Only studies investigating the association between Asn40Asp (A118G) and opioid dependence were considered for inclusion in the meta-analysis. In addition, studies had to meet the following criteria: 1. have a case-control design; 2. report Asn40Asp allele and/or genotype frequency for a group of individuals with a diagnosis of opioid dependence and also a group for a control sample; 3. present original data; and 4. have samples that were in Hardy–Weinberg equilibrium, meaning that the samples are considered representative of the populations from which they were derived. Sixteen studies met these criteria including our own unpublished data (Coller et al unpublished) and were included in the meta-analysis (for summary see Table 1), providing a total of 2324 opioid-dependent subjects and 2845 control subjects.1,2,8,11–14,16–23 It should be stated that poly-drug dependency was not an exclusion criteria per se. In reality, opioid dependents may also abuse and be dependent on other drugs (for example alcohol, nicotine, cocaine). Therefore, as long as the case subjects in the studies were dependent on opioids, regardless of the presence of other dependencies, that study was included in the meta-analysis.
