Although meta-regression failed to highlight ethnicity as a significant moderating effect of the heterogeneity, it is well known that the prevalence of the A118G polymorphism is dependent on ethnicity.5,9 Hence the data were reanalyzed following stratification by majority ethnicity, either Asian (Chinese, Indian, Malaysian) or Caucasian as these were the two prominent ethnicities studied in the literature. Results indicated that there was acceptable heterogeneity within the Caucasian studies (Figure 3, I-squared = 6.6%; χ2 (6) = 6.43, P = 0.38). In contrast however, there was still substantial heterogeneity within the Asian studies (I-squared = 73.7%; χ2 (4) = 15.22, P = 0.004). Grouping the studies with regard to Caucasian and Asian ethnicity did not alter the association between genotype frequencies and opioid dependence, with a pooled OR (95% CI) for Caucasian studies of 1.03 (0.54–1.98), P = 0.93, and for Asian studies of 1.62 (0.79–3.34), P = 0.19.
