- A long-acting barbiturate
- The phenyl side group on position R2 of its pyrimidine ring has anticonvulsant activities
- It can be used to treat all types of epilepsy (except absence seizures) including status epilepticus
- Of note, the anti-epileptic drugs phenytoin and primidone are metabolised to phenobarbital
Phenobarbital
Phenobarbital
- Odourless, white, crystalline powder
- Available for the oral route; onset in 30mins
Absorption
- Oral bioavailability approximately 90%
Distribution
- Volume of distribution of 0.7L/kg
- 50% protein bound
Metabolism
- 75% hepatic (CYP2C19)
- 25% excreted unchanged in urine
Excretion
- Clearance 11ml/kg/min
- Half life 3-5hrs
- Renal elimination is increased by alkalising urine as it is a weak acid
Respiratory
- Respiratory depression
Cardiovascular
- Cardiovascular depression
Neurological
- Sedation
- Impairment of cognition and motor skills
- Hyperkinesia
- Behavioural disturbance
Haematological
- Megaloblastic anaemia
Other
- Powerful enzyme inducer, interacting with warfarin, other anticonvulsants and the OCP
- Hypersensitivity reactions
- Osteomalacia