one third of alcohol users and half of nicotine users had an additional SUD, compared to more than 80% of cannabis users and 90% of cocaine users. The accelerated speed of transition from cannabis or cocaine use to dependence given a previous history of a SUD may be mediated by conditioned learning processes, faster neuroadaptations (Leri et al., 2003; Schlaepfer et al., 2008) and drug interactions leading to slower biotransformation, decreases in adverse drug effects and overall enhanced drug effects (Bradberry et al., 1999; Kapusta et al., 2007; Leri et al., 2003; Mayer, 1984). Individuals dependent on more than one substance appear to have higher genetic liability and alleles of several genes have been associated with polysubstance abuse (Agrawal et al., 2008; Drgon et al., 2006; Schlaepfer et al., 2008). Yet, the simultaneous use of multiple drugs is not entirely explained by genetic factors, with environmental influences playing an important role (Agrawal et al., 2004). Exposure to areas with high levels of social disadvantage facilitates access not only to one but to several drugs, (Storr et al., 2004; Wagner and Anthony, 2002b), which may normalize polysubstance use behaviors. The rapid transition from cannabis and cocaine use to dependence emphasizes the
