Generic Name

Primidone

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Classification

FDA Approved Indications

• Epilepsy (partial seizures, generalized tonic-clonic seizures)

Mechanism of Action

Primidone is metabolized into phenobarbital and phenylethylmalonamide, which enhance GABA-mediated neuronal inhibition, stabilizing neuronal membranes and reducing seizure activity.

Dosage and Administration

Adult: Initial dose: 125-250 mg at bedtime; titrate slowly. Maintenance doses vary.

Pediatric: Dosing based on weight and age, typically starting at 50 mg/day divided doses.

Geriatric: Start at lower dose due to increased sensitivity and risk of adverse effects.

Renal Impairment: Use cautiously; dosage adjustments may be necessary.

Hepatic Impairment: Monitor closely; dosage adjustments recommended.

Pharmacokinetics

Absorption: Well absorbed from gastrointestinal tract.

Distribution: Widespread, crosses blood-brain barrier.

Metabolism: Metabolized in the liver into active metabolites (phenobarbital).

Excretion: Primidone and metabolites excreted primarily in urine.

Half Life: Primidone: approximately 3-7 hours; active metabolites longer.

Contraindications

• Hypersensitivity to primidone, barbiturates, or related drugs.
• Porphyria.

Precautions

• Use cautiously in hepatic impairment, pregnancy, lactation, and with other CNS depressants. Monitor blood counts.

Adverse Reactions - Common

• Drowsiness, dizziness (Common)
• Nausea, vomiting (Common)
• Ataxia, nystagmus (Common)

Adverse Reactions - Serious

• Agranulocytosis, aplastic anemia (Serious but rare)
• Severe hypersensitivity reactions (Rare)
• Stevens-Johnson syndrome (Rare)

Drug-Drug Interactions

• Other CNS depressants, warfarin, oral contraceptives, strong CYP450 inducers or inhibitors.

Drug-Food Interactions

N/A

Drug-Herb Interactions

N/A

Nursing Implications

Patient/Family Teaching

• Take medication exactly as prescribed.
• Do not stop suddenly—risk of withdrawal seizures.
• Report signs of allergic reactions or blood dyscrasias.
• Avoid alcohol and other CNS depressants.

Special Considerations

Black Box Warnings:

• Potential for dependence, withdrawal seizures if discontinued abruptly.

Genetic Factors: N/A

Lab Test Interference: May alter liver function tests, blood counts.

Overdose Management

Signs/Symptoms: Respiratory depression, ataxia, altered mental status, coma.

Treatment: Supportive care, airway management, activated charcoal if recent ingestion, hemodialysis in severe cases.

Storage and Handling

Storage: Store at room temperature, away from moisture and light.

Stability: Stable under recommended storage conditions.