Synonyms: Gardenal
Clinical significance
Phenobarbital is a barbiturate medicine widely used to treat epilepsy.
It works by enhancing the inhibitory action of GABA (gamma-aminobutyric acid) and reducing the excitatory effects of glutamate. It is well absorbed when taken by mouth and is metabolised by the liver, where it induces enzymes. There is good correlation between the dose given and the blood concentration. Its toxic effects mainly affect the central nervous system (drowsiness, agitation, confusion, ataxia, lethargy). It can also cause low blood pressure and respiratory depression.
Clinical indications
Monitoring of treatment.
Sample type
The patient must have a blood sample taken
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes. For periodic monitoring of the blood concentration of the medicine, so that results can be compared over time, the blood sample should always be taken the same number of hours after taking the medicine.
Notes
Valproate, phenytoin and oxcarbazepine increase its concentration; carbamazepine can have a variable effect. Vigabatrin and folate reduce its concentration.
When taken by mouth, bowel disturbances (diarrhoea) can reduce absorption of the medicine and therefore its blood concentration.