Problematic opioid use is a significant problem in Ireland and across the world. However, measuring the prevalence of opioid use is challenging. Given the nature of this population, a simple head count is not feasible and general population surveys are known to be ineffective at capturing this ‘hidden’ population. Because people who use drugs fear stigmatisation and are often marginalised in society, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) recommends the use of indirect approaches, such as the capture–recapture (CRC) method, to estimate the prevalence of problematic (high-risk) drug users.