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Phenytoin Zero-Order Kinetics — SCE Neurology MCQ

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ModerateEpilepsy & Seizure DisordersPhenytoin Zero-Order KineticsSCE Neurology

A 55-year-old man with epilepsy on phenytoin 300 mg daily has a level of 8 mg/L. His dose is increased by 50 mg. One week later he has nystagmus, ataxia, and slurred speech with a level of 35 mg/L. What pharmacokinetic principle explains this?

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Correct answer: E — Zero-order (saturation) kinetics

Phenytoin follows zero-order (saturation) kinetics at therapeutic doses. Small dose increases cause disproportionately large rises in serum concentration once hepatic enzymes are saturated. Clinical features of toxicity include nystagmus (first sign), ataxia, dysarthria, and drowsiness. A: First-order kinetics would produce proportional increases. C: The relationship is non-linear. D: Enzyme induction does not explain the disproportionate rise. E: Phenytoin is hepatically metabolised.

Reference: BNF – Phenytoin; Clinical Pharmacology