Teens who use marijuana or smoke cigarettes are at increased risk of having a psychotic experience by age 18, according to a new study.
Late adolescent onset cannabis use was associated with a roughly threefold increased risk for a psychotic episode after adjusting for potential confounders, and early-onset use was even more strongly associated with increased risk, the analysis found.
The adjusted risk associated with cigarette smoking was much less robust.
The study, published online in JAMA Psychiatry, suggests that marijuana use by teens, and cigarette smoking to a lesser extent, may be independent risk factors for psychotic episodes later in adolescence.
“We found little evidence that psychotic experiences in childhood led to increased cannabis use,” wrote Hannah Jones, PhD, of Bristol Medical School in the U.K., and colleagues. “As other observational studies have indicated, the self-medication hypothesis does not appear to adequately explain the association between cannabis use and psychosis. Such a relationship for tobacco use is also not well supported by our data.”
Cannabis use and cigarette smoking have been associated with an increased risk for psychotic outcomes in previous studies, but the researchers noted that teasing out the potentially causal effects of one from the other has been difficult because dual use is so prevalent.
“One approach that can help inform causal inference is to use behavioral patterns of cannabis and cigarette use over time to identify classes of individuals with different substance use profiles across a developmental period, rather than relying on patterns of cannabis and cigarette use at a single point in time,” the researchers explained.
The team used longitudinal latent class analysis (LLCA) to identify subgroups of adolescents based on similar patterns of cigarette and cannabis use behavior over time to examine the association of different classes with the subsequent onset of psychotic experiences. Longitudinal classes were derived using 5,300 participants in the University of Bristol’s Avon Longitudinal Study of Parents and Children — also known as the Children of the 90s study — which is charting the health of 14,500 families in the Bristol area.
Data were collected periodically starting in the fall of 1990, with collection ongoing. The data were analyzed during a 10-month period between 2016 and 2017. Cannabis and cigarette smoking use data were summarized using LLCA to identify longitudinal classes of substance use, and associations between classes and psychotic experiences at age 18 were assessed.
The study participants had at least three measures of cigarette and cannabis use from ages 14 to 19. Potential confounders included sex, family history of schizophrenia or depression, family history of drug use, maternal and/or paternal smoking during pregnancy, maternal education, highest parental social class, IQ at age 8, childhood trauma or experiencing bullying between the ages of 7 and 9, emotional and behavioral problems at age 9, and alcohol use at age 12.
The main findings from the analysis were the following:
- Both early-onset and late-onset cannabis use classes were associated with psychotic experiences at age 18 and were only minimally attenuated after adjusting for potential confounders
- There was inadequate evidence to support an association between either early-onset or late-onset cigarette-only use and psychotic experiences in the adjusted analyses
- Adjustment for potential confounders did not alter the estimate of association for either the early- or late-onset cannabis use class but resulted in an approximately 60% attenuation of the estimate for the early-onset cigarette-only class
Study strengths, Jones and colleagues said, included the use of a large, well-characterized cohort with multiple measures of exposures and psychotic experience data over time. A significant limitation, however, was the inability to investigate the impact of cannabis or cigarette use on psychotic disorders due to inadequate power.
“While psychotic experiences in the population are relatively poor predictors of psychotic disorder, they represent the key characteristic of such disorders, and understanding their etiology almost certainly has relevance to understanding the etiology of clinically defined psychosis,” the researchers wrote.