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

Common

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

References

  • Epocrates
  • Medscape