2023-05-31 16:37:51
One Danish study on “Schizophrenia and Cannabis” made the last few weeks in the media the round and was even in the context of the parliamentary questioning of the Minister of Health by a CDU MPs picked up. It serves the opponents of legalization in Germany as a supposedly clear sign to distance themselves from this project. The physician Chuck Dinerstein points out in his that the relationship between cannabis and schizophrenia is by no means as clear-cut as it appears at first glance in the Danish study Contribution to the American Council On Science and Health (ACSH) does not.
The starting point of the Danish study are assumptions about a connection between Cannabis Use Disorder (CUD) and schizophrenia, which were expressed in older studies. The aim of the new Danish study is to to differentiate this presumed connection with regard to age and gender and to work out specific risk factors.
For this purpose, data from the Danish health system from 1972 to 2021 was evaluated, in which persons aged 16 to 49 are recorded. Correlations between the occurrence of schizophrenia and various demographic and medical factors such as gender, origin, other mental illnesses or various consumption disorders (alcohol, cannabis, other drugs) were then shown:
It is important to note that these are statistical relationships only make no statements about a causal relationship. Statistically, more men than women suffer from schizophrenia, 9.1% are not born in Denmark, 5.8% of all sufferers have an alcohol use disorder, 0.9% have a cannabis use disorder and 3.4% have another use disorder, etc.
However, the authors of the study subsequently assume a causal relationship between a cannabis use disorder and the occurrence of schizophreniacalculate age- and sex-specific risk factors and conclude that, if the assumption is correct that 15% of cases of schizophrenia in men could be prevented by avoiding CUD:
“Assuming causalityapproximately 15% of recent cases of schizophrenia among males in 2021 would have been prevented in the absence of CUD; by contrast, among females, 4% of recent cases of schizophrenia would have been prevented if they did not have CUD.”
The chicken and egg problem
In his contribution to the ACSH, Dinerstein underscores the fact that this assumption of a one-sided causal relationship is questionable with two older studies on the subject of schizophrenia and substance abuse. So shows one another Danish study from 2019which also uses data from the Danish health system, a statistical association between the presence of a schizophrenic disorder and the development of a substance use disorder:
“A diagnosis of schizophrenia was positively associated with the risk of developing substance abuse … primarily associated with an increased risk of abuse of cannabis, alcohol, stimulants and other substances”.
One further study examined the chronological sequence of the development of a substance use disorder in persons affected by schizophrenia. 27.5% already had a substance use disorder before the onset of the first schizophrenic episode. In 34.6%, this developed in the same month as the first schizophrenic episode, and in 37.9%, the substance use disorder only developed after the schizophrenic illness. In addition, the authors point out that causality is not substantiated in either direction by the data:
“Drug abuse preceded the first symptom in 27.5%, followed it in 37.9%, and emerged within the same month in 34.6% of the cases. The study demonstrates a remarkable association between first-episode schizophrenia and substance abuse, but a unidirectional causality is not supported, nor is a specific psychotic disorder in comorbid cases.”
Cannabis might be a trigger or risk factor for a schizophrenic episode to occur. But a general statement that 15% of cases of schizophrenia in men are directly related to a cannabis use disorder is not scientifically tenable, Dinerstein concludes.
In order to be able to really make this statement, more should be known about the exact circumstances and mechanisms of the development of schizophrenia. Otherwise, self-medication aimed at reducing the symptoms of the disease would be an equally plausible explanation for the observed statistical association between schizophrenia and substance use disorders.
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