Use of cannabis linked to earlier psychosis

Cannabis use can bring forward the onset of psychosis by years, a major Australian study has revealed. 

The comprehensive meta-analysis from researchers at the University of NSW, published in Biological Psychiatry, analysed 149 international studies involving more than 71,000 people. 

It found that cannabis users develop psychosis an average of two-and-a-half years earlier than non-users. 

Crucially, the research shows that this age gap widens as users get older - stretching to a shift of more than six years for individuals who develop psychosis in their 30s.

Psychosis is a condition affecting up to 3 per cent of adults that causes delusions, hallucinations, and disordered thinking.

It usually emerges in late adolescence or early adulthood and is a symptom of schizophrenia and other mental illnesses.

While health warnings about cannabis use have traditionally focused on teenagers with developing brains, researchers say the new data exposes a dangerous misconception that older users are out of the woods.

UNSW lead author Carly Stevens, from the School of Biomedical Sciences, said comparing different life stages provided vital clarity on the link between cannabis use and mental health outcomes.

“What we’ve found challenges the assumption that you’ll be fine if you wait to smoke weed until your mid-20s, that’s just not the case,” Stevens said. 

“There’s been so much emphasis on adolescents not smoking cannabis, which is important, but we need to broaden the warnings for everyone.”

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The study highlights that for people experiencing their first episode of psychosis in their mid-to-late 20s, cannabis users were affected 4.43 years earlier than non-users. For those aged 30 and older, the gap expanded to 6.3 years.

While a couple of years might seem minor on paper, researchers say the real-world impact on a young adult’s trajectory can be devastating.

“People may miss out on study, jobs opportunities, travel and relationships in those years, and treatment can come with physical side effects that can make life harder as well,” Stevens said.

Chief investigator and psychiatrist Conjoint Professor Matthew Large noted that while ethics prevent direct experimental testing to conclusively prove causation, the consistency of the findings strongly points toward a cumulative risk that builds over time - similar to the long-term harms of alcohol or tobacco.

The researchers also raised concerns regarding the rising popularity of prescription medical cannabis and high-potency edibles, which can mask potential psychiatric risks under a therapeutic banner.

With psychotic disorders accounting for up to 3 per cent of healthcare expenditure in developed nations, experts are now calling for expanded public health campaigns, clearer metrics for measuring cannabis exposure, and broader education to ensure consumers fully understand the mental health risks before using the drug.