The “Dude” smoked a lot of weed—and amusing as his ennui might be in the movie—he might have been a little, well off, is the polite way to put it.
In 2008, two Australian researchers, Wayne Hall and Louisa Degenhardt, published a short paper in World Psychiatry asking a question that was then mostly argued about in the abstract: does cannabis use contribute to causing psychosis? Their answer was careful. Regular use predicted roughly a doubling of the risk of schizophrenia. The relationship survived statistical controls. It was unlikely to be self-medication. And it was biologically plausible, because the cannabinoid system talks to the dopamine system, and psychosis lives in that conversation.
Eighteen years on, I have read what the field did with that paper. The short version: Hall and Degenhardt were right, and the numbers have hardened
A 2016 meta-analysis pooling nearly 67,000 people found the heaviest users at almost four times the odds of psychosis compared with non-users, with a dose–response gradient — more use, more risk — in every study included (Marconi et al., Schizophrenia Bulletin). The large European EU-GEI study then identified the variable that has changed most since 2008: potency. Daily use of high-potency cannabis — flower at 10% THC and above, which is to say, much of what is sold today — carried nearly five times the odds of a first episode of psychosis. The researchers estimated that if high-potency cannabis were removed, roughly one in eight first episodes across their sites might not have happened; in Amsterdam, one in two (Di Forti et al., The Lancet Psychiatry, 2019).
Denmark keeps registers on everyone, and the Danish data show what this means at the scale of a nation. The share of schizophrenia attributable to cannabis use disorder has risen from about 2% in the mid-1990s to 6–8% today — not because the per-user risk changed, but because use and potency rose around it (Hjorthøj et al., JAMA Psychiatry, 2021). Ontario tells the same story across its legalization years: 3.7% before, 10.3% after, rising steadily rather than jumping at the law itself (Myran et al., JAMA Network Open, 2025).
And then there is the finding I think about most in my consulting room. When a psychotic episode is induced by cannabis, it does not always end when the drug wears off. In the Danish registers, 41% of people with a cannabis-induced psychosis were later diagnosed with schizophrenia — the highest conversion rate of any substance, higher than amphetamines, higher than hallucinogens (Starzer et al., American Journal of Psychiatry, 2018). A meta-analysis puts the pooled figure at one in three (Murrie et al., 2020).
I want to be honest about the counterweight, because the people I work with deserve the whole picture. Genetics research finds the causal arrow runs in both directions — liability to schizophrenia also predicts cannabis use, and the measured genetic effect of simply trying cannabis on schizophrenia is small. The fairest reading of the whole literature is the one the evidence keeps pointing to: cannabis is a component cause. Neither necessary nor sufficient. The risk is not spread evenly across everyone who has ever smoked; it concentrates — in daily use, in high potency, in use that begins young (in Ontario data, the association lives almost entirely in the adolescent window), and in people carrying a vulnerability they usually do not know they have.
Which is why the clinical question is never really "cannabis: yes or no?" The clinical questions are: How much? How strong? How young did it start? And what is it doing to this person's sleep, motivation, and mind?
Most people who use cannabis will never become psychotic, and alarm is not a treatment. But cannabis use can become a disorder — that is a clinical fact, not a moral one — and for the person whose use is daily, escalating, and tangled up with anxiety, flatness, or frightening experiences, the way out is not willpower speeches. It is understanding what the drug has been doing, and building a life the drug is no longer holding together.
That is the work I do. At Snowcreek Psychotherapy I work with preteens, teenagers, and young men on emotional regulation, and with adults — including adults whose cannabis use has stopped being a choice they feel they are making. If that is you, or your son, insight starts here.
Start with a free 15-minute consultation. No commitment, no pressure — a short conversation about where you are and whether this work fits. And because the evidence should not sit in a journal where nobody can use it, I keep a small library for the people I work with: the cannabis detox supplement battery I use in practice — the vitamin regimen, free — and access to my research database on cannabis and the mind, the studies themselves, kept current. Both live in a password-protected area of this site, and consultation clients receive the password.
Call 802-397-1239 or write l.roen@snowcreek.info, and we will set the fifteen minutes.
Snowcreek Psychotherapy — Llewelyn-Roen Prowe, MA · East Hardwick, Vermont · 802-397-1239 · l.roen@snowcreek.info · www.snowcreek.info
