A new study in the journal JAMA Psychiatry highlights a possible link between cannabis use disorder and schizophrenia. Researchers examined the medical records of more than seven million people in Denmark, and found that the number of schizophrenia cases that may be attributable to cannabis addiction has increased three- to four-fold in the past 25 years.
To date, no direct causal link between cannabis and schizophrenia has been found – although the study authors note that both the use and potency of cannabis in Denmark have increased dramatically over the past few decades. Should such an association actually exist, then this trend should be reflected in an accompanying rise in the proportion of schizophrenia cases being attributable to the use of the drug.
Using data from the national health registry, the researchers analyzed the records of every Danish person born before 2001, who was 16 or older at any time between 1972 and 2016. In doing so, they found that two percent of schizophrenia cases in 1995 were associated with cannabis addiction, yet this rose to four percent at the turn of the millennium and has increased to between six and eight percent in the years since 2010.
“The proportion of cases of schizophrenia associated with cannabis use disorder has increased 3- to 4-fold during the past 2 decades, which is expected given previously described increases in the use and potency of cannabis,” write the study authors. To clarify, these percentages represent “an estimate of the proportion of cases of schizophrenia that would have been prevented if no individuals had been exposed (in this case to cannabis use disorder).”
Interpreting these figures, the researchers go on to explain that such an observation is “not actual proof of a causal link” between cannabis addiction and psychosis, but that it “provides evidence of the theory of cannabis being a component cause of schizophrenia.”
It’s also important to note that this study does not link regular cannabis use to schizophrenia, but relates only to cannabis use disorder. This is defined as problematic use of the drug, including an inability to cease or reduce consumption, as well as a steady increase in intake and tolerance.
Nevertheless, the authors state that their findings have “important ramifications regarding legalization and control of use of cannabis,” and call for more research into the potential harms of the drug.
A number of leading academics have reacted to the study – including some who question the conclusions drawn by the authors. For example, Professor David Curtis of the University College London Genetics Institute explained that “while it is true that people diagnosed with schizophrenia are also more likely to be diagnosed with cannabis use disorder, the reasons for this association are far from clear and it is extremely difficult to interpret results such as these with confidence.”
“If, as the authors suggest, cannabis use disorder can cause schizophrenia then there should have been a quite dramatic increase in schizophrenia incidence and we simply do not see that. So far as I can see, this study does not really provide support for the hypothesis that cannabis use causes schizophrenia.”