Is the dosing for phenobarbital (phenobarbital) intravenous (IV) to oral administration 1:1?

Phenobarbital IV to Oral Conversion Ratio

Yes, the dosing for phenobarbital intravenous (IV) to oral administration is 1:1 (100% bioavailability), meaning the same dose can be used when converting between these routes. 1, 2

Evidence for 1:1 Conversion

• Studies have demonstrated that phenobarbital has complete bioavailability (approximately 100%) when administered orally compared to intravenous administration 2
• Research shows that phenobarbital's pharmacokinetic properties remain consistent across different routes of administration, allowing for equivalent dosing between IV and oral routes 2
• In a study of epileptic patients and normal subjects, the absolute bioavailability of phenobarbital was found to be essentially complete, supporting a 1:1 conversion ratio 2

Pharmacokinetic Considerations

• Phenobarbital is primarily metabolized in the liver by CYP2C9 with minor metabolism by CYP2C19 and CYP2E1 3
• Approximately 25% of the phenobarbital dose is excreted unchanged in the urine 3
• The half-life of phenobarbital is approximately:
  • 100 hours in adults 3
  • 103 hours in term infants 3
  • 141 hours in preterm infants 3

Clinical Application

• When converting from IV to oral phenobarbital:
  • Maintain the same mg-for-mg dose 2
  • No dose adjustment is necessary when switching between routes 1
• For status epilepticus treatment:
  • IV loading dose is 20 mg/kg (maximum 1000 mg) 4
  • The same dose can be used orally when IV access is not available or when transitioning to maintenance therapy 4

Route-Specific Considerations

• Oral administration may have slightly variable absorption timing compared to IV:
  • Peak serum concentrations occur 0.5-4 hours after oral administration 2
  • IV administration provides immediate bioavailability 2
• Rectal administration has also been shown to be reliable with good bioavailability, making it a viable alternative when oral or IV routes are not available 1

Common Pitfalls to Avoid

• Do not confuse phenobarbital with other barbiturates that may have different bioavailability profiles 5
• Remember that while the dose remains the same, the onset of action will be delayed with oral administration compared to IV 2
• Monitor for therapeutic levels regardless of administration route, with therapeutic serum concentrations typically up to 40 μg/ml 3
• Be aware that patients with refractory seizures may require higher serum concentrations (up to 100 μg/ml) 3