Eligible studies had to meet the following criteria: they (1) were published in peer-reviewed journals; (2) contained original data; (3) presented sufficient data to calculate the odds ratio (OR) with confidence interval (CI) and P value; (4) were association studies investigating the specific SNP considered here; (5) described or referenced appropriate genotyping methods; (6) investigated alcohol, heroin, cocaine, or methamphetamine dependence (or abuse) diagnosed by valid published criteria. For the studies investigating alcohol-related liver disease, cirrhosis, or chronic pancreatitis, the cases were considered as alcoholics with the alcohol-related diseases. Cirrhosis was diagnosed by histological, clinical, radiological, and (or) endoscopic findings; (7) had no description of known comorbidity with major psychiatric disorders for the participants (this information was not available in all the studies); and (8) used unrelated individuals in case-control studies. Authors were contacted in cases where we determined it would be useful to have additional information regarding their studies.
