Acyclovir Dosing Recommendations
For herpes zoster (shingles), administer acyclovir 800 mg orally 5 times daily for 7-10 days, initiated within 72 hours of rash onset; for primary varicella (chickenpox), use 800 mg orally 4 times daily for 5 days in adults and 20 mg/kg (maximum 800 mg) 4 times daily for 5 days in children over 2 years, starting within 24 hours of rash onset. 1
Herpes Zoster (Shingles)
Standard dosing: 800 mg orally every 4 hours (5 times daily) for 7-10 days 1. This regimen significantly shortens time to lesion scabbing, healing, and cessation of pain when initiated within 72 hours of rash onset, with maximum benefit if started within 48 hours 2, 1. Adults over 50 years show greater clinical benefit 1.
The 800 mg five-times-daily regimen is superior to lower doses (400 mg five times daily), demonstrating significantly shortened viral shedding, accelerated healing to 50% scabbing, reduced new lesion formation, and decreased acute pain severity 3. Post-zoster pain prevalence is also reduced, particularly in the first three months following treatment 3.
Primary Varicella (Chickenpox)
Adults and Children >40 kg
800 mg orally 4 times daily for 5 days 1. Treatment must begin within 24 hours of rash onset for efficacy 1.
Children 2-12 years (≤40 kg)
20 mg/kg per dose orally 4 times daily (maximum 800 mg per dose) for 5 days 1. This regimen shortens time to 50% healing, reduces maximum lesion count, decreases fever, anorexia, and lethargy 1, 4.
Important caveats for healthy children
The AAP does not recommend routine acyclovir for otherwise healthy children with varicella, as clinical benefit is modest 2. However, oral acyclovir should be considered for persons at increased risk for moderate-to-severe varicella: those aged >12 years, persons with chronic cutaneous or pulmonary disorders, those receiving long-term salicylate therapy, and those on short/intermittent/aerosolized corticosteroids 2.
Immunocompromised patients
Intravenous acyclovir is indicated for varicella-zoster infections in immunocompromised patients 1. Oral acyclovir is not adequate for this population.
Genital Herpes Simplex
Initial episode
200 mg orally every 4 hours, 5 times daily for 10 days 1
Chronic suppressive therapy
400 mg orally twice daily for up to 12 months, followed by re-evaluation 1. Alternative regimens range from 200 mg 3 times daily to 200 mg 5 times daily 1.
Intermittent therapy for recurrences
200 mg orally every 4 hours, 5 times daily for 5 days, initiated at earliest prodrome 1
Special Populations
Pregnancy
Acyclovir is FDA Category B 2. Oral acyclovir prophylaxis is not routinely recommended during pregnancy 5, 6. However, for pregnant women with frequent, severe recurrences of genital HSV, acyclovir prophylaxis may be indicated 5, 6. For serious viral-mediated complications (e.g., pneumonia), intravenous acyclovir should be considered 2. A prospective registry of 596 first-trimester exposures showed birth defect rates approximating the general population 2.
Renal Impairment
Dose adjustment is mandatory based on creatinine clearance 1:
For 800 mg every 4 hours regimen (herpes zoster):
- CrCl >25 mL/min: 800 mg every 4 hours, 5 times daily
- CrCl 10-25 mL/min: 800 mg every 8 hours
- CrCl 0-10 mL/min: 800 mg every 12 hours 1
For 200 mg every 4 hours regimen:
- CrCl >10 mL/min: 200 mg every 4 hours, 5 times daily
- CrCl 0-10 mL/min: 200 mg every 12 hours 1
For 400 mg every 12 hours regimen:
- CrCl >10 mL/min: 400 mg every 12 hours
- CrCl 0-10 mL/min: 200 mg every 12 hours 1
Hemodialysis patients: Administer an additional dose after each dialysis session 1. The mean plasma half-life during hemodialysis is approximately 5 hours, resulting in 60% decrease in plasma concentrations after 6-hour dialysis 1.
Peritoneal dialysis: No supplemental dose needed after dosing interval adjustment 1
Geriatric Patients
Acyclovir plasma concentrations are higher in geriatric patients due to age-related renal function decline 1. Dose reduction is required in geriatric patients with underlying renal impairment using the creatinine clearance-based adjustments above 1.
Neonates and Infants
Pharmacokinetic data show elimination half-life decreases sharply during the first month after birth, from 10-15 hours to 2.5 hours 7. For neonates <1 month: 24 mg/kg/day divided into 3 doses; for infants 1-24 months: 24 mg/kg/day divided into 4 doses 7. Bioavailability is approximately 12% in this population 7.
Critical Safety Considerations
Acute Kidney Injury (AKI)
AKI occurs in 37-52% of patients receiving intravenous acyclovir for CNS infections 8. Supra-therapeutic dosing is significantly associated with AKI (adjusted OR 12.9) 9. Maintain adequate hydration and urine flow, particularly at high doses 10. Monitor renal function and adjust doses accordingly.
Timing of Initiation
Treatment effectiveness is time-dependent: For herpes zoster, initiate within 72 hours (ideally 48 hours) of rash onset 2, 1. For varicella, initiate within 24 hours of rash onset 1. Delayed treatment significantly reduces clinical benefit.
Not for Prophylaxis
Acyclovir is not indicated for prophylactic use among otherwise healthy children, adolescents, or adults after varicella exposure 2. Vaccination is the preferred prevention method. For immunocompromised persons, VZIG (not acyclovir) is recommended for post-exposure prophylaxis 2.