Acyclovir Dosing
The appropriate dose of acyclovir depends critically on the indication, route of administration, and patient factors including renal function and immune status.
Intravenous Acyclovir Dosing
CNS Infections (Encephalitis)
For HSV encephalitis in adults, use acyclovir 10 mg/kg IV every 8 hours for 14-21 days 1. For VZV encephalitis, the same dose of 10 mg/kg IV three times daily is recommended, though some experts suggest 15 mg/kg three times daily if renal function is normal, given VZV's lower sensitivity to acyclovir 1. However, most clinicians use the 10 mg/kg dose and monitor renal function frequently 1.
Pediatric CNS Disease
- Neonatal HSV (CNS or disseminated): 20 mg/kg IV every 8 hours for 21 days 2. High-dose acyclovir (60 mg/kg/day divided into three doses) has demonstrated improved survival in neonatal disseminated disease compared to standard dosing 3.
- Children beyond neonatal period with CNS disease: 10 mg/kg IV every 8 hours for 21 days 2
- Pediatric dosing considerations: Recent pharmacokinetic data suggests 10 mg/kg every 6 hours for children with normal renal function, and 15-20 mg/kg every 6 hours for those with augmented renal clearance (eGFR >250 ml/min/1.73 m²) 4
Severe Mucocutaneous Disease
For severe genital herpes or disseminated infection requiring hospitalization, use acyclovir 5-10 mg/kg IV every 8 hours for 5-7 days 5, 6.
Oral Acyclovir Dosing
First Episode Genital Herpes
Acyclovir 400 mg orally three times daily for 7-10 days is the preferred regimen 6. Alternative regimens include:
- 200 mg orally five times daily for 7-10 days 5, 6
- For herpes proctitis: 400 mg orally five times daily for 10 days 5
Recurrent Genital Herpes
Multiple equivalent options exist 5:
- 200 mg orally five times daily for 5 days
- 400 mg orally three times daily for 5 days
- 800 mg orally twice daily for 5 days
Suppressive Therapy
For patients with frequent recurrences (≥6 episodes/year), use acyclovir 400 mg orally twice daily 5. An alternative is 200 mg orally 3-5 times daily, titrated to the lowest effective dose 5.
Herpes Zoster (Shingles)
Acyclovir 800 mg orally five times daily for 7-10 days, initiated within 72 hours of rash onset 7. Treatment is most effective when started within 48 hours, particularly in adults over 50 years 7.
Chickenpox (Varicella)
For children: 20 mg/kg (maximum 800 mg) orally four times daily for 5 days, started within 24 hours of rash onset 7. Treatment is most beneficial in adolescents and adults who tend to have more severe disease 7.
Prophylaxis in Transplant Recipients
Low-dose acyclovir 400 mg orally twice daily is effective for HSV/VZV prophylaxis in both autologous and allogeneic stem cell transplant recipients 8, 9. Continue for at least one year post-transplant, with extended use while on immunosuppressive therapy 8.
Critical Dosing Considerations
Renal Impairment
Dose adjustment is mandatory in renal dysfunction 7. The drug is renally cleared, and failure to adjust increases risk of neurotoxicity and nephrotoxicity. Maintain adequate hydration throughout therapy 7.
Immunocompromised Patients
HIV-infected and other immunocompromised patients may require:
- Higher doses (e.g., 400 mg orally 3-5 times daily for mucocutaneous disease) 6
- Longer treatment duration 1
- Consideration of acyclovir resistance if lesions persist despite therapy 6
Pregnancy
Acyclovir 400 mg three times daily starting at 36 weeks gestation reduces viral shedding, recurrences, and cesarean deliveries in women with history of genital herpes 10. Acyclovir remains pregnancy category B 10.
Safety Monitoring
- Neutropenia: Monitor ANC at least twice weekly during high-dose IV therapy, particularly in neonates where 21% may develop neutropenia 3
- Nephrotoxicity: Supra-therapeutic dosing significantly increases AKI risk (adjusted OR 12.9) 11. Avoid concurrent nephrotoxic agents when possible 7
- Neurotoxicity: Risk increases with renal impairment and inadequate dose adjustment 7
Common Pitfalls
- Inadequate dosing in encephalitis: Approximately 35% of patients receive sub-therapeutic doses, which may worsen outcomes 11
- Failure to adjust for renal function: This is the most common cause of acyclovir toxicity
- Delayed initiation in herpes zoster: Treatment must begin within 72 hours, ideally within 48 hours, for maximal benefit 7
- Topical acyclovir use: This is substantially less effective than oral therapy and should be avoided 5