DRD2 and substance abuse: Results indicate that the increased phenotype of cases requires an individual to be either heterozygous at both loci or homozygous at locus B with homozygous risk factor A1A1 present. We hypothesize that overlapping expressions of CNR1 and DRD2 are the causes of CNR1-DRD2 interactions in cases of substance abuse, and the different polymorphisms of CNR1 and DRD2 genes may have decisive roles in the nature of these interactions in terms of promoting or alleviating the cannabis addiction risk factor of the individual.