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Phenytoin toxicity interaction — GPhC CRA MCQ

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ModerateCNS TherapeuticsPhenytoin toxicity interactionGPhC CRA

A 64-year-old man with epilepsy takes phenytoin and attends after starting co-trimoxazole from an urgent care centre. He reports unsteady gait and slurred speech. What is the most important interaction to flag?

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Correct answer: E — Co-trimoxazole can increase phenytoin exposure and toxicity risk

Phenytoin has a narrow therapeutic index and toxicity can present with ataxia, slurred speech and nystagmus. Co-trimoxazole can increase phenytoin exposure in some patients. The patient requires prescriber review and consideration of phenytoin levels and an alternative antibiotic.

Reference: BNF