This article was originally published in Psychology Today and is republished here with permission.
If you carry a genetic or neurobiological vulnerability to psychosis, using cannabis may increase the likelihood that psychosis will emerge, particularly during adolescence and young adulthood. Cannabis-induced psychosis is not always temporary. Numerous studies have shown that many affected individuals develop schizophrenia-spectrum disorders.
Few investigators have done more to shape contemporary thinking about cannabis and psychosis than Deepak D’Souza, M.D., the Vikram Sodhi ’92 Professor of Psychiatry at Yale University School of Medicine. His work demonstrated that delta-9-tetrahydrocannabinol (THC), the principal psychoactive component of cannabis, can induce transient psychotic symptoms in healthy volunteers and worsen existing symptoms in patients with schizophrenia. For more than two decades, D’Souza’s research, scientific publications, and public statements have challenged assumptions about the psychiatric safety of cannabis.
D’Souza’s trailblazing research helped move the field beyond the simple observation that cannabis use and psychosis are associated. The more important question became whether cannabis could cause psychotic disorders. His studies also found that abstinence reduces relapse risk, whereas continued cannabis use is associated with poorer clinical outcomes and a diminished treatment response.
Today, D’Souza is asking a different question. Amid concerns about increasingly potent commercial cannabis and unprecedented levels of THC exposure, he’s raised the possibility that cannabis may trigger a lifelong psychosis in some individuals. Recently, D’Souza and colleagues published a study comparing patients hospitalized with first-episode psychosis who had documented cannabis exposure with similar patients who had no evidence of cannabis exposure.
The study examined 119 men hospitalized with first-episode psychosis, including 66 with toxicology-confirmed cannabis exposure and 53 without cannabis exposure. Patients in the cannabis-associated group exhibited fewer negative symptoms, such as affective flattening and loss of motivation, while showing comparable levels of hallucinations and delusions. They also displayed more depressive and manic features, denoting a clinical presentation differing from classic deficit-form schizophrenia.
While both groups entered the hospital with similar levels of cognitive impairment, only the cannabis-associated group demonstrated significant cognitive improvement after four weeks of treatment and cannabis abstinence. The investigators also detected distinct EEG patterns that may reflect differences in cortical excitation and inhibition. Together, these findings suggest cannabis-associated psychosis may differ from non-cannabis-associated psychosis in ways that extend beyond clinical symptoms alone.
D’Souza summarized the evolution of his thinking in a recent conversation with me:
“Schizophrenia (SCZ) or, as Bleuler in 1911 appropriately coined, the ‘group of schizophrenias’, is heterogeneous on a number of levels, including its phenomenology and clinical manifestation, environmental risk factors, micro-scale molecular genetics, transcriptomics, proteomics, and macro-scale alterations in brain structure, function, and connectivity. Several attempts have been made to identify and tease apart SCZ subtypes based on clinical manifestations, genetics, and biomarkers. It is a bit too early to make any definitive conclusions, but our findings raise the fascinating possibility that there may be a cannabis subtype of psychotic disorders.”
D’Souza’s study, however, included only men and followed patients for just four weeks. The long-term trajectory of cannabis-associated psychosis remains unclear. Some cases may evolve into schizophrenia-spectrum or mood disorders, whereas others may not. Whether cannabis-associated psychosis represents a distinct subtype of psychotic illness remains an open question.
No single research finding has yet proven that cannabis-associated psychosis is biologically distinct. However, stellar investigators at the Institute of Psychiatry, Psychology and Neuroscience at King’s College London, including Marta Di Forti and Robin Murray, have provided clear evidence linking cannabis exposure to psychotic disorders. Their studies demonstrated that daily use of high-potency cannabis is associated with markedly increased odds of developing psychosis and showed that a substantial proportion of first-episode psychosis cases are attributable to exposure to potent cannabis products.
What is becoming increasingly difficult to dismiss is the possibility that cannabis-associated psychosis represents more than a temporary drug-induced syndrome. It may, in some individuals, constitute a persistent psychotic illness triggered by cannabis exposure. As D’Souza told me:
“Our findings need to be replicated. Furthermore, it is important to collect longer-term follow-up data to understand whether the long-term course and prognosis of this proposed subtype is distinct.”
Earlier debates about cannabis and psychosis regularly revolved around polarized positions. One camp argued that cannabis merely unmasked schizophrenia in genetically vulnerable individuals. Another viewed cannabis-induced psychosis as a transient intoxication-related phenomenon fundamentally distinct from schizophrenia. Increasingly, the evidence suggests neither formulation is entirely adequate. Nevertheless, the convergence of clinical, cognitive, and somatic differences raises the possibility that cannabis exposure may be associated with a recognizable psychosis subtype rather than simply serving as a trigger for conventional schizophrenia.
Several high-quality epidemiologic studies support D’Souza’s concerns and challenge the longstanding belief that cannabis psychosis is usually benign and self-limited. A Danish registry study found cannabis-induced psychosis had one of the highest conversion rates to schizophrenia-spectrum disorders among substance-induced psychoses, with approximately 41 percent of affected individuals later receiving a schizophrenia diagnosis. Meta-analyses have similarly demonstrated substantial progression rates from cannabis-induced psychosis to chronic psychotic illness.
Individuals with substantial cannabis exposure regularly develop psychosis at younger ages than non-users. Some studies have found fewer negative symptoms and better cognitive functioning than in patients with non-cannabis-associated schizophrenia. Reviews by Yücel and colleagues and Løberg and Hugdahl suggest the pathway to psychosis in cannabis-exposed patients may involve less severe neurodevelopmental impairment than typically seen in primary schizophrenia. Cannabis-associated psychosis may not be ordinary schizophrenia.
The strongest epidemiologic support for a novel cannabis-related psychosis pathway comes from the fact that high-potency cannabis is associated with markedly increased odds of psychotic disorder, meta-analyses documenting a dose-response relationship between cannabis exposure and psychosis risk, evidence that abstinence reduces relapse risk, and continued cannabis use is associated with poorer clinical outcomes and diminished treatment response.
What psychiatry currently labels “schizophrenia” may represent a final common clinical pathway reached through multiple genetic and environmental routes. Cannabis could plausibly be one of those routes. If so, the key question becomes whether cannabis-associated psychosis differs meaningfully from schizophrenia occurring without cannabis exposure.
D’Souza’s findings are particularly relevant today, with earlier cannabis initiation, more frequent use, and commercial products containing THC concentrations far higher than those available in the past. Complementing these observations, studies from the London groups have shown that daily use of high-potency cannabis is associated with substantially increased odds of developing psychosis. These findings raise concerns that modern cannabis products are contributing to the increasing incidence of psychotic illness.
The treatment implications are equally important. Existing evidence suggests cannabis-associated psychosis responds to antipsychotic medications as primary psychosis does. However, continued cannabis use substantially increases the risks of relapse, rehospitalization, medication nonadherence, and treatment failure. These findings also underscore the importance of cannabis cessation and treatment of cannabis and other co-occurring substance use disorders.
Cannabis use is consistently associated with an increased risk of psychosis and with exacerbations of psychotic symptoms in both healthy individuals and people with psychotic disorders.
Rather than asking whether cannabis can produce psychosis, we may need to ask what kind of psychosis it produces, in whom, and under what circumstances. Psychiatry must also move beyond the simplistic question of whether cannabis “causes” schizophrenia. The more important questions concern cannabinoid-system mechanisms, individual vulnerability, and timing of exposure. Cannabis may not create an entirely separate disease entity, but growing evidence suggests it can dictate the timing, expression, and long-term course of psychotic illness in vulnerable individuals.