High‐intensity cannabis use is associated with retention in opioid agonist treatment: a longitudinal analysis

Background and Aims Cannabis use is common among people on opioid agonist treatment (OAT), causing concern for some care providers. However, there is limited and conflicting evidence on the impact of cannabis use on OAT outcomes. Given the critical role of retention in OAT in reducing opioid‐related morbidity and mortality, we aimed to estimate the association of at least daily cannabis use on the likelihood of retention in treatment among people initiating OAT. As a secondary aim we tested the impacts of less frequent cannabis use. Design Data were drawn from two community‐recruited prospective cohorts of people who use illicit drugs (PWUD). Participants were followed for a median of 81 months (interquartile range = 37–130). Setting Vancouver, Canada. Participants This study comprised a total of 820 PWUD (57.8% men, 59.4% of Caucasian ethnicity, 32.2% HIV‐positive) initiating OAT between December 1996 and May 2016. The proportion of women was higher among HIV‐negative participants, with no other significant differences. Measurements The primary outcome was retention in OAT, defined as remaining in OAT (methadone or buprenorphine/naloxone‐based) for two consecutive 6‐month follow‐up periods. The primary explanatory variable was cannabis use (at least daily versus less than daily) during the same 6‐month period. Confounders assessed included: socio‐demographic characteristics, substance use patterns and social–structural exposures. Findings In adjusted analysis, at least daily cannabis use was positively associated with retention in OAT [adjusted odds ratio (aOR) = 1.21, 95% confidence interval (CI) = 1.04–1.41]. Our secondary analysis showed that compared with non‐cannabis users, at least daily users had increased odds of retention in OAT (aOR = 1.20, 95% CI = 1.02–1.43), but not less than daily users (aOR = 1.00, 95% CI = 0.87–1.14). Conclusions Among people who use illicit drugs initiating opioid agonist treatment in Vancouver, at least daily cannabis use was associated with approximately 21% greater odds of retention in treatment compared with less than daily consumption.