# Using decision trees to examine risk profiles for cannabis use among large samples of underage youth before and after cannabis legalization in Canada

## TL;DR

This study examines how cannabis use and its risk factors among underage youth in Canada changed before and after legalization.

## Contribution

The study uses decision trees to identify evolving risk profiles for cannabis use in youth before and after legalization.

## Key findings

- Cannabis never use increased, while current use decreased over four years of legalization.
- Risk factors for cannabis use shifted, with mental health conditions becoming more prominent post-legalization.
- Pre- and post-legalization risk profiles differed in complexity and composition.

## Abstract

•Cannabis never use increased in a 4-year period spanning cannabis legalization.•Current cannabis use decreased in a 4-year period spanning cannabis legalization.•Risk factors for current cannabis use changed from pre- to post-legalization.•Internalizing mental health conditions were important risk factors post-legalization.

Cannabis never use increased in a 4-year period spanning cannabis legalization.

Current cannabis use decreased in a 4-year period spanning cannabis legalization.

Risk factors for current cannabis use changed from pre- to post-legalization.

Internalizing mental health conditions were important risk factors post-legalization.

This paper compares risk profiles for cannabis use among large samples of youth in the school years preceding (2017–18, T1) and four years following (2021–22, T2) cannabis legalization in Canada.

COMPASS Study data from students across 85 secondary schools that participated in both the T1 and T2 waves were used. A novel classification tree approach examined current cannabis use (past 30-day), modelling complex interactions among multiple risk factors simultaneously in the T1 and T2 samples.

At T1, 15.0 % of students reported current cannabis use, compared to 12.3 % of students at T2. The classification tree at T1 identified six unique risk profiles. The highest risk group (Pr = 0.269) was large (30.4 % of the sample) and comprised students who placed lower value on getting good grades and spent 45 min or more per day texting. The classification tree at T2 identified 11 unique risk profiles. The highest risk group (Pr = 0.27) was large (18.8 % of the sample) and comprised students who again placed lower value on getting good grades but also reported not eating breakfast daily and having elevated anxiety.

Cannabis never use increased and current cannabis use slightly decreased among underage youth in a 4-year period spanning cannabis legalization. The relative importance ranking of risk factors for predicting current cannabis use changed considerably from T1 to T2. This suggests that prevention efforts need to adapt over time to target the relevant risk factors associated with cannabis use.