Daily Marijuana Use Linked to Psychosis, Study Finds

Henrietta Brewer
March 22, 2019

Across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first episode psychosis, compared with people who had never used cannabis, the findings showed.

Previous studies have suggested a link between heavy marijuana use and an increased risk of psychosis.

The researchers found the biggest link between daily cannabis use and having a psychotic disorder was in Amsterdam, where the chances were seven times higher than for those who had never used the drug.

The researchers obtained data from 901 patients with first-episode psychosis who presented to mental health services between May 2010 and April 2015 across 11 sites, including London, Amsterdam and Paris.

Participants were made to answer a questionnaire to know if they used cannabis in their lifetime; if the answer was affirmative, they were requested to give details on their pattern of use.


After controlling for education, drinking habits, and other factors, the researchers found that there was a 40 percent greater chance of developing a psychotic disorder among people who had used the drug a few times a week, as compared with those who had rarely or never tried cannabis. This increased to five times more likely for daily use of high potency cannabis. So the more people who used the drug daily; and the more who used high-potency marijuana, the higher the rate of psychosis. For example, the team found that in Amsterdam, half of new cases are connected to the drug, while in London, just under a third are.

Di Forti concluded that marijuana users should "avoid high-potency types and daily use, and seek a doctor's advice if concerned about experiencing psychotic symptoms".

Suzanne Gage, a psychologist and epidemiologist at the University of Liverpool, in an accompanying commentary with the study said, "What this paper has done that's really nice is they look at rates of psychosis and cannabis use in lots of different places where underlying rates of psychosis are different".

As more states legalize marijuana, people are starting to believe it's A-OK to smoke pot on the reg. The estimates on potency also do not include the proportion of cannabidiol (CBD), another important component of cannabis.

So how common is high-potency weed, anyway?


Other European cities in Spain, Italy and France on the other hand have less than 10 percent THC content in most popular cannabis products on the market.

However, the study doesn't prove causality, cautions Dr. Diana Martinez, a psychiatrist and addiction researcher at Columbia University. It is perfectly possible that the association between cannabis and psychosis is bidirectional.

Psychosis is usually triggered by factors like genetics and the environment.

"The use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms", said lead author Marta Di Forti, a professor at the Institute of Psychiatry, Psychology and Neuroscience at King´s College London. They should be aware that using high-potency cannabis comes with a risk, she says.


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