Cannabis is the most widely used illicit substance among adolescents in the U.S. Of 2.1 million recent initiates, 63% were younger than 18 when they first used (SAMSHA, 2007). Early onset use is associated with dependence among many users, with youths ages 12-17 constituting the majority of admissions to treatment facilities for cannabis abuse (Chen et al., 2004; Hartman et al., 2008). Excessive alcohol consumption is also prevalent among adolescents, as close to a third of 16-17 year-olds report drinking in the past month with 20% transitioning to chronic use by age 20 (SAMSHA, 2007). Despite the frequent comorbidity of marijuana and alcohol use (MJ+ALC) in adolescence (Schweinsburg et al., 2008a), it is unclear how protracted use may affect brain structure and function during this period of continued neuromaturation (Giedd, 2004; Giedd et al., 1999; Gogtay et al., 2004; Hasan et al., 2007; Lenroot and Giedd, 2006; Paus, 2005; Paus et al., 1999).
