What is the intravenous (IV) to oral (PO) conversion for acyclovir?

IV to PO Conversion of Acyclovir

The appropriate IV to PO conversion ratio for acyclovir is 5:1, meaning that oral acyclovir doses should be approximately 5 times higher than IV doses to achieve similar therapeutic effects due to the oral bioavailability of approximately 20%. 1

Dosing Considerations

Standard Dosing Equivalence

• IV acyclovir: 5-10 mg/kg every 8 hours 2, 3
• Equivalent oral dosing: 800 mg orally 5 times daily 4

Disease-Specific Conversions

For Herpes Simplex Virus Encephalitis:

• IV therapy: 10 mg/kg every 8 hours for 14-21 days 2
• When switching to oral: 800 mg orally 5 times daily 4

For Severe HSV infections:

• Begin with IV acyclovir 5-10 mg/kg every 8 hours
• Switch to oral therapy once lesions begin to regress at 20 mg/kg (maximum 400 mg/dose) 3 times daily 3

Factors Affecting Conversion Decisions

Bioavailability Considerations

• Oral acyclovir has approximately 20% bioavailability 1
• This low bioavailability necessitates the 5:1 conversion ratio
• Absorption is not affected by food intake

Renal Function Adjustments

When converting from IV to oral in patients with renal impairment, adjust both formulations according to creatinine clearance:

Creatinine Clearance (mL/min)	800 mg Oral Dose Adjustment
>25	800 mg every 4 hours, 5x daily
10-25	800 mg every 8 hours
<10	800 mg every 12 hours

4

Clinical Pearls and Pitfalls

When to Convert from IV to PO

• Convert from IV to oral therapy when:
  • Patient can tolerate oral medications
  • Clinical improvement is observed
  • Lesions begin to regress 3
  • No signs of severe disease progression

Common Pitfalls

• Failing to adjust dosage in renal impairment can lead to crystalluria and elevated serum creatinine 5
• Inadequate hydration during IV therapy increases risk of renal complications
• Premature conversion to oral therapy in severe infections may result in treatment failure
• Failure to recognize that higher oral doses are needed due to lower bioavailability

Special Populations

• For immunocompromised patients with HSV infections:

  • Higher doses may be required (400 mg orally 3-5 times daily) 2
  • Consider longer treatment duration until complete healing of lesions 3

• For HIV-infected patients:

  • Higher oral doses are recommended (400 mg orally 3-5 times daily) 2
  • Monitor closely for clinical response

Monitoring After Conversion

• Evaluate response within 72 hours after switching to oral therapy 3
• Monitor for adequate hydration to prevent crystalluria
• Watch for neurological symptoms which may indicate inadequate CNS penetration or neurotoxicity 3
• For herpes simplex encephalitis, consider CSF PCR at the end of therapy to confirm clearance 2

Remember that the 5:1 conversion ratio is based on the approximately 20% bioavailability of oral acyclovir, and this fundamental pharmacokinetic property guides appropriate dosing when transitioning from IV to oral therapy.