>>33 I'm not saying that consumption of cannabis causes a mental disorder entirely on its own. However, several studies have shown that cannabis increases the vulnerability towards lasting mental illnesses (e.g. psychosis or shizophrenia); meaning that it may cause hereditary and environmental risk factors to develop into such a chronic disease whereas non-consumption would not cause the disease to surface or only in a less severe form. You are welcome to read the articles I linked below.
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>>34 Here you are.
"Cannabis use may increase the risk of psychotic disorders and result in a poor prognosis for those with an established vulnerability to psychosis. [...] Results confirm previous suggestions that cannabis use increases the risk of both the incidence of psychosis in psychosis-free persons and a poor prognosis for those with an established vulnerability to psychotic disorder."
van Os J1, Bak M, Hanssen M, Bijl RV, de Graaf R, Verdoux H.: Cannabis use and psychosis: a longitudinal population-based study. Am J Epidemiol. 2002 Aug 15;156(4):319-27."[...] A dose-response relationship was found between cannabis exposure and risk of psychosis, and this association was independent from potential confounding factors such as exposure to other drugs and pre-existence of psychotic symptoms. However, the diagnostic specificity is weak, as cannabis exposure may be a risk factor for the occurrence of a large spectrum of psychiatric disorders, ranging from schizophrenia to mood and anxiety disorders. [...]"
Verdoux H1, Tournier M.: Cannabis use and risk of psychosis: an etiological link?. Epidemiol Psichiatr Soc. 2004 Apr-Jun;13(2):113-9."Various lines of evidence suggest an association between cannabis and psychosis. Five years ago, the only significant case-control study addressing this question was the Swedish Conscript Cohort. Within the last few years, other studies have emerged, allowing the evidence for cannabis as a risk factor to be more systematically reviewed and assessed. [...] For psychotic symptoms, a dose-related effect of cannabis use was seen, with vulnerable groups including individuals who used cannabis during adolescence, those who had previously experienced psychotic symptoms, and those at high genetic risk of developing schizophrenia. In conclusion, the available evidence supports the hypothesis that cannabis is an independent risk factor, both for psychosis and the development of psychotic symptoms. Addressing cannabis use, particularly in vulnerable populations, is likely to have beneficial effects on psychiatric morbidity."
Semple DM1, McIntosh AM, Lawrie SM.: Cannabis as a risk factor for psychosis: systematic review. J Psychopharmacol. 2005 Mar;19(2):187-94."Evidence from 6 longitudinal studies in 5 countries shows that regular cannabis use predicts an increased risk of a schizophrenia diagnosis or of reporting symptoms of psychosis. These relations persisted after controlling for confounding variables, such as personal characteristics and other drug use. The relation did not seem to be a result of cannabis use to self-medicate symptoms of psychosis. A contributory causal relation is biologically plausible because psychotic disorders involve disturbances in the dopamine neurotransmitter systems with which the cannabinoid system interacts, as demonstrated by animal studies and one human provocation study. [...] It is most plausible that cannabis use precipitates schizophrenia in individuals who are vulnerable because of a personal or family history of schizophrenia."
Degenhardt L1, Hall W.: Is cannabis use a contributory cause of psychosis?. Can J Psychiatry. 2006 Aug;51(9):556-65."[...] Therefore, all the studies that assessed a dose-effect relationship showed this link between cannabis use and the emergence of psychosis or psychotic symptoms. The fact that all causal criteria were present in the studies suggests that cannabis use may be an independent risk factor for the development of psychosis. Results seem to be more consistent for vulnerable individuals with the hypothesis that cannabis use may precipitate psychosis, notably among vulnerable subjects. In particular, early onset of cannabis use during adolescence should be an environmental stressor that interacts with a genetic predisposition to induce a psychotic disorder.
[...] The objective of this article was to examine whether cannabis use can be an independent risk factor for chronic psychotic disorders, by using established criteria of causality. Data extracted from the selected studies showed that cannabis use may be an independent risk factor for the development of psychotic disorders. Early screening of the vulnerability to psychotic disorder should permit improved focus on prevention and information about the specific risks related to cannabis use among this population."
Le Bec PY1, Fatséas M, Denis C, Lavie E, Auriacombe M.: Cannabis and psychosis: search of a causal link through a critical and systematic review. Encephale. 2009 Sep;35(4):377-85. doi: 10.1016/j.encep.2008.02.012. Epub 2008 Jul 9."Over the past 30 years evidence has been growing that cannabis use increases the risk for psychosis which could develop into schizophrenia in a proportion of cases. Over the past decade many studies have been published which clarify the association between cannabis use and psychosis. The aim of this review is to examine this association. A systematic search yielded 14 cohort studies carried out in 9 cohorts and 9 case-control studies. When the results of these studies are taken together they unambiguously support that cannabis use is an independent risk factor for psychosis and may also give rise to chronic psychotic disorders like schizophrenia. A dose dependent link is present because more frequent use associates with greater risk. The studies also show that cannabis-use in adolescence is associated with greater risk of developing psychosis than commencing the use of cannabis in adulthood. [...]"
Jonsson AJ, Birgisdottir H, Sigurdsson E.: Does the use of cannabis increase the risk for psychosis and the development of schizophrenia?. Laeknabladid. 2014 Sep;100(9):443-51.