Phenytoin

General

• Type: Anticonvulsants
• Dosage Forms: capsules 50, 100, 200, 300 125/5 mL IM/IV
• Common Trade Names: Dilantin, Phenytek

Adult Dosing

Status epilepticus

• Loading dose: 15-20mg/kg IV; if ineffective, can give 10mg/kg IV 20 min after first dose
• Maintenance dose: 100mg PO/IV q 6-8hrs
• Start 12 hours after loading dose

Seizure disorder

• 300-400mg/day divided into 2-3 doses/day (Max 400mg/dose)
• May give 4-7mg/kg/day divided into 2-3 doses/day
• Consider loading dose: 15-20mg/kg divided into 2-3 doses given 2-4 hrs apart
• Adjust dose on response and serum levels
• Available in extended or immediate release

Seizure prophylaxis (neurosurgery)

• Loading dose: 10-20mg/kg IV in 3 doses 2-4 hrs apart (max 400mg/dose)
• 4-6mg/kg/day IV divided into 2-3 doses/day
• Can give IM route, though IV prefered
  • 100-200mg IM q 4 hrs x 1 week
  • 150% PO dose by IM route if previously on medication

Pediatric Dosing

Status epilepticus

• Loading dose: 15-20mg/kg IV; if ineffective, can give 10mg/kg IV 20 min after first dose
• Max 1500mg/day
• Start maintenance 12 hrs after loading dose

Seizure disorder

All doses are divided into 2-3 doses/day; always start at 5mg/kg/day and titrate based on effect and serum levels

• <6 mo: 5-8mg/kg/day PO/IV
• 6mo-4 yrs: 8-10mg/kg/day PO/IV
• 4-7yrs: 7.5-9mg/kg/day PO/IV
• 7-10 yrs: 7-8mg/kg/day PO/IV
• 10-16 yrs: 6-7mg/kg/day PO/IV
• >16 yrs use adult dosing
• For immediate release. Extended release available for ages >7 at same dosages.

Special Populations

• Pregnancy Rating: D
• Lactation: Likely safe; limited information. Use caution.
• Renal Dosing
  • Adult: No oral loading regimen
  • Pediatric: No adjustment
• Hepatic Dosing
  • Adult: No oral loading regimen
  • Pediatric: Decrease dose, amount not defined

Contraindications

• Allergy to class/drug
• Sinus brady/SA block/2nd-3rd AV block
• Adams-Stokes syndrome
• Avoid abrupt withdrawl
• Caution:
  • CV disease
  • Hypotension
  • Alcohol use
  • HLA-B 1502 positive
  • Renal impairment
  • Hepatic impairment
  • Porphyria
  • Diabetes mellitus
  • Thyroid disease
  • Depression
  • Elderly

Adverse Reactions

See phenytoin toxicity for overdose

Toxicity symptoms by phenytoin level^

Level	Sypmtoms
>10	Usually no symptoms
10-20	Occasional mild nystagmus
20-30	Nystagmus
30-40	Ataxia, slurred speech, Nausea/vomiting
40-50	Lethargy, confusion
>50	Coma, seizure (rare)

^Provides a rough guide only; neither sensitive nor specific

Serious

• V-Fib (IV)
• Hypotension (IV)
• CV collapse (IV)
• AV conduction disturbance (IV)
• Hepatotoxicity
• Thrombocytopenia
• Leukopenia
• Agranulocytosis
• Pancytopenia
• Megaloblastic anemia
• Stevens-Johnson syndrome
• Toxic epidermal necrolysis
• Drug rash (with eosinophilia)
• Tissue necrosis (IV)
• Purple glove syndrome (IV)
• Anaphylaxis
• Lymphoma
• Lupus
• Osteomalacia
• Toxic delirium
• SI
• Periarteritis nodosa
• Withdrawal seizures with abrupt discontinuation

Common

• Nausea/vomiting
• Rash
• Nystagmus
• Ataxia
• Slurred speech
• Dizziness
• Confusion
• Paresthesia
• Blurred vision
• Somnolence
• Constipation
• headache
• Insomnia
• Gingival hyperplasia
• Tremor
• Lymphadenopathy
• Coarse facies
• Hyperglycemia
• Osteomalacia
• Phlebitis (IV)
• Peyronie disease
• Taste change

Pharmacology

• Half-life: 7-42 hrs
• Metabolism: liver; CYP450: 2C9 and 2C19
• Excretion: bile and urine
• Mechanism of Action: effects neuronal voltage-dependent NA and CA channels

See Also

• Anticonvulsants
• Seizure levels and reloading
• Phenytoin toxicity

References

• Epocrates
• Medscape

Authors

Colin Hoff, Ross Donaldson, Neil Young, Daniel Ostermayer