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In the News: The Link Between Cannabis Use and Psychosis

Written by Evolve's Behavioral Health Content Team​:

Alyson Orcena, LMFT, Melissa Vallas, MD, Shikha Verma, MD, Ellen Bloch, LCSW, Lianne Tendler, LMFT, Megan Johnston, LMFT Meet The Team >

A study posted on March 19th, 2019 by the U.K. medical journal The Lancet – “The Contribution of Cannabis Use to Variation in the Incidence of Psychotic Disorder Across Europe” caused quite a stir in the U.S. media in the weeks following its publication.

The headlines generated by several high-volume, high-exposure media outlets trended toward warnings like “Daily Marijuana Use and Highly Potent Weed Linked to Psychosis” and “Daily Use of High Potency Marijuana Linked to Higher Rates of Psychosis, Study Finds” and “Smoking Strong Marijuana Daily Increases Risk of Psychosis.”

Important Disclaimer: Evolve does not condone the use/misuse/excessive us of any intoxicant, legal or otherwise. This is not a pro-cannabis post: it’s a pro-science, pro-knowledge, pro-understanding-of-how to-read-science-articles post.

The reason we’re writing this post is to separate fact from fiction. We also want to point out the difference between speculation and proof. In order to do this, it’s crucial to understand the difference between correlation and causation. Sometimes media outlets publish headlines that blur the lines between the two. When these lines get blurred, it can lead to misunderstanding and confusion. In the end, headlines can decrease, rather than increase, general knowledge of a subject that may be important. And for parents of teenagers who may experiment with marijuana, understanding the facts is certainly important.

Correlation v. Causation

Here’s a quick refresher on the difference between correlation and causation, from a helpful webpage published by the Australian Bureau of Statistics:

Correlation is a statistical measure that describes the relationship between two or more variables. Correlation between variables does not mean a change in one variable causes the change in the other variable.

Causation indicates that one variable or event is the result of the other variable or event. This is simple cause and effect.

For example, a cause and effect relationship is like this: cigarette smoking causes in increased risk of lung cancer. Whereas a correlative relationship goes something like this: people who smoke cigarettes are more likely to have an alcohol use disorder. In example one, the smoking actually causes cancer. In example two, smoking cigarettes does not cause an increased risk of developing an alcohol use disorder.

The Lancet Study and the Media Headlines

With regards to The Lancet study on cannabis and psychosis, what our readers should know is that the data in the study is correlative, not causative. It states there’s a statistical relationship between daily use of high potency cannabis and psychosis, but it does not state that the use of cannabis – high potency, daily use, or otherwise – causes psychosis.

In fact, in supplementary material published by the Lancet alongside their study, the Lancet concedes:

“Does this mean we can now be sure that (daily and high potency) cannabis use causes psychosis? Unfortunately, not all the evidence utilising different methods is consistent about causality. For example, studies using genetic data have found evidence possibly consistent with shared genetic etiology between risk of psychosis and likelihood to use cannabis.”

That’s a fancy way of saying there may well be other factors at play other than cannabis use. Such as genetics, for example. They cite this recent study, which indicates:

“Individuals with a stronger genetic predisposition to schizophrenia are more likely to initiate cannabis use, use cannabis more regularly, and consume more cannabis over their lifetime.”

In a nutshell, what the two quotes above mean is that the risk for developing psychosis or having a psychotic break may have existed before any subjects in the used cannabis. Further, they mean that a genetic predisposition to schizophrenia may also predispose an individual to regular cannabis use.

Another research scientist, Dr. Diana Martinez, interviewed by National Public Radio (NPR) goes further:

“You can’t say that cannabis causes psychosis. It’s simply not supported by the data.”

In order to to prove causation, researchers need at least two things. First, a control group, and second, evidence collected over years. Ideally the evidence wouls span from before the time the subjects first used cannabis to the time when they first experienced psychotic episodes. They’d also need detailed medical histories of all participants. Finally, they’d need reliable data on dosage and frequency of use, which is difficult to obtain from voluntary self-reporting.

Advice for Parents

With the trend towards nationwide legalization of marijuana, parents need to know the real facts about the dangers of any kind of marijuana use. Whether it’s occasional use, long-term use, or something in between, parents need data, not headlines. We have decades of data on alcohol, amphetamines, cocaine, heroin and prescription opioids. The data on marijuana is scarce in comparison. That’s why parents need to read headlines carefully. It does not help to make a snap judgment about an important issue based on scant information. Or worse, based on headlines that – accidentally or intentionally – imply connections that are scientifically inaccurate.

We found a well-reasoned, practical tip about how to parents can discuss the dangers of marijuana use with their teenagers in the comprehensive article Marijuana and Psychosis published by the Child Mind Institute:

“The best thing parents can do is arm their kids with the facts — not scare tactics or threats, but the facts: that occasional or infrequent use of pot is much safer than regular use and that daily use could be setting them up for real trouble. And that we just don’t know enough about the risks to the developing adolescent brain and the long-term effects of marijuana.”

While fact-based and logical, we feel that statement does not go far enough. Here’s how we’d re-write it:

“We just don’t know enough about the risks to the developing adolescent brain and the long-term effects of marijuana use. Data seems to show that infrequent use is safer than daily use. However, until more research is done and the scientific community reaches consensus, the best choice for teenagers is to avoid recreational marijuana use altogether.”

Our Behavioral Health Content Team

We are an expert team of behavioral health professionals who are united in our commitment to adolescent recovery and well-being.

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