Drug Interaction Check — ORE Part 1 MCQ

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EasyPharmacology & TherapeuticsDrug Interaction CheckORE Part 1

A patient on long-term phenytoin therapy presents with a serum phenytoin level in the therapeutic range. They require a dental extraction and you plan to prescribe amoxicillin post-operatively. Is there a significant drug interaction?

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Correct answer: C — No significant interaction between amoxicillin and phenytoin

Amoxicillin has no clinically significant interaction with phenytoin. Phenytoin is metabolised by CYP2C9/CYP2C19, and problematic interactions occur with drugs that inhibit these enzymes (e.g. fluconazole, isoniazid, cotrimoxazole), not with amoxicillin. Macrolide antibiotics do interact with phenytoin and increase its levels, but penicillins do not. Option B (reduces levels) and C (increases levels) are incorrect because amoxicillin does not alter CYP enzyme activity. Option E (contraindicated) and E (makes antibiotic ineffective) are implausible; these combinations are routinely prescribed in UK dental and medical practice without adverse outcome. This tests understanding of CYP-mediated interactions rather than trivia.

Reference: Gautam P, Gautam R, Rattan A, et al. Cytochrome P450 isozymes and antiepileptic drug interactions. Published 1997 in Drug Metabolism Reviews. https://pubmed.ncbi.nlm.nih.gov/8806399/