The association between chronic pain and non-fatal overdose in people who inject drugs: Longitudinal cohort study in Montreal, Canada, 2017 to 2025.

BACKGROUND AND AIMS: The prevalence of chronic pain is elevated among people who inject drugs. Chronic pain may motivate drug use, potentially increasing the risk of non-fatal overdose. We aimed to estimate the association between chronic pain and non-fatal overdose in people who inject drugs. DESIGN: Prospective cohort study of people who inject drugs recruited in community settings [the Hepatitis Cohort (HEPCO) study], with data collection every three months between August 2017 and November 2025. SETTING: Montreal, Canada. PARTICIPANTS: Participants were people who inject drugs (past 6-month injecting), aged 18 years or older, living in the greater Montreal area and speaking French or English. MEASUREMENTS: The outcome was defined as having at least one non-fatal overdose in the past 3 months, and the exposure was defined as experiencing chronic pain in the past 3 months. Covariates were age; gender; past 3-month psychological distress (K10), housing status, opioid use, benzodiazepine use and opioid agonist treatment; past-month days of alcohol use; and interview era (pre-November 2020 vs. November 2020 onwards). A lagged analysis was conducted to explore temporality, based on chronic pain status reported at the visit preceding the outcome assessment. The analysis was not pre-registered and the results should therefore be regarded as exploratory. RESULTS: The primary analysis included 4790 study visits from 887 participants and 355 reports of non-fatal overdose. At baseline, 81.4% of the sample was men, median age was 45 years and 46.8% reported chronic pain in the past 3 months. Chronic pain in the past 3 months was associated with a 40% increase in past 3-month non-fatal overdose [adjusted prevalence ratio (PR) = 1.40; 95% confidence interval (CI) = 1.13-1.75]. Similar results were observed in the lagged analysis (adjusted PR = 1.52, 95% CI = 1.18-1.96). SUMMARY: In a Montreal, Canada, cohort of people who inject drugs, chronic pain was associated with experiencing non-fatal overdose. We interpret this finding within a reciprocal model of pain and substance use, in which chronic pain increases the likelihood of using drugs in ways that increase overdose risk, and pain is potentiated by drug use and withdrawal.