Drug markets are unpredictable with new substances emerging sporadically and established drugs varying in potency and quality. This unpredictability can result in people who use drugs being at increased risk of experiencing drug-related health harms. To counteract this, early warning systems (EWS) have been established in a number of regions and countries including the European Union (EU), United States (US), New Zealand, Netherlands, Scotland and Sweden. An EWS can identify changes in the drug market, such as new drug use patterns and substances in circulation that are contaminated, adulterated, or missold. They can enable effective and early dissemination of information and encourage harm reduction behaviour, thus protecting public health. Ireland’s National Drug and Alcohol Strategy: ‘Reducing Harm, Supporting Recovery: A health-led response to drug and alcohol use in Ireland 2017–25’ acknowledges the need to develop analytical capacity, data collection and public warning mechanisms. Strategic Action 3.3.38 of the strategy aims to strengthen the response to the illegal drug market, including the changing nature of new psychoactive substances. This evidence brief, comprising three descriptive country case studies, has been prepared by the Health Research Board (HRB) Evidence Centre with the aim of informing any future work that may be undertaken regarding the establishment of an EWS in Ireland.