A total of 2324 opioid-dependent (case) and 2845 control subjects from 16 studies were included in the meta-analysis. The age (mean ± SD) and percentage of males in the opioid-dependent and control groups were 33 ± 8 and 34 ± 10 years, respectively, and 78% and 56%, respectively. Table 2 summarizes the A118G allele and genotype data from each individual study.
