primidone

Absorption: 80–100% absorbed from the GI tract when administered orally.

Distribution: Widely distributed to tissues

Metabolism/Excretion: Converted to phenobarbital and another active anticonvulsant compound by the liver.

Half-Life: 3–7 hr.

Time/Action Profile ⬆ ⬇

(anticonvulsant effect)

ROUTE	ONSET	PEAK	DURATION
PO	4–7 days	7–10 days	8–12 hr

Contraind./Precautions ⬆ ⬇

Contraindicated in:

Hypersensitivity to primidone or phenobarbital;
Porphyria;
Lactation: Lactation.

Use Cautiously in:

All patients (may ↑ risk of suicidal thoughts/behaviors);
Severe hepatic impairment (dosage adjustment required);
OB: Safety not established in pregnancy;
Geri: Appears on Beers list. risk of physical dependence, tolerance to sleep benefits, and overdose in older adults. Avoid use in older adults.

Adv. Reactions/Side Effects ⬆ ⬇

CV: edema, orthostatic hypotension

Derm: alopecia, rash

EENT: visual changes

GI: anorexia, hepatitis, nausea, vomiting

Hemat: blood dyscrasias, folic acid deficiency, megaloblastic anemia

Neuro: ataxia, drowsiness, vertigo, excitement (↑ in children), SUICIDAL THOUGHTS

Resp: dyspnea

Interactions ⬆ ⬇

Drug-drug:

May ↓ levels and effectiveness of hormonal contraceptives, chloramphenicol, acebutolol, propranolol, metoprolol, timolol, doxycycline, corticosteroids, tricyclic antidepressants, phenothiazines, and quinidine.
Additive CNS depression with other CNS depressants, includingalcohol, antihistamines, opioid analgesics, and sedative/hypnotics.
Concurrent use with phenobarbital may lead to phenobarbital toxicity.

Drug-Natural Products:

Concomitant use of kava, valerian, skullcap, chamomile, or hops can ↑ CNS depression.
See sedative interactions.
St. John's wort may affect primidone levels and effectiveness; avoid use.

↓absorption of folic acid.

Route/Dosage ⬆ ⬇

PO (Adults and Children >8 yr): Initial dose of 100–125 mg at bedtime for 3 days, then 100–125 mg twice daily for 3 days, then 100–125 mg 3 times daily for 3 days, then maintenance dose of 250 mg 3–4 times daily (not to exceed 2 g/day).
PO (Children <8 yr): Initial dose of 50 mg at bedtime for 3 days, then 50 mg twice daily for next 3 days, then 100 mg twice daily for 3 days, then maintenance dose of 125–250 mg 3 times daily (10–25 mg/kg/day).

Availability ⬆ ⬇

(Generic available)

Tablets: 50 mg; 125 mg; 250 mg

Assessment ⬆ ⬇

Assess location, duration, frequency, and characteristics of seizure activity. Institute seizure precautions.
Assess patient for allergy to phenobarbital, because it is a metabolite of primidone.
Assess patient for signs of folic acid deficiency (mental dysfunction, unusual tiredness or weakness, psychiatric disorders, neuropathy, megaloblastic anemia). May be treated with folic acid.
Monitor closely for notable changes in behavior that could indicate the emergence or worsening of suicidal thoughts or behavior or depression.

Lab Test Considerations:

CBC and comprehensive metabolic panel should be monitored every 6 mo throughout course of therapy. May cause leukopenia and thrombocytopenia. May cause decreased serum bilirubin concentrations.
- Monitor serum folate concentrations periodically during therapy.

Toxicity and Overdose:

Serum primidone and phenobarbital (a major metabolite of primidone) levels should be routinely monitored. Therapeutic blood levels for primidone—5–10 mcg/mL; for phenobarbital—15–40 mcg/mL.
- Signs of primidone or phenobarbital toxicity include ataxia, lethargy, changes in vision, confusion, and dyspnea.

Implementation ⬆ ⬇

When switching from alternative anticonvulsant medication to primidone or when adding primidone to existing regimen, increase primidone dose gradually while decreasing or continuing other anticonvulsant doses to maintain seizure control. The switch to primidone alone should take at least 2 wk. Dose adjustment is usually made at bedtime.
PO: May be administered with food to minimize GI effects. Low initial doses (25 mg twice daily) have been used in patients experiencing nausea and vomiting. Tablets may be crushed and mixed with food or fluids for patients with difficulty swallowing.

Patient/Family Teaching ⬆ ⬇

Instruct patient to take medication at the same time each day exactly as directed. If a dose is missed, take as soon as remembered unless within 1 hr of next dose. Abrupt withdrawal may lead to status epilepticus. Instruct patient to read the Medication Guide before starting and with each Rx refill, changes may occur.
May cause drowsiness or dizziness. Caution patient to avoid driving or other activities requiring alertness until response to medication is known. These symptoms usually diminish in frequency and intensity with continued use of the medication. Do not resume driving until health care professional gives medical clearance based on control of seizure disorder.
Caution patient to avoid taking alcohol or other CNS depressants concurrently with this medication.
Caution patient to avoid sudden changes in position to decrease orthostatic hypotension.
Advise patient to notify health care professional if skin rash, unsteady gait, joint pain, fever, changes in vision, dyspnea, pregnancy, or paradoxical excitement (especially in children or the elderly) occurs. Advise patient and family to notify health care professional if thoughts about suicide or dying, attempts to commit suicide, new or worse depression, new or worse anxiety, feeling very agitated or restless, panic attacks, trouble sleeping, new or worse irritability, acting aggressive, being angry or violent, acting on dangerous impulses, an extreme increase in activity and talking, or other unusual changes in behavior or mood occur.
Instruct patient to notify health care professional of medication regimen prior to treatment or surgery.
Rep: Advise female patients to use an additional nonhormonal method of contraception during therapy and until next menstrual period. Instruct patient to notify health care professional if pregnancy is planned or suspected and to avoid breastfeeding. Encourage patients who become pregnant to enroll in the North American Antiepileptic Drug Pregnancy Registry by calling 1-888-233-2334 or on the web at www.aedpregnancyregistry.org. Enrollment must be done by patients themselves.
Advise patient to carry identification describing medication regimen at all times.
Emphasize the importance of routine exams to monitor progress.

section name header

Pronunciation ⬇

Classifications ⬆ ⬇

Indications ⬆ ⬇

Action ⬆ ⬇

Pharmacokinetics ⬆ ⬇