Overview
Primidone is an anticonvulsant medication used primarily for the management of various types of epileptic seizures. It functions through conversion to phenobarbital and phenylethylmalonamide, providing antiepileptic effects. The drug requires careful dose titration and monitoring due to potential side effects and interactions.
Mechanism of Action
Primidone is metabolized to two active metabolites: phenobarbital and phenylethylmalonamide (PEMA). Phenobarbital enhances GABA-mediated inhibitory neurotransmission by prolonging chloride channel opening, while PEMA contributes to anticonvulsant effects through less understood mechanisms. This dual action provides broad-spectrum seizure control.
Indications
- Control of grand mal (tonic-clonic) seizures
- Management of psychomotor (complex partial) seizures
- Treatment of focal epileptic seizures
- May be effective for grand mal seizures refractory to other anticonvulsants
Common Doses
- 50 mg tablets
- 250 mg tablets
Dosage
Initial adult dosage typically starts at 100-125 mg at bedtime, gradually increased over several weeks. Maintenance doses range from 250 mg to 2 g daily in divided doses. Pediatric dosing is weight-based and requires careful titration. Dosage must be individualized based on therapeutic response and tolerability.
Contraindications
- Patients with porphyria
- Patients with hypersensitivity to phenobarbital
- History of barbiturate hypersensitivity
Side Effects
- Ataxia and vertigo (common early effects, often transient)
- Nausea, anorexia, vomiting
- Fatigue, drowsiness, sedation
- Hyperirritability, emotional disturbances
- Diplopia, nystagmus
- Sexual impotency
- Morbilliform skin eruptions
- Rare: granulocytopenia, agranulocytosis, red-cell hypoplasia/aplasia
- Megaloblastic anemia (rare, responds to folic acid)
Interactions
- Enhanced CNS depression with alcohol, opioids, benzodiazepines
- Reduced effectiveness of oral contraceptives, anticoagulants, corticosteroids
- Phenobarbital induces CYP450 enzymes, affecting many medications
- Valproic acid may increase phenobarbital levels
- Possible interactions with other enzyme-inducing anticonvulsants
Counseling Points
- Do not stop medication abruptly due to seizure risk
- Take with food if gastrointestinal upset occurs
- Avoid alcohol and CNS depressants
- Use effective contraception due to drug interactions
- Report signs of infection, unusual bleeding, or severe drowsiness
- Regular blood monitoring may be required
- May impair ability to drive or operate machinery