There are several plausible explanations for the association between early cannabis use and later other drug use and problems. One possibility is that cannabis serves as a “gateway” to other substance use and problems, and that earlier onset increases the likelihood of progression to problems (Hall & Lynskey, 2005; Kandel, 2003a, 2003b; Kandel et al., 1992; Pedersen & Skrondal, 1998). Alternatively, it is possible that both early use and the development of later problems are influenced by an underlying shared vulnerability (Morral et al., 2002), which may be genetic and/or environmental. There is evidence of familiality for cannabis use, abuse, and dependence (Kendler, Karkowski, Neale, & Prescott, 2000; Lynskey et al., 2002; Maes et al., 1999; Rhee et al., 2003; Tsuang et al., 1999), and that familial influences on cannabis abuse/dependence overlap with the influences on other drug abuse/dependence (Fu et al., 2002; Kendler, Jacobson, Prescott, & Neale, 2003; Kendler, Myers, & Prescott, 2007; True et al., 1999; Tsuang et al., 1998; Xian et al., 2008).
