A vast number of studies have been reported the association of variants of the opioid system genes and drug addiction-related phenotypes, but the results are not always consistent. This inconsistency may be explained by several factors, including inconsistency in phenotyping, severity of diagnosis, small sample size, inadequate statistics, ethnic heterogeneity and population stratification, large phenotype range, and different diagnostic criteria. The majority of studies have used individual single nucleotide polymorphism (SNP) analysis, and several studies have used hypothesis-based multi-SNP arrays that are based on linkage disequilibrium (LD)-tagging SNPs and capture a substantial proportion of common genetic variation (e.g. Hodgkinson et al. 2008; Levran et al. 2008; Levran et al. 2009; Maher et al. 2011). In this article, we will define SNP as a variation in a single nucleotide of any allele frequency (see dbSNP). Linkage studies and genome-wide association studies are beyond the scope of this review. The studied populations include Asian (Japanese, Chinese, Taiwanese, and Indians) (As); European (E); European American (EA); African (including African Americans, AA); Native American (NA); and Hispanic (His). Some studies have mixed populations and only a few studies have applied methods to control for population stratification.
