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A new longitudinal study, published on March 1 in BMJ, suggests that early use of marijuana may trigger psychotic symptoms and prolonged use could increase the risk for psychotic disorders in later life. There has been a well documented correlation between marijuana use and psychosis; it has been unclear if smoking marijuana preceded psychotic symptoms.
“There are indications that especially people who start using cannabis at an early age (<16 years) are likely to become chronic users and thereby increase their risk for mental health problems, such as psychotic symptoms,” Rebecca Kuepper, research psychologist and PhD student at Maastricht University in the Netherlands said.
This study was uniquely designed to look specifically at the sequential association between incident cannabis use and incident psychotic symptoms. For 10 years, the researchers tracked 1,923 individuals from the general population of Germany, aged 14 to 24 years at baseline, who had no history of psychotic symptoms or cannabis use at the beginning of the study. They found that those who started smoking cannabis during the study had nearly twice the chance of reporting psychotic symptoms during follow-up as those who remained cannabis free. This was true even after accounting for potentially confounding factors, such as age, sex, socioeconomic status, use of other drugs, and other psychiatric diagnoses. In addition, cannabis users who reported psychotic symptoms and continued to use cannabis were more apt to have their symptoms linger than those who stopped smoking it. The continued use of marijuana increased the risk for psychotic symptoms more than two-fold.
These results suggest that incident cannabis use increases the risk for the onset of psychotic symptoms “and, if used continuously, increases the risk that those symptoms will persist,” Ms. Kuepper said. Wayne Hall, PhD, of University of Queensland, Australia, and coauthor of a linked commentary, stated that the pattern of results “makes it unlikely that cannabis use is a form of self-medication of psychotic symptoms and more likely to be a contributory cause of psychotic symptoms.” In the study, psychotic experiences did not predict later cannabis use. Given the current findings and those of earlier studies, “it is likely that cannabis use precipitates schizophrenia in people who are vulnerable because of a personal or family history of schizophrenia,” Dr. Hall and Dr. Louisa Degenhardt, co-authors of the commentary, note.