 Nalfurafine may be a biased agonist of the KOR or a KOR subtype-selective agonist.Indeed, it has been found to act as a biased agonist of the KOR, preferring activation of β-arrestin signaling "in vitro", but paradoxically, β-arrestin appears to be responsible for KOR agonist-induced aversion, and nalfurafine furthermore shows paradoxical effects "in vivo" that are not consistent with its "in vitro" profile.As such, more research is needed to clarify the distinct mechanisms and effects of this drug.