Marijuana and psychosis linked in a study

By Dr. Ananya Mandal, MDMar 3 2011

A new study has shown that smoking marijuana during adolescence or young adulthood can trigger psychotic symptoms, and continued use may increase the risk for psychotic disorder later in life. The study was published online March 1 in the British Medical Journal.

Rebecca Kuepper, research psychologist and PhD student at Maastricht University in the Netherlands, who worked on the study said, “Young people should be advised to be careful and consider that when it comes to using cannabis; frequent and continued use is especially detrimental to mental health…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.”

There have been earlier studies that show that smoking marijuana can increase the risk for psychotic symptoms. But it had been unclear whether smoking marijuana precedes psychotic symptoms. For this study researchers tracked 1923 individuals from the general population of Germany, aged 14 to 24 years at baseline for 10 years. These people had no history of psychotic symptoms or cannabis use at baseline.

Results showed that those who started smoking cannabis during the study had roughly 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. The incidence rate of psychotic symptoms between baseline and 3.5 years was 31% in cannabis smokers versus 20% in non-smokers; between 3.5 years and 8.4 years rates were 14% and 8%, respectively. Also 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. Continued use of cannabis increased the risk for persistent psychotic symptoms more than 2-fold.

Wayne Hall, of University of Queensland, Australia, and co-author of a linked commentary, said 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. Dr. Hall and co-author Louisa Degenhardt of the Burnet Institute in Melbourne, Australia in an accompanying commentary note that a causal link between cannabis use and incident psychotic symptoms is biologically plausible.

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. Degenhardt note adding, “The major challenge is to deter enough young people from using cannabis so that the prevalence of psychosis is reduced.”

The study was funded in part by the German Ministry of Research, Education, and Technology.

Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said of the latest study, “It is one of ten prospective studies all pointing in this same direction… This study adds a further brick to the wall of evidence showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia…It adds new information by showing that it is those who show psychotic symptoms within a few years of initiating cannabis use who are especially likely to develop persistent psychotic symptoms if they persist in their use of cannabis.” Peter Kinderman, professor of clinical psychology at Liverpool University, said the study “offers more evidence that cannabis use is a risk factor for psychosis and recommends a cautious and thoughtful approach to cannabis legislation”.