Objective To improve the understanding of problematic opioid use, monitoring of patterns of use is essential. We investigated changes in first-time registrations for opioid agonist treatment (OAT) in Ireland from 1999 to 2019.

Methods Data on individuals aged 15–64 years registering for OAT in Ireland for the first time from 1999 to 2019 were retrieved from the Central Treatment List, the database of all individuals receiving OAT in Ireland. Secular changes in numbers and rates of first-time registrations by age and sex were analyzed.

Results The overall incidence of first-time treatments for opioid use decreased from 40.7 per 100,000 in 1999 to 23.0 per 100,000 in 2019 (p < .001). The downward trend, seen in both sexes, was largely accounted for by declines among 15–29-year-olds, particularly those aged 24 years and younger, in whom there was an over 80% drop in first-time registration for OAT. In contrast, first-time registrations for OAT among 35–64-year-olds increased.

Conclusion The trends observed indicate a sustained decline in opioid use disorder in Ireland in younger individuals. These favorable findings support the view that recent drug policy in Ireland has contributed in part to the decline in heroin use among this group.