Cannabis’ Impact On Mental Health: Correlation Or Causation?

New research suggests that cannabis may not have as many negative effects on mental health as previously believed, and that it’s merely correlated with certain mental health conditions rather than the cause.

A twin study discovered that while there are some connections between cannabis use and certain issues like cannabis use disorder and the use of other drugs, these are mainly influenced by genetics and environment rather than directly caused by cannabis itself. 

This challenges widespread simplistic views on cannabis’s impact.

Twin study

The study, conducted by Stephanie Zellers and her team at the Institute for Molecular Medicine Finland, University of Helsinki, analysed data from over 4,000 individuals. 

They found that while cannabis use was associated with some negative outcomes, these associations weakened when comparing identical twins with differing cannabis use, indicating that shared genetic and environmental factors play a significant role.

While the study doesn’t completely dismiss potential adverse effects of cannabis, such as cannabis use disorder and increased tobacco and illicit drug use, it suggests that these effects might be more nuanced and influenced by individual predispositions. 


Zellers emphasises the importance of moderation and awareness of potential risks for individuals who choose to use cannabis.

However, the study’s scope was limited to moderate cannabis use, and heavier use might have different consequences. 

Also, it couldn’t examine certain potential outcomes like psychosis or impacts on physical health. 

However, previous studies have shown that mental health conditions like psychosis and schizophrenia are not caused by cannabis use. 

Correlation not causation 

This study from 2021 on twins found that THC use has little impact on rates of psychosis. Looking at nearly 3,000 sets of twins over their lifetime, researchers found no evidence of a causal effect of cannabis on psychosis rates. 

Published in 2018, this study looked at the genome of nearly 200,000 people and found that those who are genetically predisposed to schizophrenia are more likely to smoke weed, possibly as a form of self-medication.

In Cannabis and Psychosis: a Critical Overview of the Relationship, researchers looked at recent research on cannabis and psychosis, including studies done with schizophrenic patients and studies of first-episode psychosis.

They found that the evidence suggests that cannabis does not in itself cause a psychosis disorder.

“Rather, the evidence leads us to conclude that both early use and heavy use of cannabis are more likely in individuals with a vulnerability to psychosis,” the study says. 

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