Phenobarbital

Drug Information: Phenobarbital

Detailed information regarding Phenobarbital, a medication commonly used in clinical settings. Phenobarbital is a barbiturate derivative that acts as a central nervous system depressant. It is primarily used as an anticonvulsant to manage various types of seizures and is sometimes used to treat anxiety or insomnia, although less frequently due to the availability of safer and more effective alternatives. It’s important to note that Phenobarbital carries a risk of dependence and withdrawal symptoms with prolonged use.

Category

Neurological Drugs – Anticonvulsant

Mechanism of Action

Phenobarbital’s primary mechanism of action involves enhancing the inhibitory neurotransmission mediated by gamma-aminobutyric acid (GABA). GABA is the major inhibitory neurotransmitter in the central nervous system. Phenobarbital binds to the GABA-A receptor, a ligand-gated ion channel that, upon activation by GABA, allows chloride ions to flow into the neuron, causing hyperpolarization and reducing neuronal excitability. Unlike benzodiazepines, which bind to a different site on the GABA-A receptor complex and increase the *frequency* of channel opening, Phenobarbital increases the *duration* of chloride channel opening, thereby prolonging the inhibitory effect of GABA. At high concentrations, Phenobarbital can also directly activate the GABA-A receptor in the absence of GABA. Additionally, Phenobarbital can inhibit excitatory neurotransmission by blocking AMPA receptors, further contributing to its anticonvulsant and sedative effects. The combined effect of enhancing GABAergic inhibition and reducing glutamatergic excitation results in a significant decrease in neuronal firing, which is crucial for its therapeutic effects in controlling seizures and providing sedation.

Clinical Uses

Phenobarbital is primarily used for the treatment of seizures, particularly generalized tonic-clonic seizures and partial seizures. It is also used in the emergency treatment of status epilepticus, although benzodiazepines are often preferred as first-line agents due to their faster onset of action. In infants, Phenobarbital is sometimes used to treat neonatal seizures. Beyond its anticonvulsant properties, Phenobarbital has been used, though less commonly today, as a sedative and hypnotic for short-term treatment of anxiety and insomnia. It has also been used to manage withdrawal symptoms from other barbiturates or benzodiazepines. However, due to the availability of safer and more effective alternatives with fewer side effects and lower risk of dependence, the use of Phenobarbital for sedation and anxiety has significantly decreased. Its use is now largely restricted to seizure management, particularly in resource-limited settings where newer anticonvulsants may not be readily available.

Dosage

Dosage of Phenobarbital varies depending on the indication, patient age, and renal/hepatic function. It is available in oral (tablets and elixir) and injectable forms.

Adults:

• Anticonvulsant: Typical initial oral dose is 60-200 mg daily, usually given as a single dose at bedtime. Dosage adjustments are made based on serum drug levels and seizure control. For status epilepticus, a loading dose of 10-20 mg/kg IV may be administered.
• Sedative: 30-120 mg daily in divided doses.

Pediatrics:

• Anticonvulsant: Loading dose of 15-20 mg/kg IV for neonatal seizures. Maintenance doses range from 3-5 mg/kg/day, usually divided into two doses. Oral maintenance dose is 2-3 mg/kg/day, divided into 1-2 doses.

Dosage should be individualized and monitored closely by a healthcare professional. Regular monitoring of serum Phenobarbital levels is crucial to ensure therapeutic efficacy and minimize the risk of toxicity.

Indications

Common indications for Phenobarbital include:

• Generalized tonic-clonic seizures
• Partial seizures
• Status epilepticus (emergency treatment)
• Neonatal seizures
• Sedation (less common due to safer alternatives)
• Management of barbiturate or benzodiazepine withdrawal

Contraindications

Contraindications for Phenobarbital use include:

• Hypersensitivity to barbiturates
• Severe respiratory disease
• Severe hepatic impairment
• Porphyria
• Pregnancy (risk of fetal harm)
• Breastfeeding
• History of substance abuse (particularly barbiturates)

FAQ

Q: What is Phenobarbital used for?

A: Phenobarbital is primarily used as an anticonvulsant to treat seizures. It can also be used as a sedative or to manage barbiturate withdrawal.

Q: How does Phenobarbital work?

A: Phenobarbital enhances the effect of GABA, an inhibitory neurotransmitter in the brain, which helps to reduce neuronal excitability and control seizures.

Q: What are the common side effects of Phenobarbital?

A: Common side effects include drowsiness, dizziness, confusion, impaired coordination, and respiratory depression.

Q: Is Phenobarbital addictive?

A: Yes, Phenobarbital can be addictive, especially with prolonged use. It can lead to physical and psychological dependence.

Q: Can I drink alcohol while taking Phenobarbital?

A: No, alcohol should be avoided while taking Phenobarbital, as it can enhance the sedative effects and increase the risk of respiratory depression.

Q: What should I do if I miss a dose of Phenobarbital?

A: If you miss a dose, take it as soon as you remember. However, if it’s almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not double the dose to catch up.

Q: How long does it take for Phenobarbital to start working?

A: The onset of action varies depending on the route of administration. Intravenous Phenobarbital works quickly for emergency seizure control. Oral Phenobarbital may take longer to reach a steady state in the body for chronic seizure management.

Q: What are the signs of Phenobarbital overdose?

A: Signs of overdose include severe drowsiness, confusion, slow or shallow breathing, loss of coordination, and coma. Seek immediate medical attention if an overdose is suspected.