The incidence of psychotic disorders has increased in more recent birth cohorts in Ontario, suggesting the need for early psychosis interventions.

“Conventional wisdom is that the incidence of psychotic disorders has been stable over time in high-income countries,” study author Daniel T. Myran, MD, MPH, Gordon F. Cheesbrough Research Chair in the Department of Family and Community Medicine at North York General Hospital in Toronto, told Medscape News Canada. But when his team was evaluating the association between cannabis legalization and changes in schizophrenia rates, he noted “we saw that new cases of psychosis were increasing in younger individuals.” That prompted the investigation into rates by birth cohort.

“The size and consistency of the birth-cohort differences were surprising,” he said. “People born in the 1980s and later were not only more likely to be diagnosed with psychotic disorders but also were diagnosed at younger ages, resulting in substantially more individuals affected by early adulthood. While some of this may reflect improvements in access to care, the magnitude of the differences raises concern that broader changes in risk factors, such as early-life exposures, environmental factors, and changing behaviors, including substance use, are also contributing,” he said.

The study was published online on February 2 in CMAJ.

Early Identification and Intervention Key

The researchers conducted a retrospective cohort study using linked health administrative data on 12,231,314 people born in Ontario between 1960 and 2009. They identified diagnoses of schizophrenia spectrum disorder (SSD) and psychosis not otherwise specified (NOS) — which collectively are considered psychotic disorders — between 1992 and 2023 and compared changes in incidence, cumulative incidence, and prevalence across birth cohorts in 5-year increments.

Incidence was defined as diagnosis with no diagnosis in the previous 5 years. Age-period-cohort (APC) models were used to compare birth cohort effects.

A total of 152,587 (0.9%) participants were diagnosed with a psychotic disorder during the study period. Overall, the researchers found birth cohort increases in the incidence of these disorders and decreases in the age of diagnosis.

Between 1997 and 2023, the annual incidence of psychotic disorders increased by 60% among participants aged 14-20 years and was stable or declined among those aged 21-50 years.

In addition, the APC models indicated that the incidence of schizophrenia was 70% higher among participants born from 2000 to 2004 compared with those born from 1975 to 1979. Increases in psychosis NOS across birth cohorts were greater than increases in SSD.

Furthermore, the rate of participants who had been diagnosed with a psychotic disorder at age 20 was 104% higher for those born from 2000 to 2004 than for those born from 1975 to 1979 (0.55% vs 0.27%). The rate was 37.5% higher at age 30 for those born from 1990 to 1994 than for those born from 1975 to 1979 (1.32% vs 0.96%).

Birth cohort effects were comparable among boys and girls and in sensitivity analyses looking at diagnoses in outpatient or in hospital settings.

As with all birth cohort studies, these findings were subject to APC confounding, the authors wrote. The authors also were unable to observe more recent birth cohorts at older ages and therefore noted that the observed trends may not persist over the long term.

“While more research is essential to clarify potential mechanisms, the findings suggest that health systems may need to plan for increased demand for early psychosis intervention and community-based supports,” Myran said.

Even investments in prevention for a factor not responsible for the increases in psychosis, such as reducing adverse childhood experiences, “may help with the overall burden of psychosis and offer other mental health and societal benefits,” he said.

“Clinicians play an important role in early identification and intervention,” he added. “This includes rapid initiation of care or referral for individuals with a first episode of psychosis and discussion of known risk factors (e.g, substance use) in those with high risk.”

Screening and Referral

Commenting on the study for Medscape News Canada, Katherine M. Keyes, PhD, professor of epidemiology and inaugural director of the Susan Lasker Brody Center for Population Mental Health at the Columbia Mailman School of Public Health in New York City, said, “There’s been decades of research about the link between high-potency cannabis use and incidence of psychosis, especially among vulnerable people. And in the US, at least, there’s also been an increase, especially among young people, in the use of other kinds of psychoactive drugs, such as psilocybin. These drugs don’t induce psychosis in everyone who takes them, but there are patterns of use that are more high-risk than others.”

That said, like the authors, Keyes noted that the increase in incidence may have to do, at least in part, with changes in how clinicians are diagnosing psychosis and with how frequently people are seeking care for the condition. In analyses of administrative healthcare databases, she said, “there’s always going to be a conflation of the underlying incidence of the condition and help-seeking for that condition. That doesn’t undermine the importance of this study and the findings,” but use of multiple data sources could potentially capture more people.

Clinicians could help mitigate the consequences of the increase by becoming familiar with the psychosis literature and aware of the symptoms and signs, especially among adolescents and young adults, Keyes said. These symptoms could include unusual thoughts and perceptions, out-of-body experiences, and perceptions of persecution. “There are screening scales that can alert clinicians if a young person seems to be experiencing or is on the road to experiencing psychotic symptoms, and as rates are increasing, clinicians could ensure that screening is part of routine clinical care.”

Furthermore, she added, “many areas now have robust programs for early psychosis intervention, and it’s important to be familiar with referral pathways.”

“We’re in a period in the US and in many other countries where we’re reevaluating our policies and the legalities of various psychoactive drugs,” Keyes continued. “We need to be investigating not only the intended consequence of legalizing cannabis, for example, but also the unintended consequences, and evaluating the health effects from a public health standpoint. This study emphasizes the need to be investing in that type of research for young people’s health, and the need for the healthcare system to understand and respond to new environmental challenges.”

Funding for this study was provided by the Charité Universitätsmedizin Berlin. Myran was supported by a Canada Research Chair, the University of Ottawa, and North York General Hospital. Myran and Keyes reported having no relevant financial relationships.

Marilynn Larkin, MA, is an award-winning medical writer and editor whose work has appeared in numerous publications, including Medscape Medical News and its sister publication MDEdge, The Lancet (where she was a contributing editor), and Reuters Health.