Generic Name

Phenytoin Sodium

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Classification

FDA Approved Indications

• Partial seizures
• Tonic-clonic seizures
• Status epilepticus (adjunct)

Mechanism of Action

Stabilizes neuronal membranes by reducing neuronal excitability through inhibition of voltage-gated sodium channels, thus decreasing seizure activity.

Dosage and Administration

Adult: Initial dose: 100 mg orally three times daily; adjust based on serum levels and clinical response.

Pediatric: Initial dose: 5 mg/kg/day divided into two to three doses; dosage adjusted based on serum levels.

Geriatric: Start at lower doses due to increased sensitivity and comorbidities; monitor serum levels closely.

Renal Impairment: Adjust dose accordingly; monitor serum levels.

Hepatic Impairment: Use with caution; metabolism may be impaired, leading to increased serum levels.

Pharmacokinetics

Absorption: Variable; peak serum levels occur 1-2 hours after oral dose, but may be delayed in some formulations.

Distribution: Extensively protein-bound (~90%), primarily to serum albumin.

Metabolism: Metabolized in the liver via hepatic enzymes, mainly CYP2C9 and CYP2C19.

Excretion: Excreted primarily in urine; metabolites also excreted.

Half Life: Initially 8-60 hours, decreased to approximately 7-22 hours with chronic use due to enzyme induction.

Contraindications

• Hypersensitivity to phenytoin
• Bradycardia or sinoatrial block

Precautions

• Cardiac disease, pregnancy, liver disease, history of depression or mental health issues, monitor for skin reactions, gingival hyperplasia, hirsutism, osteoporosis. Use with caution during pregnancy due to teratogenicity (fetal hydantoin syndrome).

Adverse Reactions - Common

• Gingival hyperplasia (Common)
• Nystagmus (Common)
• Ataxia (Common)
• Diplopia (Common)
• Nausea and vomiting (Common)

Adverse Reactions - Serious

• Stevens-Johnson syndrome (Rare)
• Blood dyscrasias (aplastic anemia, agranulocytosis) (Rare)
• Hepatotoxicity (Rare)
• Cardiac arrhythmias and hypotension (especially with IV use) (Rare)

Drug-Drug Interactions

• -warfarin
• oral contraceptives
• carbamazepine
• valproic acid
• phenytoin can induce hepatic enzymes, decreasing the effectiveness of many drugs.

Drug-Food Interactions

• decreased absorption with enteral feeding or antacids containing aluminum or magnesium.

Drug-Herb Interactions

• St. John's Wort may reduce phenytoin levels.

Nursing Implications

Patient/Family Teaching

• Take medication exactly as prescribed, at the same times each day.
• Inform about potential side effects and when to seek medical help.
• Maintain good oral hygiene to prevent gingival hyperplasia.
• Avoid alcohol and CNS depressants unless approved by a healthcare provider.

Special Considerations

Black Box Warnings:

• Potential for cardiovascular risk with rapid IV administration; administer slowly (not exceeding 50 mg/min in adults).

Genetic Factors: Slow and rapid metabolizers; genetic testing can predict metabolism rate.

Lab Test Interference: Can interfere with indirect Coombs test, may cause false-positive or negative results.

Overdose Management

Signs/Symptoms: Dizziness, nystagmus, ataxia, mental confusion, coma, severe hypotension, arrhythmias.

Treatment: Supportive care, Activated charcoal if ingestion was recent, Intravenous bicarbonate for cardiac toxicity, Hemodialysis in severe cases, keep airway clear and monitor cardiac function.

Storage and Handling

Storage: Store at room temperature, away from moisture, heat, and light.

Stability: Stable for 2-3 years under proper storage conditions.