OPRM1 and Respiratory insufficiency due to muscle weakness: Despite its prevalence, the <i>in vivo</i> effects of furanylfentanyl are not well characterized, especially regarding respiratory depression.<h4>Objectives</h4>To characterize furanylfentanyl relative to fentanyl <i>in vitro</i> and compare the effects of both drugs in mouse models of antinociception and respiratory depression.<h4>Methods</h4>The affinity and potency of fentanyl and furanylfentanyl were determined in CHO cells overexpressing MOR.