Cannabis increases the risk of psychotic episodes, but legalization does not always have the same consequences

Government strategies for addressing cannabis use have been a constant debate in recent decades within regulatory and public health circles. It is acknowledged that consumption, especially at an early age, increases the risk of psychosis. At the same time, the experiences of various countries after changing their laws show that not all forms of legalization produce the same outcomes: where cannabis is freely commercialized on the market, problematic use increases, but that is not the case in places where the state controls sales.

These are the main findings of two international reviews just published in The Lancet Psychiatry. One examines the relationship between cannabis and various mental health disorders; the other studies what has happened following regulatory changes introduced between 2000 and 2025. Read together, they outline a complex chain: regulation alters accessibility, price, potency, and supply; those factors can promote frequent use, and it is precisely that pattern that shows the strongest association with psychosis, although there is no evidence of causation that legalization causes an increase in episodes.

The mental health review concludes there is credible evidence that regular cannabis use is a contributing cause of psychosis. That phrase is important: it does not mean cannabis is necessary or sufficient to cause a psychotic disorder, but that it can act alongside genetic, environmental, and personal factors. The risk increases the earlier use begins, the more frequent it is, the longer it continues and the higher the concentration of THC, its psychoactive substance.

In other words: most people who use cannabis will not develop psychosis, but in some it will act as a “trigger,” according to Celso Arango, head of the Child and Adolescent Psychiatry Department at Hospital Universitario La Paz in Madrid. “Some have a predisposition that might never have manifested, but use precipitates a first episode. Sometimes symptoms remit; other times the disorder persists even after stopping cannabis and progresses to a chronic illness,” explains the psychiatrist, who did not take part in the studies.

In Arango’s unit, around 70% of adolescents admitted for a psychotic episode test positive for cannabis. This is the experience of a specialized service and is not a figure that can be extrapolated to the whole population, but it illustrates how often the two phenomena coincide in clinical practice. His longitudinal studies of patients who have experienced first psychotic episodes have also observed a better prognosis among those who stop using than among those who continue.

Causality is harder to establish for depression and anxiety. “Some people use cannabis because they perceive it temporarily reduces distress, fear, or feeling down. This self-medication hypothesis may explain part of the association,” emphasizes Rafael Maldonado, professor of pharmacology at Pompeu Fabra University, who warns against drawing hasty conclusions, since the same genetic vulnerability may favor both use and mental disorder, and certain social or environmental circumstances can increase both risks.

Bipolar disorder occupies an intermediate position. Cannabis is associated with earlier onset, more frequent episodes, more severe symptoms and a poorer response to treatment, but there are fewer studies and possible confounding factors have been less well controlled. Maldonado believes this relationship deserves much more attention, because it is barely present in the public debate.

Not all legalization is the same

The second review shows that the legal change by itself explains little. What seems decisive is how access is organized.

Uruguay established a system subject to strict state control: it limits distribution channels, products, prices and the potency of cannabis sold in pharmacies. The two available studies found no increases in use among adolescents and young adults, and one observed reductions in several indicators of risky use.

Canada and much of the United States have developed far more commercial markets, though with important differences between jurisdictions. The expansion of shops, falling prices, and the arrival of products with high THC concentrations have been associated with increased use among adults and with cannabis use disorder.

“The consequences of changing legal status depend on how it is done,” Maldonado says. In his view, any reform should learn from countries that have already gone down this path and place public health ahead of commercial interests.

The reviews do not allow the claim that legalization automatically causes more schizophrenia. There is no consistent association with the overall incidence of psychotic disorders. Increases in emergency visits and hospitalizations related to psychosis have been observed in places such as Ontario and Colorado, especially during phases of greater commercialization.

“The easier it is to buy it, administer it, and serve it, the greater the accessibility will be, use will increase and there will be more harmful consequences,” Arango summarizes.

This also applies to so-called medical cannabis. Poorly regulated medical access may offer little benefit, increase harm, and in practice facilitate non-medical use. For Arango, medication status should not be decided by a parliamentary vote — “it would be like politicians voting on the effect of ibuprofen,” he illustrates — but through clinical trials and the ordinary evaluation procedures.

That, the psychologist criticizes, has happened in some places, such as in some U.S. states. There, the sale of cannabis for certain pathologies has been allowed without it having passed the tests required of other medicines. In Spain, the use of medicines derived from cannabis for therapeutic purposes is approved, but very limited and only in cases where evidence has been demonstrated.

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