OPRM1 and substance dependence: Even though the association of the A118G OPRM1 variant with specific substances is debatable (Crist and Berrettini, 2014; Sloan et al., 2018), a collaborative meta-analysis (n = 28,689 subjects with European ancestry) indicated that the G allele had a protective effect on general substance dependence (OR = 0.90; 95% CI: 0.83–0.97) (Schwantes-An et al., 2016).