RI-CLPMs use a multilevel approach to test for within-person differences that inform on the extent to which an individual’s increase in cannabis use precedes an increase in that individual’s psychosis symptoms, and vice versa.
The approach provides the most rigorous test of causal predominance between two outcomes, said Conrod.
“One of the problems in trying to assess a causal relationship between cannabis and mental health outcomes is the chicken or egg issue. Is it that people who are prone to mental health problems are more attracted to cannabis, or is it something about the onset of cannabis use that influences the acceleration of psychosis symptoms?” she said.
The study revealed statistically significant positive cross-lagged associations, at every time point, from cannabis use to psychosis symptoms reported 12 months later, over and above the random intercepts of cannabis use and psychosis symptoms (between-person differences). The statistical significances varied from P < .001 to P < .05.
Cannabis use, in any given year, predicted an increase in psychosis symptoms a year later, said Conrod.
A limitation of the study was that cannabis use and psychosis symptoms were self-reported and were not confirmed by clinicians. However, as the authors note, previous work has shown positive predictive values for such self-reports of up to 80%.
Commenting on the findings for Medscape Medical News, Robert Milin, MD, child and adolescent psychiatrist, addiction psychiatrist, and associate professor of psychiatry, University of Ottawa, said…”The study is at the forefront because it is specifically looking to measure psychosis symptoms and cannabis use in adolescents, and the model they are using strengthens the study,”
That model uses “refined measures or improved measures to look at causality, vs what we call temporal associations,” he said.