Here's some stuff from UptoDate. From what I've learned in our classes there seems to be definitive evidence that marijuana increases the risk for psychotic illnesses.
LONG TERM MEDICAL EFFECTS OF CANNABIS USE Psychotic disorders — Substantial evidence indicates that cannabis use causes psychosis, beyond the transient psychosis sometime seen during intoxication:
●A meta-analysis of 83 observational studies found that onset of psychosis (schizophrenia-related illness) occurred significantly earlier (2.7 years) in patients who used cannabis, compared with patients who did not [51]. By contrast, alcohol use was not associated with a significantly earlier age of onset of psychosis. ●A systematic review of 35 longitudinal studies found a significantly increased risk of psychosis for those who ever used cannabis compared to those who did not (adjusted OR 1.41, 95% CI 1.20-1.65) [52]. The review also found a dose response relationship, with a two-fold increase in risk of psychosis for people who used cannabis most frequently (OR 2.09, 1.54-2.84). The outcome of psychosis ranged from self reported symptoms to clinical diagnoses of schizophrenia, and the study adjusted for confounding factors and excluded cohorts with identified mental illness or substance use problems. ●A review of neurochemical studies suggested cannabis use may cause psychosis through its effects on dopamine release [53]. ●There is evidence that genetic factors may influence the risk of psychosis in adults who used cannabis as adolescents [54,55]. This may help explain why the vast majority of cannabis users do not develop psychosis. Cannabis use may exacerbate symptoms in patients with established psychotic illnesses such as schizophrenia. A systematic review of longitudinal studies (albeit with limitations) found cannabis use was consistently associated with increased relapse of psychosis, rehospitalization, and poor compliance with treatment [56].
Large M, Sharma S, Compton MT, et al. Cannabis use and earlier onset of psychosis: a systematic meta-analysis. Arch Gen Psychiatry 2011; 68:555.
Moore TH, Zammit S, Lingford-Hughes A, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet 2007; 370:319.
Fergusson DM, Poulton R, Smith PF, Boden JM. Cannabis and psychosis. BMJ 2006; 332:172.
Caspi A, Moffitt TE, Cannon M, et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biol Psychiatry 2005; 57:1117.
Henquet C, Rosa A, Delespaul P, et al. COMT ValMet moderation of cannabis-induced psychosis: a momentary assessment study of 'switching on' hallucinations in the flow of daily life. Acta Psychiatr Scand 2009; 119:156.
Zammit S, Moore TH, Lingford-Hughes A, et al. Effects of cannabis use on outcomes of psychotic disorders: systematic review. Br J Psychiatry 2008; 193:357.