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Phenytoin Toxicity — RACP Adult Medicine MCQ

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EasyClinical PharmacologyPhenytoin ToxicityRACP Adult Medicine

A 45-year-old man with epilepsy on phenytoin is found to have a phenytoin level of 35 mg/L (therapeutic range 10–20 mg/L). He reports nystagmus and ataxia. What characteristic of phenytoin's pharmacokinetics explains why small dose increases can produce large increases in serum levels?

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Correct answer: D — Zero-order (saturable) elimination at therapeutic doses

Phenytoin exhibits non-linear (Michaelis-Menten) pharmacokinetics because hepatic metabolism becomes saturated at therapeutic doses, resulting in zero-order elimination. This means that even small dose increments can cause disproportionately large increases in plasma levels, leading to toxicity. Dose adjustments should be made in small increments (25–50 mg) with monitoring.

Reference: AMH – 2025 – Antiepileptics; eTG – 2025 – Neurology