No difference in genotype frequencies between opioid-dependent and control groups was observed, with a pooled OR (95% CI) from the 13 studies of 1.28 (0.77–2.11), P = 0.34 (Figure 1). In addition, there was no evidence of publication bias as indicated by the funnel plot (Figure 2) and Egger’s test value (t = −0.12, P = 0.91). The high level of heterogeneity (I-squared = 52.4%; χ2 (12) = 25.18, P = 0.014) suggested the presence of some potential moderator variable(s). However, meta-regression models failed to identify significant moderating effects by the following potential explanatory variables: 50% or more participants being Caucasian (yes/no) P = 0.66; 50% or more participants being Asian (yes/no) P = 0.33; mean age (continuous) P = 0.51; type of drug dependence (opioid/heroin) P = 0.07; and percentage of males (continuous) P = 0.13.
