Cannabis is the most commonly used illegal drug in the United States (Johnston, O'Malley, & Bachman, 2003), and research has consistently indicated that cannabis use is associated with an increased risk of other illegal drug use and problems (Donovan & Jessor, 1985; Hammer & Vaglum, 1990; Jessor, Chase, & Donovan, 1980; Kandel, Davies, Karus, & Yamaguchi, 1986; Kandel, Yamaguchi, & Chen, 1992; Morral, McCaffrey, & Paddock, 2002). Furthermore, multiple studies have shown that the risk of substance abuse/dependence is greater among early illegal substance users than among later-users (Anthony & Petronis, 1995; Grant & Dawson, 1998; Hall & Lynskey, 2005; Lynskey et al., 2003).
