Next, these analyses demonstrate that the association between a common variant in the chromosome 8p11region is specific to nicotine dependence and does not represent a general substance dependence liability locus. These meta-analyses found that the nicotine dependence result is robust, even after adjusting for comorbidity with other substances. In contrast, none of the other three dependence diagnoses (alcohol, cocaine, marijuana) displayed a statistically significant association in the meta-analysis. The lack of association results must be interpreted with knowledge of the limitations of power. The sample sizes for the individual strata were modest. For the modest effect sizes expected for complex phenotypes such as alcohol and other drug dependence, large samples are needed to confidently rule out an association. Thus, while the evidence points to this locus having a specific risk effect associated with nicotine dependence, more modest genetic risks for alcohol, marijuana, or cocaine may be present, but beyond the limits of detection in this study.
