The subject of cannabis use and schizophrenia has been explored in depth in the past several years. There has been evidence of a clear link between cannabis use and psychotic episodes and a link between episodes of psychosis and the later onset of schizophrenia. But until now, no one has looked at how cannabis-related psychosis increases the risk for the development of schizophrenia. In addition, little research has looked at how this risk varies over time. To address these voids in research, Edison Manrique-Garcia of the Department of Public Health Sciences, Division of Social Medicine at the Karolnska Institutet in Sweden recently led a study that analyzed data from more than 50,000 individuals who had reported their experience with cannabis use throughout adolescence. The individuals remained part of a cohort that was evaluated over a 35-year period. It was from these data that Manrique-Garcia looked at the specific relationships between cannabis use, psychosis, and eventual schizophrenia.
The study revealed that the individuals who used cannabis regularly were almost four times more likely to develop schizophrenia than those who never used cannabis and more than twice as likely to experience a brief psychosis episode. The results also showed that the risk for future psychosis and schizophrenia weakened over the long-term. Manrique-Garcia said, “Of the cases related to cannabis use, 60% occurred during the first decade compared with 45% among non-users of cannabis.” However, the findings also demonstrated a clear relationship between dose and risk. In particular, those who used the highest amounts of cannabis for the longest periods of time had the highest risk of schizophrenia. This risk was increased by early episodes of psychosis, regardless of whether they were cannabis induced or not. The individuals who experienced episodes of cannabis-induced psychosis and those who had non-cannabis-related psychotic episodes were equally at risk for schizophrenia. But Manrique-Garcia points out that the individuals with cannabis-related psychosis may not have experienced any psychotic episodes if they had not used cannabis. Further research is needed to determine if this would ultimately decrease their risk for the later development of schizophrenia.
Manrique-Garcia, E., Zammit, S., Dalman, C., Hemmingsson, T., Andreasson, S. (2012). Cannabis, schizophrenia and other non-affective psychoses: 35 years of follow-up of a population-based cohort. Psychological Medicine, 42.6, 1321-1328.
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