DB09173 (Butyrfentanyl or butyrylfentanyl) is a potent, short-acting mu opioid receptor agonist, and an analog of fentanyl, differing by only one methyl group. It has no approved medical indications but is being used for recreational purposes, with cases of fatal overdoses reported in Europe and the United States. Pre-clinical studies of butyrfentanyl are scarce; however, the few available studies suggest that butyrfentanyl is about 30 times less potent than fentanyl itself, and has significant antinociceptive properties, as demonstrated by the acetic acid writhing test in rodents. DB09173 is being abused for its opioid effects. As with other mu-opioid agonists, it can induce respiratory depression which may lead to death and numerous deaths have been reported. No studies of butyrfentanyl dependence or cross-dependence conducted in humans could be identified.

6-alpha-Naloxol is active metabolite of naloxone. 6-alpha-Naloxol was shown to be neutral antagonist at the mu receptor in vitro, with no affect on cAMP levels or GTPitalic gammaS binding, regardless of morphine pretreatment. It elicits withdrawal behaviour and conditioned place aversion in morphine pretreated rodents.