The A118G variant has emerged as a prime candidate for genes associated with the risk of addiction; however, the vast discrepancy between published results needs to be addressed. This may be because the majority of the current literature is restricted to data collected from classical case-control studies, which may possibly lead to false negatives or false positives depending on the types of approach used, thus making results difficult to compare.9 Meta-analysis allows a substantially larger sample size to be analysed and therefore allows a greater statistical power. Two recent meta-analyses have attempted to elucidate the nature of the relationship between the A118G polymorphism and either opioid9 or substance5 dependence. Although neither study found an overall association, both studies only evaluated an allelic association. Furthermore, additional case-control studies have been conducted since these publications,13,14 including data from our laboratory. Therefore, capitalizing on the power of meta-analysis, the aims of the present study were to reinvestigate the association between A118G allele and genotype frequencies and the occurrence of opioid dependence.
