Phenobarbital is a barbiturate, nonselective, central nervous system depressant which is primarily used as a sedative hypnotic and also as an anticonvulsant in subhypnotic doses.

Pharmacology

Phenobarbital, the longest-acting barbiturate, is used for its anticonvulsant and sedative-hypnotic properties in the management of all seizure disorders except absence (petit mal). Phenobarbital acts on GABAA receptors, increasing synaptic inhibition. This has the effect of elevating seizure threshold and reducing the spread of seizure activity from a seizure focus. Phenobarbital may also inhibit calcium channels, resulting in a decrease in excitatory transmitter release. The sedative-hypnotic effects of phenobarbital are likely the result of its effect on the polysynaptic midbrain reticular formation, which controls CNS arousal.

Phenobarbital is indicated for the following conditions:

• Sedatives
• Hypnotics for the short-term treatment of insomnia
• Pre-anesthetics
• Long-term anticonvulsants for the treatment of generalized tonic-clonic and cortical local seizures. And, in the emergency control of certain acute convulsive episodes, (those associated with status epilepticus, eclampsia meningitis, tetanus, and toxic reactions to Strychnine or local anesthetics).

Suggested doses of Phenobarbital for specific indications are as follows:

Pediatric Oral Dosage:

• Preoperative: 1 mg to 3 mg/kg.
• Anticonvulsant: 1 mg to 6 mg/kg per day

Adult Oral Dosage:

• Daytime sedative: 30 mg to 120 mg daily in 2 to 3 divided doses.
• Bedtime hypnotic: 100 mg to 320 mg.
• Anticonvulsant: 50 mg to 100 mg 2 to 3 times daily.

Pediatric Injection Dosage:

• 15 to 20 mg/kg IV over 10 to 15 min.
• Preoperative Sedation:1 to 3 mg/kg IM/IV;
• Anticonvulsant: 4 to 6 mg/kg IM/IV per day, for 10 days. Alternatively, use 10 to 15 mg/kg IM/IV per day to reach therapeutic level more quickly. Maxium IV rate 60 mg/min.

Adult Injection Dosage:

• Insomnia: 100 to 320 mg IM/IV;
• Convulsions: 100 to 320 mg IV. Repeat if needed (maximum, 600 mg per day);
• Status Epilepticus: 10 to 20 mg/kg IV. Repeat if needed.

Phenobarbital is contraindicated in patients with known Phenobarbital sensitivity or a history of latent porphyria.

The most common adverse reaction is somnolence. Other less frequent adverse reactions are agitation, confusion, hyperkinesia, ataxia, CNS depression, nightmares, nervousness, psychiatric disturbance, hallucinations, insomnia, anxiety, dizziness, thinking abnormality, apnea, bradycardia, hypotension, nausea, vomiting and constipation.

Overdose Effects

The toxic dose of barbiturates varies considerably. In general, an oral dose of 1 gram of most barbiturates produces serious poisoning in an adult. Death commonly occurs after 2 to 10 grams of ingested barbiturate. Acute overdosage with barbiturates is manifested by CNS and respiratory depression Treatment of overdosage is mainly supportive and immediate hospitalization is necessary.

Tolerance and psychological and physical dependence may occur with continuing use. Phenobarbital should be administered with caution to patients who are mentally depressed, have suicidal tendencies, or a history of drug abuse. In patients with hepatic damage, Phenobarbital should be administered with caution and initially reduced doses.

Pregnancy & Lactation

Pregnancy Category D. Phenobarbital can cause fetal damage when administered to a pregnant woman. If this drug is used during pregnancy, or if the patient becomes pregnant while taking this drug, the patient should be in risk of the potential hazard to the fetus. Caution should be taken when Phenobarbital is administered to a nursing woman since small amounts of Phenobarbital are excreted in the milk.