Acyclovir IV Equivalent Dose
For a 400 mg oral acyclovir tablet, the intravenous equivalent is 5 mg/kg IV every 8 hours, which typically translates to approximately 350-400 mg IV every 8 hours for an average 70 kg adult.
Dosing Rationale
The conversion from oral to IV acyclovir is not a simple 1:1 ratio due to bioavailability differences. Oral acyclovir has approximately 15-20% bioavailability 1, meaning only a fraction of the oral dose reaches systemic circulation.
Standard IV Dosing by Indication
For mucocutaneous HSV infections (the typical indication for oral 400 mg):
- 5-10 mg/kg IV every 8 hours is the standard dose 2, 3
- For severe disease requiring hospitalization: 5-10 mg/kg IV every 8 hours for 5-7 days 2, 3
For HSV encephalitis:
- 10 mg/kg IV every 8 hours for 14-21 days 4
- Some centers use 20 mg/kg IV every 8 hours in neonates with improved outcomes 4
For VZV encephalitis:
- 10-15 mg/kg IV every 8 hours 5
Weight-Based Calculation
For an average 70 kg adult:
- 5 mg/kg = 350 mg IV every 8 hours
- 10 mg/kg = 700 mg IV every 8 hours
The FDA label confirms these dosing ranges and notes that reconstituted acyclovir provides 50 mg/mL concentration 6.
Important Clinical Considerations
Renal Adjustment is Critical
Acyclovir clearance is highly dependent on renal function 6. For patients with creatinine clearance <10 mL/min, the dose should be reduced to 200 mg every 12 hours for oral therapy 7. IV dosing requires similar adjustments based on creatinine clearance.
Bioavailability Evidence
Research demonstrates that oral acyclovir 400 mg five times daily produces equivalent clinical outcomes to 5 mg/kg IV three times daily in herpes zoster 8. This supports the 5 mg/kg IV dose as roughly equivalent to 400 mg oral when accounting for bioavailability.
A study comparing oral valacyclovir 1000 mg to IV acyclovir 5 mg/kg showed equivalent AUC exposure with 60% bioavailability 9, further validating the dosing conversion principles.
Administration Requirements
- Must be given as 1-hour infusion to prevent nephrotoxicity 6
- Adequate hydration is essential
- Monitor renal function closely, especially with doses >1 mg/kg or in patients with baseline renal impairment
- Supra-therapeutic dosing significantly increases acute kidney injury risk (adjusted OR 12.9) 10
Immunocompromised Patients
For HIV-infected or other immunocompromised patients with severe HSV disease, higher doses may be needed 3:
- Consider 400 mg oral 3-5 times daily equivalent
- For severe IV cases: 5 mg/kg IV every 8 hours minimum
Pediatric Dosing
In children, 10 mg/kg IV every 8 hours is standard for moderate-to-severe disease 11. Recent pharmacokinetic data suggests 10 mg/kg every 6 hours may be optimal for children with normal renal function, and 15-20 mg/kg every 6 hours for those with augmented renal clearance 12.
Common Pitfalls
- Do not use topical acyclovir as it is substantially less effective than oral or IV formulations 2
- Avoid underdosing in obese patients: use actual body weight for non-obese patients and adjusted body weight for obese patients 10
- Do not forget renal dose adjustments: failure to adjust for renal function is a major cause of toxicity
- Ensure adequate infusion time: rapid bolus administration significantly increases nephrotoxicity risk