For alcohol, polymorphisms of the gene that encodes the KOR (Opkr1) have been associated with the severity of alcohol use disorder and depression [156], whereas polymorphisms of the Pdyn gene have been associated with alcohol dependence [157] and the propensity to drink in negative emotional states [158], an effect that appears to be more prominent in men [159]. Here, OPRK1 is linked to major depressive disorder.