PHENOBARB

What it is

Phenobarb is phenobarbital 30 mg in tablet form, supplied by Arrotex Pharmaceuticals. Phenobarbital is a nonselective central nervous system depressant with a long duration of action. It may act by enhancing and/or mimicking the synaptic action of gamma-aminobutyric acid (GABA).

Approved indications

— Treatment of epilepsy.

Dosing overview

The dose of phenobarbital must be adjusted to the individual, giving consideration to the patient's age, weight and condition. Recommended dosage for adults is 60 to 240 mg per day in 2 to 3 divided doses, and for paediatric patients is 1 to 6 mg per kg per day in 2 to 3 divided doses. Phenobarbital may be given as a single daily dose as it has a long half-life. Plasma concentrations required to control epilepsy are usually about 15 to 40 micrograms/mL (65 to 170 micromoles per L), and drug levels should be monitored so as to keep within the therapeutic range.

Key safety warnings

Phenobarbital may cause fetal harm when administered to a pregnant woman, and prenatal exposure to phenobarbital may increase the risk for congenital malformations and adverse developmental outcomes. Phenobarbital should not be used in women of childbearing potential unless the potential benefit is judged to outweigh the risks following consideration of other suitable treatment options. Due to enzyme induction, phenobarbital may result in a failure of the therapeutic effect of oral contraceptive drugs. Prolonged use may lead to physical dependence and tolerance, and withdrawal symptoms are characterised by apprehension and weakness, followed by anxiety, headache, dizziness, irritability, tremors, nausea, vomiting and insomnia, visual problems, muscle twitching and tachycardia, with hallucinations, orthostatic hypotension and convulsions potentially developing after a day or two. Phenobarbital dose should be reduced gradually over a period of days or weeks, for example by reducing the total daily dose by 30 mg daily or reducing the dose daily by 10%. Mild hypersensitivity reactions have been reported in 1–3% of patients with phenobarbital, including urticaria and maculopapular, erythematous and morbilliform rashes, which resolve on discontinuation. <cite index index="1-38">More serious reactions include serum sickness, exfoliative dermatitis, erythema multiforme and Steven Johnson syndrome.</cite> The most frequent adverse effect following administration of phenobarbital is sedation that often becomes less marked with continued administration, and phenobarbital may induce mood changes and impairment of cognition and memory. Barbiturates may impair the ability to perform potentially hazardous activities requiring mental alertness and physical coordination such as driving and operating machinery.

Contraindications

Phenobarb is not to be administered to patients who are known to be hypersensitive to barbituric acid derivatives. It is not to be administered to patients with porphyria, severe respiratory depression or pulmonary insufficiency, renal impairment, hepatic impairment, sleep apnoea, uncontrolled diabetes mellitus, severe anaemia due to folate deficiency, hyperkinetic children, suicidal potential, alcoholism and drug dependency. Phenobarb is not to be used in the presence of uncontrolled pain as paradoxical excitement may be produced.

PBS listing

Phenobarb 30 mg tablet is listed on the PBS as a single item with restricted authority required status, at an ex-manufacturer price of A$5.20.

Regulatory history

Phenobarb phenobarbital 30 mg tablet was first listed on the Australian Register of Therapeutic Goods on 14 October 1991.