Over 800 candidate gene association studies have been published in this field, but only 212 (24%) of these reports met our inclusion criteria. Some papers published 20 years ago were missing raw genotype and allelic distribution data and had inconsistent use of genetic markers. In addition, since the number of available allelic contrast tests was limited, we combined all data regardless of the types of addictive drugs and the racial/ethnic composition of the group studied. The heterogeneity of the datasets was high: even after our comprehensive meta-analysis, the results were still Grades B and C, according to the criteria of the Human Genome Epidemiology Network (HuGENet) (Table 1). Protocols such as those proposed by HuGENet [24] could standardize data collection and reporting and allow for improved meta-analyses in the future.
