Several studies have tested the relationship between the A118G SNP of the OPRM1 gene and substance use disorders, particularly alcoholism and opioid dependence and the results are largely inconsistent. While some investigations have found support for the association between the A118G SNP and alcohol or opioid dependence (Kranzler et al., 1998; Schinka et al., 2002; Tan et al., 2003; Town et al., 1999), several studies have failed to find an association (Arias et al., 2005; Bergen et al., 1997; Crowley et al., 2003; Franke et al., 2001; Gelernter et al., 1999; Loh et al., 2004). However, recent laboratory-based research used an endophenotype-driven approach to studying the effects of the A118G SNP of the OPRM1 gene. Instead of measuring alcohol dependence per se, this study tested the association between this SNP and measures of subjective response to alcohol (Ray & Hutchison, 2004). Results revealed that individuals with at least one copy of the G allele showed greater subjective response to the effects of alcohol measured by subjective intoxication, sedation and stimulation, and changes in mood states, as compared to participants who
