What is the recommended dosage of Acyclovir (antiviral medication) for a patient with chickenpox, considering factors such as age, weight, and immune status?

Acyclovir Dosing for Chickenpox

For otherwise healthy children with chickenpox, administer oral acyclovir 20 mg/kg (maximum 800 mg per dose) four times daily for 5 days, initiated within 24 hours of rash onset; for adolescents and adults (≥12 years), give 800 mg orally four times daily for 5-7 days. 1, 2

Standard Dosing by Age and Weight

Children (2 years and older)

• 20 mg/kg per dose orally 4 times daily for 5 days (maximum 800 mg per dose) 1, 2
• Children over 40 kg should receive the adult dose 2
• Total daily dose: 80 mg/kg/day in four divided doses 3

Adolescents and Adults (≥12 years)

• 800 mg orally four times daily for 5-7 days 1, 2
• Alternative: 800 mg five times daily 1

Critical Timing Considerations

Treatment must be initiated within 24 hours of rash onset for optimal benefit. 1, 2, 4

• Efficacy diminishes significantly when therapy starts after the first day of rash 1, 3
• Patients presenting 24-48 hours after rash onset still derive some benefit, though less pronounced than those treated within 24 hours 3
• There is no information about efficacy when therapy is initiated more than 24 hours after symptom onset 2
• Five days of therapy is sufficient; a 7-day course provides no additional benefit in immunocompetent patients 3

Special Population Dosing

Immunocompromised Patients

• Intravenous acyclovir is indicated for immunocompromised patients with chickenpox 2
• HIV-infected children with moderate to severe disease: 10 mg/kg IV every 8 hours for 7-10 days or longer depending on clinical response 1
• Immunocompromised patients may have prolonged episodes requiring more aggressive and extended therapy 1
• Children with CNS involvement: 10 mg/kg IV three times daily for 21 days 1

High-Risk Immunocompetent Patients

The CDC and American Academy of Pediatrics recommend considering acyclovir for: 1

• Persons aged >12 years
• Those with chronic cutaneous or pulmonary disorders
• Patients on long-term salicylate therapy
• Those receiving short, intermittent, or aerosolized corticosteroids

Pregnant Women

• Acyclovir is FDA Category B 1
• Routine use is not recommended for uncomplicated varicella in pregnancy 1
• Intravenous acyclovir should be considered for serious complications such as pneumonia 1

Renal Dosing Adjustments

Dose modification is required for patients with renal impairment: 2

For 800 mg every 4 hours regimen:

• Creatinine clearance >25 mL/min: 800 mg every 4 hours (5 times daily)
• Creatinine clearance 10-25 mL/min: 800 mg every 8 hours
• Creatinine clearance 0-10 mL/min: 800 mg every 12 hours

Hemodialysis patients:

• Adjust dosing schedule to administer an additional dose after each dialysis session 2
• Mean plasma half-life during hemodialysis is approximately 5 hours, resulting in 60% decrease in plasma concentrations after 6-hour dialysis 2

Peritoneal dialysis:

• No supplemental dose appears necessary after adjusting the dosing interval 2

Expected Clinical Outcomes

When initiated within 24 hours of rash onset, acyclovir treatment results in: 4

• Fewer total varicella lesions (mean 294 vs 347 with placebo)
• Reduced proportion with >500 lesions (21% vs 38% with placebo)
• No new lesion formation after day 3 in >95% of patients
• Accelerated progression to crusted and healed stages
• Duration of fever and constitutional symptoms limited to 3-4 days

Important Caveats and Limitations

• Acyclovir does not reduce transmission of varicella or decrease school absence duration 1
• Acyclovir does not eradicate latent virus or affect subsequent risk of herpes zoster later in life 1
• No serious adverse events occurred during drug administration in pediatric trials 1
• Adequate hydration and urine flow must be maintained, particularly at higher doses 5
• Viruses shed during therapy retain susceptibility to acyclovir and normal thymidine kinase function 3

Common Pitfalls to Avoid

• Do not delay treatment beyond 24 hours of rash onset - efficacy drops substantially 1, 3, 4
• Do not extend therapy beyond 5 days in immunocompetent patients - no additional benefit demonstrated 3
• Do not use oral acyclovir as sole therapy in immunocompromised patients - IV formulation is indicated 2
• Do not forget to adjust dosing in renal impairment - accumulation can occur 2
• Do not assume treatment will prevent future zoster - latent virus remains unaffected 1