Class: Hydantoin Anticonvulsant
Dosage Forms. Capsule: 30 mg, 100 mg, 200 mg, 300 mg; Chewable Tablet: 50 mg; Suspension: 125 mg/5 mL
Common FDA Label Indication, Dosing, and Titration.
1. Seizure, generalized tonic-clonic, complex partial, or following neurosurgery, treatment, and prophylaxis: Adults, 100 mg po tid, may titrate to 200 mg po tid; Children, 5 mg/kg/d po divided into 2-3 doses, may titrate to 300 mg/d
1. Diabetic nephropathy: 5-6 mg/kg/d po
MOA. Phenytoin is a hydantoin that suppresses the spread of seizure activity mainly by inhibiting synaptic post-tetanic potentiation and blocking the propagation of electric discharge. Phenytoin might decrease sodium transport and block calcium channels at the cellular level to produce these actions.
Drug Characteristics: Phenytoin
Medication Safety Issues: Phenytoin
Drug Interactions: Phenytoin
Adverse Reactions: Phenytoin
Efficacy Monitoring Parameters. Reduction in the frequency and severity of seizures; phenytoin serum level range 10-20 mcg/mL (obtain after at least 5-7 half-lives after treatment initiation or dosage change).
Toxicity Monitoring Parameters. Emergence or worsening of depression, suicidal behavior or ideation, or unusual changes in behavior; monitor CBC and LFTs.
Key Patient Counseling Points. Do not crush extended-release capsules. Avoid activities requiring mental alertness or coordination until drug effects are realized. Report signs/symptoms of pancytopenia, hepatotoxicity, systemic lupus erythematosus, or severe skin reaction. Do not drink alcohol while taking this drug. Many drug interactions; check with pharmacist when starting new medications or OTC products.
Clinical Pearls. Highly protein bound, so albumin levels should be taken into account when measuring phenytoin concentration; dose adjustment based on free phenytoin concentration. Injectable formulation available, but not for use IM (causes “purple glove syndrome” related to tissue necrosis). Medication guide required at dispensing.