Antispasmodic

Uses

Phenobarbital is used as-

• Sedative and hence it relieves anxiety, tension and fear
• Hypnotic and hence it is used for short term insomnia
• Pre-anaesthetics
• Anti-epileptic in epilepsy with Partial seizure or Generalized Tonic-clonic seizure, status epilepticus
• Certain acute convulsive episodes

Antispasmodic is also used to associated treatment for these conditions: Alcohol Withdrawal Syndrome, Anxiety, Febrile Convulsions, Hyperbilirubinemia, Insomnia, Menopausal Symptoms, Partial-Onset Seizures, Withdrawal Symptoms, Generalized seizure, Sedation

How Antispasmodic works

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.

Dosage

Antispasmodic dosage

Adults:

• Hypnosis: 100 to 320 mg
• Sedation: 30 to 120 mg/day in 2 to 3 divided doses
• Epilepsy: 60 to 250 mg/day
• Convulsion: 50 to 100 mg/day in 2 to 3 divided doses
• Status epilepticus: IV 10-20 mg/Kg, repeat if needed

Children:

• Preoperative: 1-3 mg/Kg body weight
• Convulsion: 4-6 mg/Kg/day
• Status epilepticus: IV 15-20 mg/Kg over 10-15 minutes

Dilute with most IV infusion soln (e.g. NaCl 0.45% or 0.9%, lactated Ringer's, dextrose 5%, Ringer's).

Side Effects

Drowsiness is the most common side effect. Less common side effects are CNS depression, nervousness, agitation, psychiatric disturbance, lethargy, mental depression, ataxia, nightmares, bradycardia, apnea, nausea, vomiting, constipation, restlessness and confusion in the elderly and hyperkinesia in children.

Toxicity

CNS and respiratory depression which may progress to Cheyne-Stokes respiration, areflexia, constriction of the pupils to a slight degree (though in severe poisoning they may wshow paralytic dilation), oliguria, tachycardia, hypotension, lowered body temperature, and coma. Typical shock syndrome (apnea, circulatory collapse, respiratory arrest, and death) may occur.

Precaution

Phenobarbital is potentially habit forming if taken over an extended period of time. When being prescribed to overcome insomnia, the drug should not be used for a period longer than two weeks. Caution should be taken in patients who are mentally depressed, have hepatic damage, suicidal tendencies or a history of drug abuse.

Interaction

Phenobarbital can interact with a number of prescription and nonprescription medications including acetaminophen, anticoagulants such as warfarin, chloramphenicol, monoamine oxidase inhibitors (MAOIs), antidepressants, asthma medicine, cold medicine, anti-allergy medicine, sedatives, steroids, tranquilizers, and vitamins. Interactions with these medications can increase the drowsiness caused by phenobarbital.

Elimination Route

Absorbed in varying degrees following oral, rectal or parenteral administration. The salts are more rapidly absorbed than are the acids. The rate of absorption is increased if the sodium salt is ingested as a dilute solution or taken on an empty stomach.

Half Life

53 to 118 hours (mean 79 hours)

Pregnancy & Breastfeeding use

Pregnancy Category D. Phenobarbital can cause potential fetal damage. Their use in pregnancy alone, or in combination with other anticonvulsants, can cause coagulation defects in the newborn infant which may be preventable by the prophylactic administration of Vitamin K to the mother prior to delivery. Phenobarbital is excreted through human milk; so caution should be taken during lactation period.

Contraindication

Phenobarbital is contraindicated in patients with acute intermittent porphyria and who have a natural or idiosyncrasy to barbiturates

Special Warning

Debilitated patient: Reduce dose

Renal Impairment: Reduce dose. Severe: Contraindicated

Hepatic Impairment: Reduce dose. Severe: Contraindicated

Acute Overdose

Phenobarbital should not be used more than the dosage guide line. 1 gm Phenobarbital oral dose may cause serious poisoning and 2 gm may cause even death. Over dosage produces severe, persistent depression. Treatment includes artificial respiration, maintenance of fluid balance and antibiotics to prevent pneumonia. Alkalinisation of the urine and forced diuresis or haemodialysis have been used in cases of severe poisoning.

Storage Condition

Protect from light, store in cool and dry place. Keep out of reach of children.

Innovators Monograph

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