Background
The EUDA’s founding regulation acknowledges that the use of multiple substances is a common pattern of drug use, which increases the risks of health and social problems. The regulation tasks EUDA with monitoring ‘poly-substance use and its consequences, in particular the increased risk of health and social problems, the social determinants of drug use, drug use disorders and addictions, and the implications for policies and responses’. The Austrian National Focal Point of the REITOX Network was commissioned to conduct a dimensioning study on poly-substance use and its consequences with the aim of identifying future needs and areas for action for EUDA. The project was supervised by the EUDA/NFP Joint Working Group on poly-substance use.

Methods
Two rounds of expert consultation were conducted with experts from the EUDA network. In addition, desk-based and literature research was carried out to examine definitions, data availability and operationalizations, existing gaps, and relevant stakeholders. For each (potential) data source, recommendations on how to move forward concerning poly-substance use were elaborated.

Results
Key finding was a dynamic model for defining poly-substance use, which can be specified depending on the issue at hand. 88 (research/policy) questions in 12 thematic areas were developed. Clear priority was given to questions concerning the role of poly-substance use in connection with high-risk drug use and overdoses. Holistic research requires linking a wide variety of data sources and investigating the sociocultural, sociodemographic, and socioeconomic causes, motives, and consequences of poly-substance use.

Conclusion
A flexible and context-specific definition of poly-substance use is essential to address diverse research, and policy needs and should include legal and illegal/illicit substances and addictive behaviors. Existing data systems should be expanded to include legal substances and behavioral addictions, despite current institutional limitations, to improve comparability and resource efficiency. Research should prioritize high-risk drug users, especially in relation to overdose risks and the emergence of synthetic opioids. Real-time and delayed data sources offer valuable insights but require broader coverage and adaptation to better capture poly-substance use patterns. Targeted research projects, enhancing data collection and integration, and conducting qualitative studies to understand motivations, realities of life and long-term consequences of poly-substance use are necessary.