One underlying issue might be lack of statistical power in individual studies, if rs1799971 has a relatively small effect size. Schwantes-An et al. meta-analyzed rs1799971 genotype in the context of substance dependence (nicotine, alcohol, cannabis, cocaine, and/or opioid) in a large population of European descent (case n = 9064, control n = 7844) [17*]. They found a small (odds ratio (OR) = 0.90), but significant, effect of genotype on substance dependence risk. In analyses of the individual substances, rs1799971 was found to have similar effect sizes regardless of drug. However, none of these associations were significant due to the reduced sample size (opioid dependence: case n = 2139, control n = 5168; OR = 0.84).
