Addictive disorders represent debilitating conditions that result in productivity loss, and an increased risk for associated mental disorders as well as infectious, metabolic, proliferative, respiratory, and vascular diseases [1]. Addictions encompass substance-use disorders and compulsive behaviors [2]. The heritabilities of substance-use disorders are consistently found to be ~50 % [3] with the lowest heritability for hallucinogen and highest for cocaine use disorders, respectively [4]. The ~2:1 monozygotic:dizygotic twin concordance ratios for many substance-use disorders support additive genetic effects and multiple loci [4]. Estimates for the influence of genome-wide common variants on nicotine and alcohol dependence, and illicit drug use traits are ~30–36 % [5, 6], representing most of the estimated heritability. There is evidence for shared genetic influence across multiple substance-use disorders [6, 7].
