Primidone

Anticonvulsant, Barbiturate (parent compound), Tremor medicine

Primidone

Mysoline

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Titration strategies

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Summary

Classes

Anticonvulsant, Barbiturate (parent compound), Tremor medicine

Common uses

Partial-onset and generalized tonic-clonic seizures (FDA)▲0▼, Essential tremor (widely used and AAN-guideline-supported as first-line alongside propranolol)▲0▼

Pharmacy

Starting dose

Seizures: 100-125 mg PO at bedtime x 3 days, then BID, then TID, escalating to 750-1500 mg/day. Essential tremor: 25-50 mg PO at bedtime, titrate slowly to 250-750 mg/day

Preparations

Tablets 50, 250 mg

US FDA Max

2 g/day (seizures); typically much lower for essential tremor

Pharmacology

Routes

Oral

Onset

Anticonvulsant effect emerges with slow titration over weeks; tremor effect over weeks

Duration

BID-TID dosing

Half-life

Primidone 5-15 hours; phenobarbital active metabolite 50-150 hours; PEMA (phenylethylmalonamide) active metabolite 16 hours

Bioavailability

~90% (oral)

Pregnancy

Substantial teratogenic signal (barbiturate class effects including neonatal withdrawal and hemorrhagic disease of newborn).^{[citation needed]}

Legal status

Rx-only in US. Federally non-controlled despite being a barbiturate, a paradoxical situation given that its primary active metabolite phenobarbital is Schedule IV

Purported mechanism

Barbiturate that metabolizes to two active species: phenobarbital (the major contributor to seizure control, mediating GABA-A receptor potentiation) and PEMA (phenylethylmalonamide), which contributes to seizure control and is the leading candidate to explain the essential-tremor efficacy. Phenobarbital alone does not reliably suppress tremor at non-sedating doses, so PEMA's independent action is the proposed mechanism for the tremor indication.▲0▼ Strong CYP3A4 induction via the phenobarbital metabolite produces many interactions (reduces oral contraceptives, warfarin, many psychotropics). Essential-tremor efficacy is the unique pharmacological selling point.