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Codeine CYP2D6 ultrarapid metabolism — MRCPCH TAS MCQ

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ModeratePharmacologyCodeine CYP2D6 ultrarapid metabolismMRCPCH TAS

A child given codeine after tonsillectomy becomes excessively sleepy with shallow breathing. Genetic testing later shows high CYP2D6 activity. What is the most likely mechanism?

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Correct answer: D — Rapid conversion of codeine to morphine

Codeine is a prodrug converted to morphine by CYP2D6, and ultrarapid metabolisers can develop opioid toxicity. Prostaglandin inhibition describes non-steroidal anti-inflammatory drugs, not codeine. The pearl is that pharmacogenetic variation can make a standard dose unsafe in susceptible children.

Reference: BNFc; Nelson Textbook of Pediatrics; RCPCH Theory examination syllabi