The aim of this thesis is to evaluate key outcomes and associated risk factors for people on opioid agonist treatment (OAT) in Ireland, across multiple settings. It also examines the impact of COVID-19 on service delivery and outcomes, and seeks to identify subsequent opportunities for improving quality of care.

The thesis consists of five studies. The risk of mortality associated with transfers between settings is evaluated in a cohort receiving OAT with methadone in Dublin specialised treatment centres between 2010 and 2016, linked with mortality and community dispensing data. Factors associated with treatment dropout at 3 and 12 months are examined in the same cohort. Trends in benzodiazepines/z-drugs, opioids and gabapentinoids prescribing practices in Irish prisons are observed between 2012 and 2020, accounting for gender and opioid use disorder (OUD). The impact of COVID-19 contingency changes on OAT provision are evaluated using interrupted time series. Finally, consensus is sought from stakeholders on potential improvements of quality of care resulting from above changes, using a Delphi methodology.

The main findings suggest that remaining on OAT with methadone is protective against all-cause and drug-related mortality, regardless of transfers between settings. Dropping out of OAT with methadone is common, with increased missed doses, low dose methadone and psychiatric comorbidities associated with dropout. Incarceration rates are high, and people with OUD constitute a distinct group, with additional health needs, within Irish prison settings. An increasing trend in gabapentinoids prescribing rates in men’s prisons was also noted. The rapid changes to OAT provision associated with the COVID-19 pandemic increased access to OAT in Ireland, with no evidence of increase in dropout. Sixteen recommendations derived from the contingency changes to OAT guidelines were regarded as contributing to quality of care beyond the pandemic by a panel of expert stakeholders. In contrast, the frequency of urine drug testing and methadone dose did not reach consensus.

This thesis has contributed to knowledge in the field of OAT, also impacting policy, services and society. Future research is warranted to examine the impact of revised guidelines on OAT prescribing practices and drug related mortality in Irish settings.