Cannabis Use Is Associated with Lower Rates of Initiation of Injection Drug Use Among Street-Involved Youth: A Longitudinal Analysis

Introduction and Aims: Street-involved youth are known to be at elevated risk of initiating injection drug use. However, the impact of so-called “gateway” drugs, such as cannabis, on injection initiation is unknown. The objective of this study was to examine the association between cannabis use and initiation of injection drug use among a prospective cohort of street-involved youth in Vancouver, Canada. Design and Methods: Data for this study were collected from the At-Risk Youth Study (ARYS). From September 2005 to May 2015, participants aged 14-26 who reported illicit drug use were recruited into this open prospective cohort study. An extended Cox regression model with time-updated covariates was used to identify factors independently associated with injection initiation. Results: During the study period 481 street-involved youth were included in this study. Of these, 228 (47.4%) reported at least daily cannabis use, and 103 (21.4%) initiated injection drug use. In a multivariable analysis, ≥ daily cannabis use was associated with slower rates of injection initiation (Adjusted Relative Hazard [ARH]: 0.66, 95% Confidence Interval [CI]: 0.45 – 0.98; p = 0.038). Subanalyses revealed that cannabis use was negatively associated with initiation of injection stimulants but not initiation of injection opioids. Discussion and Conclusions: Given the expansion of cannabis legalization throughout North America, it is encouraging that cannabis use was associated with slower time to initiation of injection drug use in this cohort. This finding challenges the view of cannabis as a gateway substance that precipitates the progression to using harder and more addictive drugs.