Educational use only. Drug cards are AI-assisted study material for NCLEX preparation.
Mechanism of action
Stabilizes neuronal membranes by blocking voltage-gated sodium channels in their inactive state, suppressing repetitive firing. Used for tonic-clonic and partial seizures and for status epilepticus (IV fosphenytoin or phenytoin).
Adverse effects
Life-threatening / NCLEX-tested
- Phenytoin toxicity — therapeutic 10–20 mcg/mL (free 1–2); ataxia, nystagmus, slurred speech, confusion at higher levels
- Stevens-Johnson syndrome / toxic epidermal necrolysis (especially HLA-B*1502 carriers — Asian ancestry screening)
- Severe hypotension and cardiac arrhythmia with rapid IV push (max 50 mg/min adult, 25 mg/min elderly)
- Purple glove syndrome (limb ischemia from extravasation or rapid IV)
- Hepatotoxicity
- Bone marrow suppression
- Teratogenic — fetal hydantoin syndrome
Side effects
Common — what to teach
- Gingival hyperplasia (very common — meticulous oral hygiene)
- Hirsutism (especially in young women)
- Coarsening of facial features with chronic use
- Folate deficiency
- Mild ataxia, dizziness, drowsiness
- Acne
Food & drug interactions
Phenytoin is a strong CYP3A4 INDUCER — lowers levels of warfarin (initially raises, then lowers), oral contraceptives (failure risk), DOACs, statins, and many others. Many drugs raise phenytoin levels (cimetidine, fluconazole, isoniazid, valproate). Highly protein-bound — albumin matters. Enteral feeds reduce absorption — hold tube feeds 1–2 hours before and after dose.
Nursing implications
Assessment, monitoring, patient teaching
- Check phenytoin level (total 10–20 mcg/mL, free 1–2); correct level for albumin in hypoalbuminemia
- IV: large vein, dedicated line, NS only (incompatible with D5W — precipitates), max 50 mg/min adult / 25 mg/min elderly; cardiac monitor and BP cuff during infusion
- Stop tube feeds 1–2 hours before and after each oral dose — feeds bind phenytoin and reduce absorption dramatically
- Counsel meticulous oral hygiene from day 1 to limit gingival hyperplasia
- Teach patients NEVER to abruptly stop — status epilepticus risk
- Counsel women of childbearing age on contraception (phenytoin reduces OCP efficacy) and fetal hydantoin syndrome risk
- Teach early SJS warning: any new rash → stop and call immediately
When to hold / contraindications
- Phenytoin level > 20 mcg/mL with symptoms, or any level with toxicity symptoms
- New rash — possible SJS
- Severe bradycardia, second/third-degree AV block, or hypotension during IV infusion
- Severe hepatic dysfunction without dose review
- Pregnancy without specialist direction (risk vs benefit)
Memory anchor
Phenytoin 10–20 mcg/mL therapeutic. "Don't push fast, don't stop cold, don't mix with D5W, and stop tube feeds for an hour each side." Gum care from day 1.
Practice Phenytoin questions
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