Psychosis Tends to Follow Cannabis Use, Not Vice Versa

Longitudinal study in teens establishes predominant time sequence

by Nicole Lou,

Cannabis use preceded onset of psychosis symptoms in teens more often than the reverse, according to researchers analyzing data from a longitudinal study conducted in Quebec.

A total of 3,966 adolescents volunteered to participate annually in an online survey in which they reported their cannabis use in the past year (on a 6-point scale ranging from "never" to "every day") and any psychosis symptoms they experienced as per the Adolescent Psychotic-Like Symptoms Screener.

With the survey data, a statistical model was created showing the relationship between marijuana use and psychosis symptoms over ages 13-16 (χ²=48.22). Once the model incorporated time-lagged associations between earlier cannabis use and psychosis symptom onset 12 months later, the model acquired a better fit (χ²=26.07, P=0.001), according to Patricia Conrod, PhD, of the University of Montreal, CHU Sainte-Justine Research Center, and colleagues, reporting online in a research letter in JAMA Psychiatry.

"This analysis demonstrates a predominant association at the individual level of cannabis use frequency with increased psychosis symptoms, and not the opposite, in the general population at a developmental stage when both phenomena have their onset.

"[T]hese results emphasize the need for targeted cannabis use prevention as jurisdictions revise their cannabis regulatory policies," the researchers suggested. Canada is on the verge of legalizing marijuana nationwide, possibly as soon as this week.

"Promoting evidence-based interventions and policies that reduce access to and demand for cannabis among youth could lead to population-based reductions in risk for major psychiatric conditions," Conrod and colleagues wrote.

The investigators used data from teenagers in the Co-Venture cohort, representing 76% of all grade 7 students at 31 secondary schools in the Montreal area. Data from 3,720 were included in the analysis.

The researchers acknowledged that the study's reliance on unverified, self-reported cannabis use and psychosis symptoms were an important limitation.

Conrod and co-authors reported having no conflicts of interest.