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Fentanyl context-sensitive half-time — FRCA Primary MCQ

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HardPharmacologyFentanyl context-sensitive half-timeFRCA Primary

A patient receiving a fentanyl infusion becomes unexpectedly bradypnoeic after fluconazole is started. Renal function is unchanged. Which mechanism most plausibly accounts for the change?

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Correct answer: D — Reduced CYP3A-mediated fentanyl clearance increases opioid exposure

The best answer is “Reduced CYP3A-mediated fentanyl clearance increases opioid exposure”. Fentanyl is substantially metabolised through hepatic CYP3A pathways, so a CYP3A inhibitor can increase or prolong exposure and respiratory depression. This is not a renal-clearance interaction, and fentanyl is not dependent on plasma cholinesterase or glucuronidation to an active morphine-like metabolite. The practical response includes reassessment, dose reduction or interruption and ventilatory monitoring.

Reference: Fentanyl 50 micrograms/mL solution for injection, UK SmPC: https://www.medicines.org.uk/emc/product/3387/smpc; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf