A sweeping new review in The Lancet Psychiatry has reached a conclusion the marijuana industry has spent years working to obscure: there is no safe amount of cannabis to use. People who use cannabis “need to know that there is no universally safe level of cannabis use,” the authors write. “The only reliable way to avoid any risk for harm from using cannabis is abstinence.”
The review, led by Wayne Hall and colleagues, assessed the coherence of epidemiological, genetic, experimental, and preclinical evidence. Its central conclusion is that “regular cannabis use is a contributory cause of psychosis.” The authors cite a meta-analysis linking cannabis use to “a three-fold increase in the incidence of psychosis.”
The paper confronts the industry’s claim that mentally ill people simply use cannabis to self-medicate. The largest within-person study found cannabis was a far stronger predictor of later psychotic symptoms (OR=7.03) than psychotic symptoms were of later cannabis use (OR=0.59) — evidence that points from the drug to the disorder, not the other way around.
It also takes on the argument that some hidden factor explains the link away, reporting an E-value of 5.95, meaning an unmeasured confounder would need to be tied to both cannabis use and psychosis at a risk ratio of nearly six. And the danger is growing: the attributable risk of cannabis for schizophrenia has “increased since 2010, probably reflecting a higher prevalence of cannabis use since then.”
The heaviest burden falls on the young. The review puts the risk of cannabis use disorder at “27% … in young people who have ever used cannabis and 33% … among young people who used cannabis at least weekly.” Adolescent use predicts later depression more strongly (OR=1.33) than depression predicts later use (OR=1.03), and a cannabis use disorder diagnosis carries a suicide mortality more than three times the norm.
The authors are careful where the evidence is thinner, flagging bipolar disorder as needing more study. That restraint only sharpens the weight of a conclusion they state without hedging.
The findings arrive as the Trump administration moves to reschedule marijuana and states race to commercialize it — a verdict, from the field’s most authoritative journal, that no level of use is safe.