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Phenytoin and doxycycline — GPhC CRA MCQ

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ModerateDrug InteractionsPhenytoin and doxycyclineGPhC CRA

A patient taking phenytoin for epilepsy is prescribed doxycycline for acne. His seizures are controlled and phenytoin level was stable at last review. He asks if there is any issue. What is the most important interaction to flag?

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Correct answer: A — Phenytoin can reduce doxycycline exposure

Phenytoin is an enzyme inducer and can reduce exposure to doxycycline, potentially reducing antibiotic effectiveness. The prescriber may need to consider an alternative or treatment response monitoring. The patient should be advised to report lack of improvement.

Reference: BNF