Opioid-receptor Agonist/Norepinephrine and Serotonin Reuptake Inhibitors

Tramadol is a weak, centrally acting μ-receptor agonist/norepinephrine and serotonin reuptake inhibitor.33 After oral administration, tramadol is readily absorbed from the gastrointestinal tract. It reaches appreciable plasma concentrations in 60 minutes. Tramadol’s bioavailability is high and it is readily distributed from the vascular compartment to all tissues. Tramadol is metabolized in the liver.33 The unchanged fraction of the drug and its metabolites are excreted in the kidneys.33

Tapentadol is also a weak, centrally acting μ-receptor agonist/norepinephrine and serotonin reuptake inhibitor.34 It is rapidly absorbed after oral administration, although the bioavailability of the drug due to first-pass metabolism is low.34,35 Tapentadol is readily distributed from the vascular compartment to all tissues. The primary metabolic pathway of tapentadol is hepatic glucuronidation.34,35 Tapentadol and its metabolites are rapidly and completely excreted in the kidneys.34,35