Tramadol produces analgesia through two distinct mechanisms. A 35-year-old rapid metaboliser on tramadol for chronic pain develops unexpected opioid toxicity (respiratory depression, miosis). This is most likely because:

• A Rapid CYP3A4 metabolism converts tramadol to N-desmethyltramadol, which accumulates and causes toxicity
• B Rapid CYP2D6 metabolism converts tramadol to O-desmethyltramadol (M1), which has 200 times higher mu receptor affinity than tramadol itself ✓
• C Rapid CYP2C9 metabolism prevents norepinephrine reuptake inhibition, increasing mu-opioid pathway activity
• D Ultra-rapid CYP2D6 metabolism reduces tramadol levels and simultaneously increases serotonin reuptake inhibition

Explanation

Tramadol exerts weak mu opioid agonism itself and also inhibits norepinephrine/serotonin reuptake. CYP2D6 converts tramadol to O-desmethyltramadol (M1), which has approximately 200 times greater affinity for mu opioid receptors. Ultra-rapid CYP2D6 metabolisers produce excessive M1, causing full opioid-toxicity symptoms. This is clinically significant in breastfed neonates (whose mothers are ultra-rapid CYP2D6 metabolisers) and in post-tonsillectomy paediatric patients — the FDA issued a black-box warning against tramadol in these settings.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.