Substance use frequency and associations with chronic pain among a cohort of people who inject drugs in Montreal, Canada.

Background: The relationship between substance use and chronic pain is bidirectional. Although chronic pain and polysubstance use are highly prevalent among people who inject drugs (PWID), few studies have examined how the frequency of use of different substances relates to chronic pain. Aims: The aim of this study was to examine associations between substance use frequency and chronic pain in a sample of PWID. Methods: A cross-sectional analysis was conducted among PWID participating in a community-based cohort in Montreal, Canada. Chronic pain measures were introduced in the interviewer-administered questionnaire in February 2017. The first questionnaire was used for analyses, which covers data up to November 2022. Logistic regression analyses were conducted to examine associations between alcohol, stimulants, opioid and cannabis frequency and chronic pain. Results: Six hundred and eight participants were included; 84% were men and mean age was 44.7 years old. Prevalence of chronic pain was 48%. Age (adjusted odds ratio [aOR] = 1.38, 95% confidence interval [CI] 1.15-1.65) and regular alcohol consumption in the past month (aOR = 1.76, 95% CI 1.13-2.75) were associated with chronic pain in univariable and multivariable logistic regression models. The frequency of use for all other substances was not found to be significantly associated with chronic pain. Conclusion: The prevalence of chronic pain in our sample was high. The positive association between high frequency of alcohol use and chronic pain could be explained by alterations of pain pathways by heavy use and withdrawal episodes, potentially increasing hyperalgesia. This study underscores the importance of addressing alcohol use along with other substances among PWID.