutive days 1
- Ensure adequate hydration during therapy to prevent crystalluria 6
- Monitor for central nervous system effects, particularly in patients with any degree of renal impairment 6
Important Caveats
- Avoid salicylates (aspirin) for 6 weeks after chickenpox due to Reye syndrome risk 7
- Aciclovir treatment does not eradicate viral latency, so the patient remains at risk for herpes zoster reactivation later in life 8
- Viruses shed during aciclovir therapy retain normal susceptibility to the drug, with no evidence of resistance development during standard treatment courses 2
- Intravenous aciclovir would be indicated instead of oral therapy if the patient were immunocompromised, had severe disease, or developed complications 1, 5