It should be emphasized that significant heterogeneity exists in the current body of case-control studies investigating the association between the A118G SNP and opioid dependence, which may have prevented the association from being reliably investigated. The heterogeneity detected in the present meta-analysis was not fully accounted for, even when studies were separated into majority Caucasian and Asian ethnicity groups. In fact this stratification only reduced heterogeneity levels to within acceptable limits for one of our four analyses, namely the genotype frequency of A118G in majority Caucasian studies, but did not alter the observation that there was no association between the SNP and opioid dependence. In the case of the majority Asian analysis, this may be explained by the inclusion of an Indian study.13 This study yielded the largest OR of A118G between opioid-dependent and control groups with regard to both allele and genotype frequency (Figures 1, 2, 4, 6). As India is not part of mainland Asia, its inhabitants have a different ancestral and hence genetic background and thus it may not be a valid inclusion. However, it should be
