SAG and respiratory depression: The idea that a G protein‐biased μ receptor agonist would have a reduced ‘on target’ side effect profile arises from the observation that in arrestin‐3 (β‐arrestin 2) knockout mice, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1627, the prototypical opioid agonist, showed a reduced propensity to produce respiratory depression and constipation (Raehal et al.,2005) implying that respiratory depression resulted from μ receptor coupling to arrestin‐mediated signalling.