What is the recommended dosage of acyclovir (antiviral medication) for treating chicken pox?

Acyclovir Dosing for Chickenpox (Varicella)

For immunocompetent children aged 2-12 years with chickenpox, administer oral acyclovir 20 mg/kg (maximum 800 mg) four times daily for 5 days, initiated within 24 hours of rash onset for optimal benefit. 1, 2

Dosing by Patient Population

Immunocompetent Children (2-12 years)

• 20 mg/kg per dose orally four times daily for 5 days (maximum 400 mg/dose) 1
• Alternative dosing: 80 mg/kg/day divided into four doses (maximum 3,200 mg/day) 3
• Treatment initiated within 24 hours of rash onset provides maximum benefit; 5 days of therapy is equivalent to 7 days 3, 4

Adolescents and Adults (≥12 years)

• 800 mg orally five times daily for 5-7 days 1, 2, 5
• Adolescents and adults are at higher risk for moderate to severe varicella and should be prioritized for treatment 1

Immunocompromised Patients (All Ages)

• Intravenous acyclovir 10 mg/kg every 8 hours (or 500 mg/m² every 8 hours) for 7-10 days 1, 5
• Treatment should be initiated regardless of timing from rash onset due to higher risk of disseminated infection and mortality 1, 6
• Continue treatment until all lesions have scabbed 1

Kidney Transplant Recipients

• Either intravenous OR oral acyclovir can be used for primary VZV infection 1
• Continue treatment at least until all lesions have scabbed 1

Critical Timing Considerations

Optimal Treatment Window

• Initiate within 24 hours of rash onset for maximum efficacy in reducing lesion count, fever duration, and constitutional symptoms 1, 3, 4
• Treatment initiated between 24-48 hours still provides benefit, though less pronounced than treatment started within 24 hours 3
• After 48 hours, benefit diminishes significantly in immunocompetent patients 3

Immunocompromised Exception

• Treat regardless of timing in immunocompromised patients, as they remain at high risk for complications even with delayed treatment 1, 6

Special Populations

Pregnancy

• Acyclovir is FDA Category B with no increased birth defects documented in 596 first-trimester exposures 1, 7
• Consider acyclovir for pregnant women at increased risk for moderate to severe varicella 1, 7
• Varicella-zoster immune globulin (VZIG) within 96 hours of exposure is strongly preferred for post-exposure prophylaxis in pregnant women 6, 7
• If VZIG unavailable or >96 hours post-exposure, use acyclovir 20 mg/kg (maximum 800 mg) four times daily for 5-7 days, initiated 7-10 days after exposure 1, 6

Patients on Chronic Salicylate Therapy

• Should be considered for acyclovir treatment due to increased risk of Reye syndrome 1

Patients with Chronic Cutaneous or Pulmonary Disorders

• Should be considered for acyclovir treatment due to increased risk for moderate to severe varicella 1

Clinical Outcomes with Treatment

Expected Benefits (When Started Within 24 Hours)

• Reduction in lesion count: Mean 294 lesions vs 347 in placebo (P<0.001) 4
• Fewer patients with >500 lesions: 21% vs 38% with placebo (P<0.001) 4
• No new lesions after day 3 in >95% of treated patients vs continued formation through day 6+ in placebo 4
• Fever and constitutional symptoms limited to 3-4 days vs >4 days in 20% of placebo recipients 4
• Accelerated progression to crusted and healed stages with less itching 4

Important Limitations

• Acyclovir does not reduce transmission of varicella 1, 7
• Does not reduce duration of school absence 1
• Does not prevent complications in most studies, though complication rates were numerically lower 4, 8
• Antibody titers remain protective after treatment 1, 3, 8

Common Pitfalls to Avoid

Timing Errors

• Do not confuse the 24-hour window for chickenpox with the 72-hour window for herpes zoster 6, 9
• Missing the 24-hour window significantly reduces treatment benefit in immunocompetent patients 3, 4

Dosing Errors

• Do not underdose: The effective dose is 20 mg/kg four times daily, not the lower doses used for herpes simplex 1, 2
• Maximum single dose is 800 mg, not 400 mg 1, 2

Route Selection Errors

• Use intravenous acyclovir for all immunocompromised patients, not oral 1, 5
• Oral acyclovir is appropriate only for immunocompetent patients 1, 2

Monitoring and Safety

Adverse Events

• Most common: nausea/vomiting (2.7%), diarrhea (3.2%), malaise 2
• Serious but rare: renal failure, neurologic symptoms (particularly in elderly or renally impaired), encephalopathy 2
• Maintain adequate hydration and monitor renal function, especially with high-dose therapy 5

Viral Resistance

• Viruses shed during 5-7 day treatment courses remain susceptible to acyclovir with normal thymidine kinase function 3
• Resistance is not a concern with standard treatment duration 3