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A large prospective population based study in young people has confirmed that using cannabis risks psychosis developing later—greatly for those susceptible to psychosis and moderately for others. It also provides evidence that using cannabis might cause psychosis and is not just a consequence of it.
Young people using cannabis at baseline in the study were more likely to have psychotic symptoms four years later (odds ratio (OR) 1.67), after adjustment for a slew of confounding factors. The likelihood of having any psychotic symptoms rose with frequency of use in a dose–response way, from 0.99 for use less than once a month to 2.23 for almost daily use. The adjusted difference in risk of psychosis with cannabis use was 23.8% for young people predisposed to psychosis but 5.6% for the rest. Predisposition to psychosis at baseline, however, did not significantly affect use of cannabis during the next four years (OR 1.42).
The study analysed data on 2437 young people aged 14–24 years who were part of the random regional representative population sample in the prospective early developmental stages of psychopathology study (EDSP) in Munich, Germany.
Whether using cannabis causes psychosis has been disputed, some arguing that predisposition to psychosis may be the driving force to taking up the drug, rather than cannabis causing psychosis to be expressed. This—the first prospective study to research the issue—suggests that cannabis may be the culprit.
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