A number of explanations have been proposed for the observed association between early cannabis use and other drug use/abuse/dependence (Agrawal et al., 2007; Fergusson, Boden, & Horwood, 2006; Hall & Lynskey, 2005; Kandel, 2003a, 2003b). Although we are unable to test specific mechanisms with our data, a number of causal mechanisms could explain the association between early cannabis use and other substance use/abuse/dependence (Fergusson et al., 2006; Hall & Lynskey, 2005; Wagner & Anthony, 2002). Wagner and Anthony (2002) found support for an “exposure opportunity” process (individuals who use marijuana are more likely to have opportunities to use other illegal substances) as well as evidence that greater seeking of additional drug use opportunities could not entirely explain the increased access. Fergusson et al. (2006) also suggested that cannabis use could be causally linked with other drug use if it: 1) induces changes in brain chemistry, or 2) serves as a learning process for pleasurable effects of drugs. In fact, animal research has shown some support for changes in brain functioning as a result of substance exposures (Ellgren, Spano, & Hurd,
