Treatment retention in opioid agonist therapy: comparison of methadone versus buprenorphine/naloxone by analysis of daily-witnessed dispensed medication in a Canadian Province

BACKGROUND: The last decade has shown a remarkable increase in the rates of illicit opioid use in Canada and internationally, which is associated with large increases in opioid related morbidity and mortality. While the differences between methadone and buprenorphine/naloxone in terms of retention have been studied outside Canada, the unique location and design of this study, gives it a specific significance. OBJECTIVES: This study aims to describe the relative treatment retention rates for first episode opioid replacement treatment between methadone and buprenorphine/naloxone for patients receiving daily witnessed dispensed medications in Nova Scotia. METHODS: A longitudinal retrospective descriptive study analyzing secondary data from the Nova Scotia Prescription Monitoring Program on patients 18 years of age and older who started first episode opioid agonist therapy with methadone or buprenorphine/naloxone for opioid use disorder in Nova Scotia between 2014 and 2018. Treatment episode was defined as date of initial opioid agonist prescription until there is a gap of greater than 6 days without receiving opioid agonist medication at a pharmacy. RESULTS: One thousand eight hundred sixty-seven of whom were analyzed as they had at least 1 day in treatment. There was significant treatment dropout within the first 2 weeks of treatment, which did not show a significant difference between OAT medication (23.4% of buprenorphine/naloxone; 22.2% methadone). Median duration of retention in treatment was 58 days for those treated with buprenorphine/naloxone and 101 days for patients treated with methadone. Multivariate cox proportional hazards model showed that buprenorphine/naloxone use as compared to methadone lead to increased hazard of treatment dropout by 62% (HR = 1.62). Hazard rate of treatment dropout for patients below 25 years of age was calculated. (HR 1.53). Median duration of retention in treatment for this subgroup of patients younger than age 25 was 37.5 days for patients treated with buprenorphine/naloxone and 69 days for patients treated with methadone. CONCLUSIONS: Our data suggests that methadone is a numerically superior medication for opioid use disorder when the metric of treatment retention is viewed in isolation, for our population in Nova Scotia. However, the results should be interpreted carefully considering the number of limitations of this study. There are social/accessibility, pharmacologic/safety, and patient preference factors which are also key in decision making when prescribing opioid agonist therapy. These must all be considered when deciding on which medication to initiate for a patient beginning a new treatment episode with OAT for opioid use disorder. This study should stimulate further research into this important area in addiction medicine.