**2.3. Archetypal opioids**

The archetypal opioids (morphine) and all similar extracts, as well as other opioids, affect the cerebral opioid receptor coordination [16, 17]. Tramadol is a serotonin‐norepineph‐ rine reuptake inhibitor (SNRI) through feeble μ‐opioid receptor agonist actions, whereas buprenorphine is a partial agonist of the μ‐opioid receptor [18]. Tramadol and venlafaxine are structurally very close to codeine that exerts analgesia not by individual opioid‐like prop‐ erties but through less agonism of the μ‐opioid receptor and by acting as a serotonin‐releasing agent and a norepinephrine reuptake inhibitor [19–22]. Opioids, though very active analge‐ sics, might have certain unfriendly side effects. Those patients starting morphine might expe‐ rience vomiting and nausea [23].
