OPRM1 and cocaine dependence: [11C]carfentanil is a MOR selective PET ligand which has been used to investigate MOR availability in a range of addictions including alcohol (Bencherif et al., 2004; Heinz et al., 2005; Turton et al., 2020) and cocaine dependence (Ghitza et al., 2010; Gorelick et al., 2008) and gambling disorder (Mick et al., 2016).