Cannabis can lead to a pyschotic/schizophrenic break - especially when used over a long period of time. But it's still an "which comes first - chicken and egg "
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Based on research reviews, looking at the connection between cannabis and psychosis, two hypotheses have been developed.
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The first hypothesis is that cannabis use causes psychotic symptoms in an otherwise healthy individual that would not have occurred with abstinence. The second hypothesis is that cannabis use may precipitate psychosis in individuals who are predisposed to acquiring a psychotic disorder.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2811144/