Primidone

Trade Names:
Mysoline
- Tablets 50 mg
- Tablets 250 mg

Pharmacology

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Primidone and its metabolites (phenobarbital and phenylethylmalonamide) have anticonvulsant activity, raising seizure threshold and altering seizure patterns.

Pharmacokinetics

Absorption

Readily absorbed from the GI tract. T _max is 3 h (primidone) and 7 to 8 h (phenobarbital and phenylethylmalonamide [PEMA] metabolites). Bioavailability is 90% to 100%.

Distribution

Protein binding is negligible for primidone and PEMA; approximately 50% (phenobarbital). Distributes into breast milk. Vd is 0.64 to 0.86 L/kg.

Metabolism

HEPA, 2 active metabolites: phenobarbital (15% to 25%) and PEMA. PEMA is the major metabolite and is less active than phenobarbital.

Elimination

Excreted in the urine (40% as unchanged and remainder excreted as metabolites). Plasma t _½ is 5 to 15 h (primidone), 10 to 18 h (PEMA), and 53 to 140 h (phenobarbital).

Indications and Usage

Control of grand mal, psychomotor, or focal epileptic seizures; may control grand mal seizures refractory to other anticonvulsants.

Unlabeled Uses

Treatment of benign familial tremor (essential tremor).

Contraindications

Hypersensitivity to barbiturates; porphyria.

Dosage and Administration

If no previous treatment, initiate as follows: PO For days 1 to 3, give 100 to 125 mg at bedtime; days 4 to 6, give 100 to 125 mg twice daily; days 7 to 9, give 100 to 125 mg 3 times daily; and day 10 through maintenance dose, give 250 mg 3 times daily or 4 times daily. May increase to 250 mg 5 to 6 times/day, but do not exceed 500 mg 4 times daily (2 g/day).

PO For days 1 to 3, give 50 mg at bedtime; days 4 to 6, give 50 mg twice daily; days 7 to 9, give 100 mg twice daily; and day 10 through maintenance dose, give 125 to 250 mg 3 times daily or 10 to 25 mg/kg/day in divided doses.

Initiate at 100 to 125 mg at bedtime, gradually increasing dose to maintenance level as other drug is gradually decreased. Complete switch to primidone should occur over more than 2 wk.

General Advice

Administer without regard to meals. Administer with food if GI upset occurs.

Storage/Stability

Store tablets at controlled room temperature (59° to 86°F).

Drug Interactions

Decreased anticoagulant effects.

Effects of beta-blockers may be reduced.

Decreased primidone levels; increased concentrations of carbamazepine.

Decreased effect of corticosteroids.

Decreased doxycycline serum levels.

Contraceptive failure has been reported.

Additive CNS suppression.

Decreased effect of felodipine.

Decreased serum griseofulvin levels.

Increased primidone serum levels.

Plasma concentrations may be reduced by primidone, leading to opiate withdrawal.

Enhanced renal toxicity may occur.

Therapeutic failure of metronidazole.

Decreased nifedipine levels.

Decreased quinidine serum levels.

Decreased primidone levels.

Decreased theophylline levels.

Laboratory Test Interactions

None well documented.

Adverse Reactions

CNS

Ataxia; vertigo; fatigue; hyperirritability; emotional disturbances; drowsiness; personality deterioration; mood changes; paranoia.

Dermatologic

Morbilliform or maculopapular skin eruptions.

EENT

Diplopia; nystagmus.

GI

Nausea; anorexia; vomiting.

Genitourinary

Impotence; crystalluria.

Hematologic

Megaloblastic anemia; granulocytopenia, agranulocytosis, red-cell hypoplasia, aplasia (rarely).

Precautions

Pregnancy

Category D . Consult health care provider regarding anticonvulsant use during pregnancy.

Lactation

Excreted in breast milk.

Status epilepticus

May be precipitated by abrupt withdrawal.

Patient Information

• Advise patient to read the patient information leaflet before starting therapy and with each refill.
• Instruct patient, family, or caregiver to continue to take other medications for seizures unless advised by health care provider.
• Instruct patient to take exactly as prescribed and to not change the dose or discontinue unless advised by health care provider.
• Advise patient that medication will be started at a low dose and then gradually increased until max benefit has been obtained.
• Advise patient that each dose may be taken without regard to meals but to take with food if stomach upset occurs.
• Advise patient that the most common side effects of therapy are dizziness, drowsiness, feeling of whirling motion, and incoordination and that they generally occur early in therapy or after a dose increase and tend to disappear with continued therapy.
• Advise patient that if medication needs to be discontinued it will be slowly withdrawn over a period of several weeks unless safety concerns (eg, rash) require a more rapid withdrawal.
• Caution patient that drug may cause dizziness, drowsiness, or coordination problems and to use caution while driving or performing other tasks requiring mental alertness or coordination until tolerance is determined.
• Instruct patient to contact health care provider immediately if rash or fever occur.
• Instruct patient to inform health care provider if seizures get worse of if new types of seizures occur.
• Advise patient to avoid alcoholic beverages and other depressants while taking this medication.
• Advise patient to carry identification (eg, card, bracelet) indicating medication usage and epilepsy.

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