Effect of the COVID-19 pandemic on initiations of opioid agonist therapy in Ontario: A population-based interrupted time series analysis

• OAT initiations dropped sharply after COVID-19 onset but returned close to pre-pandemic levels. • The pandemic shifted OAT initiation from buprenorphine/naloxone toward methadone. • Flexible OAT policies may have helped maintain treatment access during public health emergencies. Maintaining access to opioid agonist therapy (OAT) was a key concern at the COVID-19 pandemic onset. Consequently, Ontario introduced guidance for changes to OAT delivery, including relaxed take-home dosing and expanded virtual care. We examined how the pandemic influenced OAT initiation trends. We conducted a population-based, retrospective interrupted time series analysis using linked administrative health data from March 2018 to January 2022 in Ontario, Canada. OAT initiations were defined as new (no OAT prescribed in the preceding two years) prescriptions for methadone, buprenorphine/naloxone, or slow-release oral morphine. Weekly initiation trends were compared before and during two pandemic periods defined by provincial emergency declarations. Analyses were stratified by medication type and sex. We identified 31,866 new OAT initiations. Immediately after the pandemic began, initiations declined significantly (-36.7/week; 95% CI: -53.3 to -20.1), then gradually returned toward baseline. Overall initiation volumes during the pandemic were not clearly different from pre-pandemic levels, although estimates were imprecise. However, prescribing patterns shifted: we estimate that methadone initiations increased by 2321 (95% CI: 1600 to 3043), while buprenorphine/naloxone initiations declined by 2009 (95% CI: −2783 to −1235) as a result of the pandemic. Sex-stratified trends were consistent with overall findings. The pandemic led to a temporary drop in OAT initiations but no long-term decline. A shift toward methadone suggests changing preferences among prescribers and patients, potentially driven by updated guidance and rising drug toxicity. These findings support the need for flexible, low-barrier OAT delivery models that can adapt to public health emergencies and evolving unregulated drug supply risks.