Tramadol is a weak opioid with additional analgesic mechanisms. Which combination of mechanisms explains its efficacy in moderate pain when pure opioid doses would be insufficient?

• A Kappa-opioid agonism plus NMDA receptor blockade
• B Mu-opioid agonism plus COX-2 inhibition
• C Delta-opioid agonism plus GABA-B potentiation
• D Mu-opioid receptor agonism plus inhibition of serotonin and noradrenaline reuptake (similar to SNRI mechanism) ✓

Explanation

Tramadol acts by two complementary mechanisms: (1) weak mu-opioid receptor agonism (parent drug and its active O-desmethyl metabolite, M1, via CYP2D6 conversion) and (2) inhibition of neuronal serotonin and noradrenaline reuptake, enhancing descending inhibitory pain pathways. The dual mechanism produces additive analgesia. This combination also explains why tramadol causes serotonin syndrome risk when combined with SSRIs/MAOIs, and seizures at high doses (lowered seizure threshold via serotonergic and opioid mechanisms).

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

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