How do you convert phenobarbital (oral) to intravenous (IV) administration?

Phenobarbital Oral to IV Conversion

The oral and intravenous (IV) formulations of phenobarbital have equivalent bioavailability, so they can be converted using a 1:1 ratio. 1

Dosing Equivalence

• Phenobarbital has nearly 100% bioavailability when administered orally, making the oral and IV doses equivalent 1
• The same dose can be used when transitioning from IV to oral administration or vice versa 1, 2
• For status epilepticus, the American Academy of Pediatrics recommends a loading dose of 20 mg/kg (maximum 1000 mg) for both IV and oral routes 3, 1

Clinical Considerations When Converting Between Routes

• When switching from IV to oral administration:

  • Maintain the same total daily dose 2
  • Consider dividing the daily dose into 2-3 administrations initially, though once-daily dosing at bedtime may be appropriate for maintenance therapy 2, 4
  • Monitor serum levels to ensure therapeutic concentrations are maintained 5

• When switching from oral to IV administration:

  • Use the same total daily dose 1, 2
  • IV administration rate should not exceed 60 mg/min for adults 2
  • Slow IV injection is essential with careful monitoring of vital signs 2

Pharmacokinetic Considerations

• The half-life of phenobarbital is approximately 100 hours in adults 5

  • In term infants: 103 hours
  • In preterm infants: 141 hours
  • In 4-week-old infants: 67 hours

• Serum phenobarbital concentration rises approximately 1.65 μg/mL for each mg/kg administered intravenously 6

Practical Application

• For maintenance therapy in adults:

  • Typical daily dose: 1-3 mg/kg/day divided into 1-3 doses 2
  • Once-daily bedtime dosing may improve compliance and reduce daytime sedation 4

• For status epilepticus:

  • Loading dose: 15-20 mg/kg IV over 10-15 minutes 3, 2
  • Non-responders may receive additional doses of 5-10 mg/kg until seizures stop 5
  • Maintenance dose: 1-3 mg/kg IV every 12 hours after resolution 3

Important Precautions

• IV administration requires careful monitoring:

  • Use larger veins to minimize irritation and thrombosis risk 2
  • Avoid small veins on the dorsum of the hand or wrist 2
  • Avoid intraarterial injection which can lead to severe complications 2
  • Monitor blood pressure, respiration, and cardiac function during administration 2

• Be aware of potential drug interactions as phenobarbital is metabolized by CYP2C9 with minor metabolism by CYP2C19 and CYP2E1 5