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phenytoin

N03AB02

Forms & Strengths

Oral Capsule: 100 mg
Chewable Tablet: 50 mg
Oral Suspension: 125 mg/5 mL
Injection: 50 mg/mL in 2 mL ampules/vials

Adult Dosing

Status Epilepticus (IV): Loading dose 15-20 mg/kg IV at a maximum rate of 50 mg/min
Maintenance (Oral/IV): 100 mg 3 times daily, adjusted based on plasma concentrations (target 10-20 mcg/mL)

Pediatric Dosing

Status Epilepticus (IV): Loading dose 15-20 mg/kg IV at a maximum rate of 1-3 mg/kg/min
Maintenance (Oral/IV): Initial 5 mg/kg/day in 2-3 divided doses; maintenance usually 4-8 mg/kg/day

Indications

Management of tonic-clonic (grand mal) and complex partial seizures
Prevention and treatment of seizures occurring during or following neurosurgery

Mechanism of Action

Promotes sodium efflux from neurons, stabilizing the neuronal membrane against depolarization
Decreases post-tetanic potentiation at synapses, limiting seizure propagation

Contraindications

Hypersensitivity to phenytoin or other hydantoins
History of previous phenytoin-induced acute hepatotoxicity
Co-administration with delavirdine
Sinus bradycardia, sinoatrial block, 2nd and 3rd degree AV block, and Stokes-Adams syndrome (for IV formulation)

Adverse Reactions

Nystagmus, ataxia, slurred speech, dizziness, confusion
Gingival hyperplasia, hirsutism, acne
Stevens-Johnson syndrome, toxic epidermal necrolysis, rash
Osteomalacia and decreased bone mineral density with long-term use
Cardiovascular collapse and severe hypotension with rapid IV administration

Drug Interactions

Strong inducer of CYP3A4, CYP2C9, CYP2C19, and UGT, decreasing efficacy of oral contraceptives, warfarin, and corticosteroids
Plasma levels increased by CYP inhibitors (e.g., amiodarone, fluconazole)
Plasma levels decreased by inducers (e.g., rifampin)
Highly protein-bound; displaced by or displaces other tightly bound drugs (e.g., valproic acid)

Curated Content: Needs Vetting Before Put to Clinical Use