Marijuana use during early adolescence has been associated with two-fold increased risk for schizophrenia (Andreasson, Allebeck, Engstrom, & Rydberg, 1987; Arseneault et al., 2002; Zammit, Allebeck, Andreasson, Lundberg, & Lewis, 2002). Longitudinal assessments of birth cohorts find that adolescent marijuana users are significantly more likely than non-users to be diagnosed with schizophrenia at follow-up. Other large epidemiologic prospective studies have replicated this association between adolescent marijuana use and persistent psychotic disorders (Henquet et al., 2005a; Stefanis et al., 2004; van Os et al., 2002). Recent meta-analyses further indicate that schizophrenia risk rises with increasingly heavy adolescent marijuana exposure, and estimate that adolescent marijuana use may account for 8–14% of schizophrenia cases (Henquet, Murray, Linszen, & van Os, 2005b; Moore et al., 2007).
