Twin and other classical genetic studies indicate that drug addiction is a complex brain disorder with strong genetic contributions [1,2]. Genetic association studies, including candidate gene studies and genome-wide association studies (GWAS), can provide insights into the genetic background of this neurobiological and behavioral disorder. Using these approaches, more than 800 publications during the past three decades have reported genomic loci and/or specific genetic variants that have been associated with susceptibility to drug addiction. It has been difficult to draw general inferences from these studies, however, because genetic association studies generated results that were sometimes inconsistent, many of these studies were modestly powered (especially when polygenic genetic architectures are considered), genomic controls are infrequent, and biases can be detected in a number of analytic strategies. In this context, meta-analysis of genetic association studies may be particularly useful, especially when the underlying genetic architecture for the disorder is relatively straightforward [3-6]. In addition, although different addictive drugs have disparate pharmacological effects, there are also similarities after acute and chronic exposure such as acute rewarding and negative emotional symptoms upon drug withdrawal
