The present meta-analysis, which included 5169 subjects (2324 of whom were opioid dependent), showed no evidence of a relationship between the A118G OPRM1 SNP and opioid dependence in terms of either genotype or allele frequencies. These results both corroborate and extend previous meta-analyses which found no association between A118G and substance or opioid dependence.5,9 These authors, however, did not study the genotype frequency of A118G, only the allele frequency. In contrast, the current meta-analysis investigated both the allele and genotype frequency of A118G, and also included data from three additional studies conducted following the publication of these two meta-analyses. In comparison to the prior meta-analysis with opioid-dependent cases, the addition of the three new studies to the present meta-analysis resulted in a larger sample size (2324 cases and 2845 controls versus 1675 cases and 2465 controls9) and therefore potentially an increased power to detect a difference between the groups. Nonetheless, the overall conclusion regarding the association remained unchanged.
