Chemical name: 5-ethyldihydro-5-phenyl-4,6 (1H, 5H)-pyrimidinedione. Structural formula:

Primidone is a white, crystalline, highly stable substance, M.P. 279-284°C. It is poorly soluble in water (60 mg per 100 mL at 37°C) and in most organic solvents. It possesses no acidic properties, in contrast to its barbiturate analog.

Primidone Tablets, USP 50 mg and 250 mg contain the following inactive ingredients: lactose monohydrate, NF; magnesium stearate, NF; methyl cellulose, USP; microcrystalline cellulose, NF; purified water, USP; sodium lauryl sulfate, NF; sodium starch glycolate, NF; talc, USP

Primidone tablets raises electro-or chemoshock seizure thresholds or alters seizure patterns in experimental animals. The mechanism(s) of primidone’s antiepileptic action is not known.

Primidone per se has anticonvulsant activity as do its two metabolites, phenobarbital and phenylethylmalonamide (PEMA). Inaddition to its anticonvulsant activity, PEMA potentiates the anticonvulsant activity of phenobarbital in experimental animals.

Primidone tablets, used alone or concomitantly with other anticonvulsants, is indicated in the control of grand mal, psychomotor, and focal epileptic seizures. It may control grand mal seizures refractory to other anticonvulsant therapy.

Primidone is contraindicated in: 1) patients with porphyria and 2) patients who are hypersensitive to phenobarbital (see ACTIONS).

The abrupt withdrawal of antiepileptic medication may precipitate status epilepticus. The therapeutic efficacy of a dosage regimen takes several weeks before it can be assessed.

Suicidal Behavior and Ideation

Antiepileptic drugs (AEDs), including Primidone, increase the risk of suicidal thoughts or behavior in patients taking these drugs for any indication. Patients treated with any AED for any indication should be monitored for the emergence or worsening of depression, suicidal thoughts or behavior, and/or any unusual changes in mood or behavior.

Pooled analyses of 199 placebo-controlled clinical trials (mono- and adjunctive therapy) of 11 different AEDs showed that patients randomized to one of the AEDs had approximately twice the risk (adjusted Relative Risk 1.8, 95% CI:1.2, 2.7) of suicidal thinking or behavior compared to patients randomized to placebo. In these trials, which had a median treatment duration of 12 weeks, the estimated incidence rate of suicidal behavior or ideation among 27,863 AED-treated patients was 0.43%, compared to 0.24% among 16,029 placebo-treated patients, representing an increase of approximately one case of suicidal thinking or behavior for every 530 patients treated. There were four suicides in drug-treated patients in the trials and none in placebo-treated patients, but the number is too small to allow any conclusion about drug effect on suicide.

The increased risk of suicidal thoughts or behavior with AEDs was observed as early as 1 week after starting drug treatment with AEDs and persisted for the duration of treatment assessed. Because most trials included in the analysis did not extend beyond 24 weeks, the risk of suicidal thoughts or behavior beyond 24 weeks could not be assessed.

The risk of suicidal thoughts or behavior was generally consistent among drugs in the data analyzed. The finding of increased risk with AEDs of varying mechanisms of action and across a range of indications suggests that the risk applies to all AEDs used for any indication. The risk did not vary substantially by age (5 to 100 years) in the clinical trials analyzed.

Table 1 shows absolute and relative risk by indication for all evaluated AEDs.

Table 1: Risk by indication for antiepileptic drugs in the pooled analysis

The relative risk for suicidal thoughts or behavior was higher in clinical trials for epilepsy than in clinical trials for psychiatric or other conditions, but the absolute risk differences were similar for the epilepsy and psychiatric indications.

Anyone considering prescribing primidone tablets or any other AED must balance the risk of suicidal thoughts or behavior with the risk of untreated illness. Epilepsy and many other illnesses for which AEDs are prescribed are themselves associated with morbidity and mortality and an increased risk of suicidal thoughts and behavior. Should suicidal thoughts and behavior emerge during treatment, the prescriber needs to consider whether the emergence of these symptoms in any given patient may be related to the illness being treated.

Patients, their caregivers, and families should be informed that AEDs increase the risk of suicidal thoughts and behavior and should be advised of the need to be alert for the emergence or worsening of the signs and symptoms of depression, any unusual changes in mood or behavior, or the emergence of suicidal thoughts, behavior, or thoughts about self-harm. Behaviors of concern should be reported immediately to healthcare providers.

The total daily dosage should not exceed 2 g. Since primidone tablets therapy generally extends over prolonged periods, a complete blood count and a sequential multiple analysis-12 (SMA-12) test should be made every 6 months.

The most frequently occurring early side effects are ataxia and vertigo. These tend to disappear with continued therapy, or with reduction of initial dosage. Occasionally, the following have been reported: nausea, anorexia, vomiting, fatigue, hyperirritability, emotional disturbances, sexual impotency, diplopia, nystagmus, drowsiness, and morbilliform skin eruptions. Granulocytopenia, agranulocytosis, and red-cell hypoplasia and aplasia, have been reported rarely. These and, occasionally, other persistent or severe side effects may necessitate withdrawal of the drug. Megaloblastic anemia may occur as a rare idiosyncrasy to primidone and to other anticonvulsants. The anemia responds to folic acid without necessity of discontinuing medication.

To report SUSPECTED ADVERSE REACTIONS, contact Carnegie Pharmaceuticals LLC at 1-732-783-7010 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.

Usual Dosage

Patients 8 years of age and older who have received no previous treatment may be started on primidone tablets according to the following regimen using either 50 mg or scored 250 mg primidone tablets:

Days 1 to 3: 100 to 125 mg at bedtime.

Days 4 to 6: 100 to 125 mg twice a day.

Days 7 to 9: 100 to 125 mg three times a day.

Day 10 to maintenance: 250 mg three times a day.

For most adults and children 8 years of age and over, the usual maintenance dosage is three to four 250 mg primidone tablets in divided doses (250 mg three times a day or four times a day). If required, an increase to five or six 250 mg tablets daily may be made, but daily doses should not exceed 500 mg four times a day.

Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations of primidone may be necessary for optimal dosage adjustment. The clinically effective serum level for primidone is between 5 to 12 mcg/mL.

Patients Already Receiving Other Anticonvulsants

Primidone tablets should be started at 100 to 125 mg at bedtime and gradually increased to maintenance level as the other drug is gradually decreased. This regimen should be continued until satisfactory dosage level is achieved for the combination, or the other medication is completely withdrawn. When therapy with primidone tablets alone is the objective, the transition from concomitant therapy should not be completed in less than 2 weeks.

Pediatric Dosage

For children under 8 years of age, the following regimen may be used:

Days 1 to 3: 50 mg at bedtime.

Days 4 to 6: 50 mg twice a day.

Days 7 to 9: 100 mg twice a day.

Day 10 to maintenance: 125 mg three times a day to 250 mg three times a day.

For children under 8 years of age, the usual maintenance dosage is 125 to 250 mg three times daily or, 10 to 25 mg/kg/day in divided doses.

Primidone Tablets, USP

50 mg: White, round, flat-faced, bevel-edged tablets, debossed “CP” above “11” on one side and bisected on the other side, in bottles of 100 (NDC 80005-117-11), 500 (NDC 80005-117-14) and 1000 (NDC 80005-117-15).

250 mg: White round, flat-faced, bevel-edged tablets, debossed “CP” bisect “118” on one side and plain on the other side, in bottles of 100  (NDC 80005-118-11), 500 ( NDC 80005-118-14) and 1000 ( NDC 80005-118-15).

Storage

Store at 20° to 25°C (68° to 77°F)

[see USP Controlled Room Temperature].

Dispense in a tight, light-resistant container with a child-resistant closure.

Manufactured and Distributed by:

Carnegie Pharmaceuticals LLC

Delran, NJ 08075, USA

Rev. 06/2024

NDC 80005-117-11

Primidone Tablets, USP

50 mg

Each tablet contains: 50 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

100 Tablets

NDC 80005-117-14

Primidone Tablets, USP

50 mg

Each tablet contains: 50 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

500 Tablets

NDC 80005-117-15

Primidone Tablets, USP

50 mg

Each tablet contains: 50 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

1000 Tablets

NDC 80005-118-11

Primidone Tablets, USP

250 mg

Each tablet contains: 250 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

100 Tablets

NDC 80005-118-14

Primidone Tablets, USP

250 mg

Each tablet contains: 250 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

500 Tablets

NDC 80005-118-15

Primidone Tablets, USP

250 mg

Each tablet contains: 250 mg of primidone, USP

PHARMACIST: Dispense the accompanying Medication Guide to each patient.

Rx Only

1000 Tablets