A patient treated with tramadol for pain has a seizure and shows signs of serotonin toxicity. Which pharmacological properties of tramadol explain both adverse effects?

• A Tramadol is a direct serotonin receptor agonist at 5-HT3 and 5-HT1A receptors and inhibits GABA-A channels, explaining both effects
• B Tramadol's active metabolite M1 is a potent kappa agonist that paradoxically increases CNS excitability in rapid metabolizers
• C Tramadol inhibits serotonin and norepinephrine reuptake (SNRI activity) and its O-desmethyl metabolite (via CYP2D6) is a mu-opioid agonist; serotonin excess causes serotonin syndrome and opioid-mediated CNS excitation lowers seizure threshold ✓
• D Tramadol inhibits glycine receptors in the spinal cord, producing myoclonus misidentified as seizure, and activates 5-HT2A receptors causing serotonin syndrome

Explanation

Tramadol has a dual mechanism: (1) The parent molecule and its enantiomers inhibit serotonin and norepinephrine reuptake (SNRI activity), elevating synaptic serotonin — placing tramadol at risk for serotonin syndrome especially when combined with other serotonergic agents (SSRIs, MAOIs, linezolid, triptans). (2) Tramadol undergoes O-demethylation by CYP2D6 to generate the active metabolite O-desmethyltramadol (M1), which is a full mu-opioid receptor agonist with ~200 times greater MOR affinity than the parent drug. The seizure risk from tramadol is multifactorial: it reduces GABA-mediated inhibition, lowers seizure threshold at the spinal cord level, and its serotonergic activity can generate epileptiform activity. CYP2D6 ultrarapid metabolizers are at greater opioid toxicity risk; poor metabolizers lose analgesic efficacy but may still experience serotonin toxicity from the parent drug.

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

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Written and medically reviewed by the StethoPrep medical team.