Phenytoin

Summary

Phenytoin is a drug that works by blocking sodium channels to reduce electrical firing in neurons and heart cells.

Clinically, it is used as an antiepileptic to treat various seizures, as well as an antiarrhythmic to treat irregular heart rhythms.

Phenytoin has a narrow therapeutic range of 10 to 20 micrograms per milliliter that requires frequent blood level monitoring.

Phenytoin’s side effects include nystagmus, hirsutism, and teratogenicity. It can also cause slurred speech, confusion, ataxia, Stevens-Johnson Syndrome, nausea/vomiting, gingival hyperplasia, and sedation.

Phenytoin has many drug interactions and reduces the effectiveness of oral contraceptives. Alcohol should be avoided while taking phenytoin.

For IV administration, phenytoin must be diluted in normal saline and given via a slow IV push. Oral phenytoin should not be stopped abruptly—it must be tapered gradually under medical supervision to avoid withdrawal seizures.

Key Points

• Phenytoin (Dilantin)
  • Drug Class
    • Hydantoin
  • Mechanism
    • Blocks sodium channels
      • Slows the rate of neuron firing
  • Clinical Use
    • Treats seizures
      • Tonic-clonic seizures
      • Partial seizures
    • Treats arrhythmias
      • Class 1B antiarrhythmic
  • Phenytoin Levels
    • Therapeutic range 10-20 mcg/mL
      • Narrow therapeutic range requires frequent blood level monitoring throughout treatment
  • Side Effects
    • Toxicity manifested by CNS effects
      • Nystagmus
        • Repetitive, uncontrolled eye movements
      • Ataxia
        • Poor coordination
      • Confusion
      • Sedation & drowsiness
      • Slurred speech
    • Gingival hyperplasia
      • Common with long-term use
      • Encourage good oral hygiene and regular dentist appointments
    • Nausea and vomiting
    • Stevens-Johnson Syndrome 
      • A rash could be a symptom of Stevens-Johnson Syndrome and should be reported immediately
    • Hirsutism
      • Excessive hair growth
    • Teratogenic
      • Pregnancy Risk Category D
        • Proven to cause fetal abnormalities, and use during pregnancy should only be used if benefits outweigh the costs
  • Interactions
    • Lots of drug interactions
      • As a CYP450 inducer, phenytoin affects the way many drugs are metabolized through the liver
      • Decreases effectiveness of oral birth control
        • Recommend other forms of birth control to prevent pregnancy
    • Avoid alcohol
  • Administration
    • IV administration
      • Dilute medication in normal saline
        • Phenytoin is extremely irritating to the veins, so diluting the solution minimizes irritation
        • Do not dilute in a solution that contains dextrose. This increases risk of precipitate formation
      • Administer via slow IV push
        • Should not be administered via continuous infusion
        • Rate should be no faster than 50 mg/mL
        • Rapid infusion increases risk for hypotension and bradycardia
    • Oral administration
      • Do not stop abruptly
        • Medication dosage should be tapered down
        • If stopped abruptly, patient is at high risk for seizures