skip to main content

Phenytoin Level Adjustment — RACP Adult Medicine MCQ

Instant feedback + full explanation. One question, done properly.

ModerateNeurologyPhenytoin Level AdjustmentRACP Adult Medicine

A 40-year-old woman on phenytoin for epilepsy has been seizure-free for 2 years. Phenytoin level is 85 μmol/L (therapeutic 40-80). She has nystagmus on lateral gaze but no ataxia. She reports some gum overgrowth. What is the most appropriate management?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Reduce phenytoin dose and recheck level in 5-7 days

Mildly supratherapeutic phenytoin (85, range 40-80) with early toxicity signs (nystagmus) and chronic side effects (gingival hyperplasia) warrants a small dose reduction. Phenytoin has zero-order kinetics — small dose changes cause disproportionate level changes. Reduce by 25-50 mg and recheck in 7 days. Consider switching to an alternative AED (levetiracetam, lamotrigine) given chronic side effects. Never abruptly cease AEDs (seizure risk).

Reference: AMH – 2025 – Phenytoin; eTG Neurology 2024