PRIMIDONE

PRIMIDONE (pri'mi-done) Apo-Primidone , Mysoline Classifications: barbiturate; anticonvulsant; Therapeutic: anticonvulsant Prototype: Phenobarbital Pregnancy Category: D

Availability

50 mg, 250 mg tablets; 250 mg/5 mL suspension

Action

Converted in body to phenobarbital. Impairs vitamin D, calcium, folic acid, and vitamin B₁₂ metabolism and utilization.

Therapeutic Effect

Antiepileptic properties result from raising the seizure threshold and changing seizure patterns.

Uses

Alone or concomitantly with other anticonvulsant agents in the prophylactic management of complex partial (psychomotor) and generalized tonic-clonic (grand mal) seizures.

Unlabeled Uses

Essential tremor.

Contraindications

Hypersensitivity to barbiturates, porphyria; ethanol intoxication, hepatic encephalopathy, pregnancy (category D), lactation.

Cautious Use

Chronic lung disease, sleep apnea; liver or kidney disease, dialysis; hyperactive children; mental status changes, major depression, suicidal ideation.

Route & Dosage

Seizures Adult/Child (≥8 y): PO 250 mg/d, increased by 250 mg/wk (max: 2 g in 2–4 divided doses) Child (<8 y): PO 125 mg/d, increased by 125 mg/wk (max: 2 g/d in 2–4 divided doses)

Administration

Oral

• Give whole or crush with fluid of patient's choice.
• Give with food if drug causes GI distress.
• Note: Transition from another anticonvulsant to primidone normally requires at least 21 wk.

Adverse Effects (≥1%)

CNS: Drowsiness, sedation, vertigo, ataxia, headache, excitement (children), confusion, unusual fatigue, hyperirritability, emotional disturbances, acute psychoses (usually patients with psychomotor epilepsy). Special Senses: Diplopia, nystagmus, swelling of eyelids. GI: Nausea, vomiting, anorexia. Hematologic: Leukopenia, thrombocytopenia, eosinophilia, decreased serum folate levels, megaloblastic anemia (rare). Skin: Alopecia, maculopapular or morbilliform rash, edema, lupus erythematosus-like syndrome. Urogenital: Impotence. Body as a Whole: Lymphadenopathy, osteomalacia.

Interactions

Drug: Alcohol, cns depressants compound CNS depression; phenobarbital may decrease absorption and increase metabolism of oral anticoagulants; increases metabolism of corticosteroids, oral contraceptives, anticonvulsants, digitoxin, possibly decreasing their effects; antidepressants potentiate adverse effects of primidone; griseofulvin decreases absorption of primidone. Herbal: Kava, valerian may potentiate sedation.

Pharmacokinetics

Absorption: Approximately 60–80% from GI tract. Peak: 4 h. Distribution: Distributed into breast milk. Metabolism: In liver to phenobarbital and PEMA. Elimination: In urine. Half-Life: Primidone 3–24 h, PEMA 24–48 h; phenobarbital 72–144 h.

Nursing Implications

Assessment & Drug Effects

• Lab tests: Perform baseline and periodic CBC, complete blood chemistry (q6mo), and primidone blood levels. (Therapeutic blood level for primidone: 5–10 mcg/mL.)
• Monitor primidone plasma levels (concentrations of primidone >10 mcg/mL are usually associated with significant ataxia and lethargy).
• Therapeutic response may not be evident for several weeks.
• Observe for S&S of folic acid deficiency: Mental dysfunction, psychiatric disorders, neuropathy, megaloblastic anemia. Determine serum folate levels if indicated.
• Be aware that presence of unusual drowsiness in breast fed newborns of primidone-treated mothers is an indication to discontinue breast feeding.

Patient & Family Education

• Avoid driving and other potentially hazardous activities during beginning of treatment because drowsiness, dizziness, and ataxia may be severe. Symptoms tend to disappear with continued therapy; if they persist, dosage reduction or drug withdrawal may be necessary.
• Avoid alcohol and other CNS depressants unless otherwise directed by physician.
• Do not take OTC medications unless approved by physician.
• Pregnant women should receive prophylactic vitamin K therapy for 1 mo prior to and during delivery to prevent neonatal hemorrhage.
• Withdraw primidone gradually to avoid precipitating status epilepticus.
• Carry medical information at all times. It needs to indicate medical diagnosis, medication(s), physician's name(s), address(es), and telephone number(s).

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Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug