In the Netherlands Mental Health Survey and Incidence Study (NEMESIS), 4,045 psychosis-free individuals and 59 individuals with a psychotic disorder were assessed at baseline, 1, and 3 years [225] using a measure of psychosis. Individuals using cannabis at baseline were nearly three times more likely to manifest psychotic symptoms at follow-up even after adjustment for a range of factors. Further, a dose–response relationship was established with the highest risk (OD = 6.8) for the highest level of cannabis use. The relationship between cannabis use and psychotic symptoms was stronger for cases with more severe psychotic symptoms. Individuals who reported psychotic symptoms at baseline were also more likely to develop schizophrenia if they used cannabis, than were individuals who did not. The attributable risk of cannabis to psychosis was estimated at 13% for psychotic symptoms and 50% for cases with psychotic disorders that required psychiatric treatment.
