An estimated 15.3 million adults in the United States met criteria for an alcohol use disorder in the past 12 months. Of those with alcohol use disorders, 2.3 million adults also met criteria for a drug use disorder (Stinson et al., 2005) with odds ratios estimated to be 7.4 for any drug use disorder, but 3.4 to 19.2 for specific drug use disorders (Khan et al., 2013; Stinson et al., 2005). Both alcohol and drug use disorders are heritable, with approximately 50% of the variance attributable to heritable factors (Bienvenu et al., 2011; Ducci and Goldman, 2012; Tsuang et al., 1998; Wang et al., 2012), although this estimate varies dramatically by substance (e.g. up to 70% heritability for cocaine dependence)(Kendler et al., 2000), age (Bergen et al., 2007; Derringer et al., 2008; Vrieze et al., 2012) and other characteristics, including comorbid psychopathology (Pickens et al., 1991).
