DRD4 and opioid use disorder: There is an inconsistent literature on DRD4 VNTR and SUD, with some studies reporting DRD4 allele variations in people with alcohol, nicotine, methamphetamine or opioid use disorders, while other studies reporting weak or no associations (see McGeary, 2009; Chen et al., 2011; Di Ciano et al., 2014).