Phenobarbital

Brand Names: Luminal

Drug Class: Barbiturate

Overview

Phenobarbital is a long-acting barbiturate used primarily as an anticonvulsant for treating generalized and partial seizures, and secondarily as a sedative. It works by enhancing GABA-mediated inhibition in the central nervous system. Due to its potential for dependence, abuse, and significant side effects, it requires careful dosing and monitoring.

Mechanism of Action

Phenobarbital is a barbiturate that acts as a positive allosteric modulator at the GABA-A receptor, enhancing GABA-mediated chloride influx, leading to neuronal hyperpolarization and reduced excitability in the central nervous system.

Indications

  • Sedative
  • Anticonvulsant for generalized and partial seizures

Common Doses

  • 15 mg
  • 30 mg
  • 60 mg
  • 100 mg

Dosage

Dose must be individualized based on patient age, weight, and clinical condition. Typical adult anticonvulsant doses range from 50-100 mg two to three times daily, with lower doses for sedation.

Contraindications

  • Hypersensitivity to barbiturates
  • History of manifest or latent porphyria
  • Marked impairment of liver function
  • Respiratory disease with dyspnea or obstruction

Side Effects

  • Somnolence (most common)
  • Drowsiness, lethargy, vertigo
  • Respiratory depression, apnea
  • Agitation, confusion, ataxia
  • Nausea, vomiting, constipation
  • Hypersensitivity reactions including skin rashes, angioedema, exfoliative dermatitis
  • Paradoxical excitement or delirium
  • Megaloblastic anemia with chronic use

Interactions

  • Alcohol and other CNS depressants produce additive CNS depression
  • Enzyme-inducing effects may reduce efficacy of oral contraceptives, warfarin, and other drugs metabolized by CYP450 enzymes
  • Valproic acid may increase phenobarbital levels

Counseling Points

  • This medication can be habit-forming - do not increase dose or frequency without medical advice
  • Avoid alcohol and other CNS depressants while taking this medication
  • Do not stop abruptly after prolonged use due to risk of withdrawal symptoms
  • May cause drowsiness - avoid driving or operating machinery until effects are known
  • Use effective contraception as phenobarbital may reduce efficacy of oral contraceptives