A new review article has been pulished in the Public Library of Science (Medicine) on the connection between Cannabis (Marijuana) and psychosis (schizophrenia is one type of psychosis).
The journal article writes:-------------------------------------------------------------http://www.schizophrenia.com/sznews/archives/003204.html
Cannabis use and psychotic symptoms and disorders are associated in the population and in persons with schizophrenia. The major explanations of this association have been that: (1) cannabis use precipitates schizophrenia in persons who are vulnerable to the disorder, (2) cannabis is used to self-medicate symptoms of schizophrenia, or (3) the association arises from uncontrolled confounding by variables that predict an increased risk of both cannabis use and schizophrenia.
The first explanation was supported by a 15-year prospective study of 50,465 Swedish conscripts that found a dose–response relationship between the risk of developing schizophrenia and the number of times cannabis had been used by age 18. These risks remained significant after statistical adjustment for confounding variables. A later 27-year follow-up of this cohort also found a dose–response relationship between the frequency of cannabis use and the risk of schizophrenia, which persisted after statistically controlling for confounding factors.
These findings have been recently replicated in: a three-year study of 4,848 young people in the Netherlands; a 4-year follow up of a cohort of 2,437 young Germans ; and two New Zealand birth cohorts. All of these studies found a relationship between regular cannabis use and psychosis that persisted after controlling for confounding variables.
In the Dutch, German and New Zealand cohorts, young people who reported psychotic symptoms at baseline were much more likely to report psychotic symptoms at follow-up if they used cannabis than were cannabis-using peers who did not report these symptoms at baseline. In one of the New Zealand studies, young people with a variant allele of the COMT gene who used cannabis had a risk of reporting psychotic symptoms that was ten times higher than young people who did not have the allele who used cannabis.
The self-medication hypothesis was not supported in the van Os or Henquet studies, both of which found that early psychotic symptoms did not predict an increased use of cannabis. These results have been supported by Verdoux et al., who found that cannabis users were more likely to report unusual perceptions after using cannabis than to report using cannabis in response to experiencing unusual perceptions, and that this relationship was stronger in individuals with a history of psychotic experiences.
I'm not certain on any bias presented by any of the involved parties. The part that I put in bold is the only thing that's really convincing to me.
Also, interesting posts from Alfa relating to the COMT disorder:
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