Phenobarbital Injection Maintenance Dose
The maintenance dose of intravenous phenobarbital for seizure control is 3-4 mg/kg/day, administered after an initial loading dose of 20 mg/kg. 1
Dosing Algorithm
Initial Loading Dose
- Administer 20 mg/kg IV as the loading dose at a rate not exceeding 100 mg/min 1, 2
- For refractory seizures, additional boluses of 5-10 mg/kg can be given until seizures stop 1
- In status epilepticus, doses up to 36 mg/kg may be required for seizure termination 2
Maintenance Dosing
- Standard maintenance: 3-4 mg/kg/day administered IV or orally once seizures are controlled 1
- This maintenance dose typically achieves therapeutic serum concentrations of 15-40 µg/mL 1, 3
- For patients with super-refractory status epilepticus, mega-dose phenobarbital (>10 mg/kg/day) may be necessary, with serum levels reaching median 151.5 µg/mL 4
Therapeutic Monitoring
Target Serum Levels
- Therapeutic range: 15-40 µg/mL for standard seizure control 1, 3
- Nonresponders may require serum concentrations up to 100 µg/mL in refractory cases 1
- Regular monitoring is recommended to maintain therapeutic range 5
Time to Steady State
- In adults, phenobarbital has a half-life of approximately 100 hours 1
- Daily maintenance dosing takes 6-9 days to reach therapeutic levels of 10 mg/L 6
- IV administration reaches equivalent serum levels 18 hours faster than oral dosing 6
Critical Safety Considerations
Cardiovascular and Respiratory Effects
- Hypotension occurs in all patients receiving phenobarbital for status epilepticus, typically requiring vasopressor support with dopamine 6, 5
- Respiratory depression is common, especially with rapid administration 5
- Infusion rate should not exceed 100 mg/min to minimize cardiorespiratory complications 2
Efficacy Limitations
- In patients with anoxic or metabolic disturbances, seizure control is achieved in less than 40% of cases 6, 5
- Infection is the most critical complication in prolonged high-dose therapy 4
Clinical Pitfalls to Avoid
- Do not use fixed doses without weight-based calculation - studies using non-weight-based dosing (e.g., fixed 300 mg daily) failed to achieve therapeutic levels 6
- Monitor for drug interactions - phenobarbital is a potent CYP3A4 inducer causing significant interactions 5
- Prolonged postictal depression of consciousness is expected but typically mild and transient 2