Substance use and disorders are highly comorbid phenomena. High rates of comorbid substance use and disorders occur more often than expected by chance as evidenced in large national and international epidemiological surveys (Degenhardt et al., 2001; Johnston et al., 2008; Substance Abuse and Mental Health Services Administration (SAMHSA), 2008). For instance, of the 22.3 million persons aged 12 or older with a past year SUD in the 2007 National Survey on Drug Use and Health survey, 3.2 million were dependent on alcohol and an illicit substance. In a recent study using biological markers of substance use, there were strong correlations between tobacco dependence, cannabis use, and alcohol abuse and dependence (Kapusta et al., 2007). This high rate of comorbidity observed among substances suggests a parsimonious mechanism (Vanyukov et al., 2003) of risk for substance use disorders.
