Andrew lost his son to cannabis. A landmark study helps us understand why

When Patrick Leunig finished high school with a great ATAR and a coveted spot in a law degree, his father assumed he was on the path to success.

“It looked like he had his whole future in front of him,” Patrick’s father, Andrew Leunig, said.

“We later found out he started smoking cannabis in Year 11, when he was 16.”

Patrick dropped out of university inside a year. By the time he was 21, he was hospitalised against his will for the first time, when the delusions and conspiracies of drug-induced psychosis made it clear he needed help.

In April 2024, Patrick was found dead of a methamphetamine overdose. It was two days before his 26th birthday.

“If he had never smoked cannabis, would he have had a mental illness? I don’t know the answer,” Leunig said.

A landmark Australian-led global study found cannabis users develop psychosis an average two-and-a-half years earlier than non-users who experience psychosis.

The largest study of its kind, published in Biological Psychiatry , involving 149 studies with data from more than 70,000 individuals, found that the younger people started smoking cannabis, the worse the clinical outcomes. They lost years of healthy life and incurred greater healthcare and societal costs .

Cannabis users in their early 20s developed psychosis 1.6 years earlier than non-users, while those over 30 developed psychoses about 6.3 years earlier, the analysis found.

University of NSW professor of psychiatry Matthew Large, one of the study’s co-authors, said the most plausible explanation is the increased years of exposure.

The cumulative toxicity of cannabis was comparable to lung cancer and tobacco – the longer someone smokes, the higher their risk, Large said.

Co-author Carly Stevens said there was a prevailing myth that “you’ll be fine if you wait to smoke weed until your mid-20s,” once your brain had developed.

“What we’ve found challenges the assumption,” she said. “There’s been so much emphasis on adolescents not smoking cannabis, which is important, but we need to broaden the warnings for everyone … no one is really safe when it comes to trying cannabis.”

Cannabis users are two to five times more likely to develop schizophrenia, with a six-year average delay between initial cannabis use and the onset of psychosis.