OPRM1 and Euphoria: This elicits three main effects; withdrawal symptoms are managed and suppressed due to the occupation of mu-opioid receptors; the long-acting effect of these drugs minimises opioid-induced euphoria; and by competitively binding to the mu-opioid receptors, additional concomitant use of non-medical or illicit opioids will have no euphoric effect [22,37,38].