primidone

Action

Unknown. May raise seizure threshold by decreasing neuronal firing after being converted to phenobarbital.

Availability

Suspension: 250 mg/5 ml

Tablets: 50 mg, 250 mg

Indications and dosages

➣ Grand mal, psychomotor, or focal epileptic seizures

Adults and children ages 8 and older: Initially, 100 to 125 mg P.O. at bedtime on days 1 to 3, then 100 to 125 mg P.O. b.i.d. on days 4 to 6, then 100 to 125 mg P.O. t.i.d. on days 7 to 9, followed by a maintenance dosage of 250 mg P.O. three or four times daily

Children younger than age 8: Initially, 50 mg P.O. at bedtime on days 1 to 3, then 50 mg P.O. b.i.d. on days 4 to 6, then 100 mg P.O. b.i.d. on days 7 to 9. For maintenance, 125 to 250 mg t.i.d. or 10 to 25 mg/kg/day in divided doses.

Dosage adjustment

• Renal impairment

Off-label uses

• Benign familial (essential) tremor

Contraindications

• Hypersensitivity to drug or phenobarbital

• Porphyria

Precautions

Use cautiously in:

• hepatic, renal, or chronic obstructive pulmonary disease

• pregnant or breastfeeding patients

• hyperactive children.

Administration

• Don't change brands. Bioequivalency problems have occurred.

☞ Don't stop therapy suddenly. Dosage must be tapered.

• Know that drug may be given alone or with other anticonvulsants.

Adverse reactions

CNS: headache, dizziness, stimulation, drowsiness, sedation, confusion, hallucinations, psychosis, ataxia, vertigo, hyperirritability, emotional disturbances, paranoid symptoms, coma

EENT: diplopia, nystagmus, eyelid edema

GI: nausea, vomiting, anorexia

GU: erectile dysfunction

Hematologic: megaloblastic anemia,

thrombocytopenia

Skin: flushing, rash

Interactions

Drug-drug. Acetazolamide, succinimide: decreased primidone blood level Carbamazepine: decreased primidone blood level, increased carbamazepine blood level

Hydantoins, isoniazid, nicotinamide: increased primidone blood level

Drug-diagnostic tests. Hemoglobin, platelets: decreased levels

Liver function tests: altered results

Patient monitoring

• Monitor primidone and phenobarbital blood levels.

• Monitor CBC and blood chemistry. Watch for evidence of blood dyscrasias.

• Assess neurologic status regularly. Stay alert for excessive drowsiness and emotional status changes.

Patient teaching

☞ Caution patient not to discontinue therapy suddenly. Advise him to discuss dosage changes with prescriber.

☞ Instruct patient to immediately report unusual bleeding, bruising, or rash.

• Tell patient drug may cause sexual dysfunction. Advise him to discuss this issue with prescriber.

• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration, vision, and alertness.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.