ANJA Presents: Cannabis, Correlation, and Causation

Ashley Robins

Aug 1, 2022


Author: Ashley Robins

Role: Marketing and Policy Lead



What’s your favorite ice cream flavor?

Did you know that indulging in that delicious scoop of ice cream is associated with a higher chance of death by drowning?

It’s true! Ice cream sales and drowning accidents are positively correlated. But does this mean that eating ice cream dooms you to drowning?


Just because something is linked to an outcome, does not mean that that something caused that outcome. In the case of the ice cream example, there are reasons that ice cream is associated with drowning accidents. Consumers are more likely to eat ice cream on hot days- the same way that they are more likely to be swimming on a hot day. There may be a relationship between ice cream and drowning, but enjoying ice cream is not a direct cause of a drowning experience.

Correlation Fallacy: Cannabis Causes Schizophrenia

Understanding the difference between correlation and causation is vital to understanding the rather ambiguous research that happens in the cannabis industry. As new research emerges, there are often unclear and contradictory results that appear across the various studies. The misconception that will be discussed today carries serious implications, but it is important to address both the good and the bad of cannabis accurately. 

Today’s misconception: Cannabis causes schizophrenia and bipolar disorder.

In a comment regarding the content of this ANJA blog post, one reader shared the following:

“It is well established by clinical studies that cannabis usage is linked to the worsening or even new developments of psychotic conditions. The way one processes cannabis differs depending on genetic makeup.” 

This reader supported their argument using the following quote from the literature review of doi:10.3390/ijerph16214149 - a 2019 article published in the International Journal of Environmental Research and Public Health.


"These were patients that did not have schizophrenia or bipolar disorder diagnoses before the incident substance-induced psychotic episode. In this registry study it was reported that 41.2% of patients with cannabis-induced psychotic disorder eventually converted to schizophrenia. A total of 47.4% of patients with cannabis-induced psychotic disorder eventually converted to either schizophrenia or bipolar disorder."



Looking at that paragraph alone, it is easy to see where concerns about the linkage between mental illness and cannabis usage come into play. However, that same study also notes that:



"Unfortunately, most of the literature with respect to documented cases consistent with CIPD (cannabis-induced psychotic disorder)  diagnoses is limited to case reports and case series and many of the often cited examples are from decades ago…"




"It is important to note that most of these registry studies are looking at cannabis use in relatively young people and subsequent schizophrenia or other psychotic outcomes. The age of onset of cannabis use appears to heavily influence the cannabis/schizophrenia association. One potential explanation for this is that cannabis use has stronger effects on developing brains and that is what leads to a stronger association with future psychoses. Genetic risk is an important part of the cannabis/schizophrenia association as well. We should expect this as schizophrenia has often been estimated as having approximately 80% heritability."



So what does this mean?

First and foremost, if you have a history or predisposition towards schizophrenia or bipolar disorder, it is best to speak with a physician before trying cannabis. While we have your wellbeing at heart, we agree that cannabis is not for everybody. That being said, saying that cannabis causes schizophrenia is the same as saying that ice cream causes drowning; correlation does not equal causation.

Importantly, the study in question notes that both the sample size and the modernity of the samples were less than optimal. Even if we were to ignore that, the study also points out a few major problems with drawing causality conclusions. Younger people who used cannabis were more likely to develop psychotic outcomes- suggesting that cannabis usage should be kept out of the hands of minors. Another major factor of schizophrenia–according to the study–is genetic predisposition. Schizophrenia has an 80% heritability factor, meaning that cannabis may catalyze or bring out pre-existing schizophrenic tendencies, but it will not create the disorder from scratch. 

In regards to the age factor: this is why it is so critical that cannabis becomes legalized and regulated. Drug dealers don’t check IDs, but dispensaries do. For anyone concerned about the health consequences of cannabis getting into the hands of minors, the best thing they can do is advocate for additional identity verification and cannabis research. ANJA strongly advocates for the safe and responsible usage of cannabis for adults 21+; when you have questions, we have ANJA.

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