Drug Guide
Phenytoin
Classification
Therapeutic: Anticonvulsant, Antiepileptic
Pharmacological: Hydantoin Derivative
FDA Approved Indications
- Partial seizures (focal seizures)
- Tonic-clonic seizures (grand mal seizures)
- Prevention of seizures following neurosurgery
Mechanism of Action
Phenytoin stabilizes neuronal membranes and inhibits excessive neuronal firing by blocking voltage-gated sodium channels, thereby reducing seizure activity.
Dosage and Administration
Adult: Initial dose: 100 mg orally three times daily. Maintenance dose varies; typically 300-600 mg/day divided into 2-3 doses. Adjust based on serum levels and clinical response.
Pediatric: Initial: 5 mg/kg/day divided into 2-3 doses; maintenance individualized based on serum levels.
Geriatric: Start at lower doses due to increased sensitivity and comorbidities; monitor serum levels closely.
Renal Impairment: Use with caution; dosage adjustments may be necessary.
Hepatic Impairment: Use cautiously; monitor liver function and serum levels.
Pharmacokinetics
Absorption: Well absorbed orally, but bioavailability can vary (about 90%).
Distribution: Widely distributed; crosses the blood-brain barrier; protein-bound (~90%).
Metabolism: Primarily hepatic via CYP2C9 and CYP2C19 pathways.
Excretion: Metabolites excreted in urine; minor unchanged drug excreted renally.
Half Life: 9-16 hours initially; decreases with chronic use to about 7-8 hours due to autoinduction.
Contraindications
- Hypersensitivity to phenytoin or hydantoin derivatives.
- Sinus bradycardia, sinoatrial block, conduction disturbances.
- Pediatric patients with lactation (for some formulations).
Precautions
- Use cautiously in hepatic impairment, cardiac disease, and in pregnancy due to teratogenicity.
- Monitor for suicidal ideation.
Adverse Reactions - Common
- Gingival hyperplasia (Common)
- Diplopia (Common)
- Ataxia (Common)
- Rash (Common)
Adverse Reactions - Serious
- Stevens-Johnson syndrome (Rare)
- Cardiac arrhythmias, hypotension (with IV use) (Rare)
- Hepatotoxicity (Rare)
- Blood dyscrasias (aplastic anemia, agranulocytosis) (Rare)
Drug-Drug Interactions
- Warfarin (decreases INR),
- Oral contraceptives (decreased efficacy),
- Valproic acid (reduces phenytoin levels),
- Carbamazepine (autoinduction and competition)
Drug-Food Interactions
- Alcohol (increases toxicity)
Drug-Herb Interactions
N/ANursing Implications
Assessment: Monitor serum phenytoin levels (target 10-20 mcg/mL), observe for signs of toxicity, check for gingival hyperplasia and rash.
Diagnoses:
- Risk for injury related to ataxia or diplopia.
- Risk for bleeding due to blood dyscrasias.
Implementation: Administer with meals to reduce gastrointestinal upset. Regularly monitor serum levels and hepatic function.
Evaluation: Therapeutic response in seizure control, monitor for adverse effects, adjust dose accordingly.
Patient/Family Teaching
- Take medication exactly as prescribed, do not alter dose without consulting provider.
- Maintain good oral hygiene to prevent gingival hyperplasia.
- Report signs of rash, confusion, bleeding, or unusual symptoms immediately.
- Avoid alcohol and abrupt discontinuation.
Special Considerations
Black Box Warnings:
- Serious dermatologic reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis).
- Risk of cardiotoxicity with IV administration.
Genetic Factors: Genetic polymorphisms in CYP2C9 can affect metabolism and serum levels.
Lab Test Interference: Can interfere with blood glucose measurement (give samples in tube separate from glucose-containing beverages).
Overdose Management
Signs/Symptoms: Ataxia, nystagmus, confusion, slurred speech, hypotension, coma.
Treatment: Supportive care, activated charcoal if ingestion is recent, hemodialysis in severe cases, and correction of electrolytes, cardiac monitoring.
Storage and Handling
Storage: Store at room temperature, away from light and moisture.
Stability: Stable when stored properly for the duration specified in the package insert.