Psychiatric comorbidity predicted transition to dependence, as previously reported (Breslau et al., 2004; Glantz et al., 2008; Kessler, 2004; Kessler et al., 1996; Zimmermann et al., 2003). The co-occurrence of mental and SUD may be due to self-medication practices or the existence of common liability factors (Leonard et al., 2007; Neale and Kendler, 1995; van Os et al., 2002). Family, twin and genome-wide studies suggest that genetic factors are largely responsible for the pattern of comorbidity of common psychiatric and substance use disorders (Burmeister et al., 2008; Kendler et al., 2003; Li and Burmeister, 2009). For example, individuals with a lifetime history of a psychotic disorder had an increased risk of transition to nicotine dependence. Recent findings have linked mutations in the neurexin 1 gene, a cell adhesion-related gene, to an increased risk for nicotine dependence or schizophrenia (Leonard et al., 2007; Nussbaum et al., 2008; Rujescu et al., 2009). Heightened impulsivity and disturbances in reward motivation are also common risk factors across several psychiatric disorders that can explain their co-occurrence (Agrawal et al., 2004; Chambers and Potenza, 2003). Prevention and treatment of mental disorders may decrease the risk of transition from substance use to dependence.
