Background and PurposeThe illicit use of fentanyl‐like drugs (fentanyls), which are μ opioid receptor agonists, and the many overdose deaths that result, has become a major problem. Fentanyls are very potent in vivo, leading to respiratory depression and death. However, the efficacy and possible signalling bias of different fentanyls is not clearly known. Here, we compared the relative efficacy and bias of a series of fentanyls.Experimental ApproachFor agonist signalling bias and efficacy measurements, Bioluminescence Resonance Energy Transfer experiments were undertaken in HEK293T cells transiently transfected with μ opioid receptors, to assess Gi protein activation and β‐arrestin 2 recruitment. Agonist‐induced cell surface receptor loss was assessed using an enzyme‐linked immunosorbent assay, whilst agonist‐induced G protein‐coupled inwardly rectifying potassium channel current activation was measured electrophysiologically from rat locus coeruleus slices. Ligand poses in the μ opioid receptor were determined in silico using molecular dynamics simulations.Key ResultsRelative to the reference ligand DAMGO, carfentanil was β‐arrestin‐biased, whereas fentanyl, sufentanil and alfentanil did not display bias. Carfentanil induced potent and extensive cell surface receptor loss, whilst the marked desensitisation of G protein‐coupled inwardly rectifying potassium channel currents in the continued presence of carfentanil in neurones was prevented by a GRK2/3 inhibitor. Molecular dynamics simulations suggested unique interactions of carfentanil with the orthosteric site of the receptor that could underlie the bias.Conclusions and ImplicationsCarfentanil is a β‐arrestin‐biased opioid drug at the μ receptor. It is uncertain how such bias influences in vivo effects of carfentanil relative to other fentanyls.