The goal of the above mentioned activities is an attempt to measure the scale of the unseen  problem – the hidden population of problem drug users. A hidden population in the context of drugs is the population of problem drug users, who, for a variety of reasons, are not included in the statistics of the healthcare system, the police or any others and due to their lifestyles are not usually covered in studies such as population surveys on drug use.

The basic problem in estimating the hidden population is the precise definition of the group. The Polish estimations are based on the national definition of the term problem drug use. According to such a definition problem drug use is understood as being long term drug use (use of illegal substances), causing serious health and social issues. Therefore, the Polish definition includes problem drug users of opiates, amphetamine, cocaine (although these cases are rarely recorded), as well as cannabis and other drugs. Such definition of the issue is due to the limited data sources for estimations. In Poland, beside population surveys, sources of data include data on the undergone treatment collected as part of the statistical system of the Institute of Psychiatry and Neurology. The data is based on ICD-10 medical classification, which determines the way the studied population is defined. Moreover, the data based on ICD-10 makes it difficult to implement a precise breakdown into substances because the majority of the cases recorded in the system belong to the category “other and mixed”.

The Polish definition is not identical with the operational definition of the European Monitoring Centre for Drugs and Drug Addiction, which defines problem drug users as long lasting, frequent or injection users of opiates, amphetamine and/or cocaine.

There is at least a few methods of estimating the number of problem drug users. The basic methods include the multiplier method, capture-recapture method and the Multivariate Indicator Method (MIM). In Poland, due to limited sources of data, the method that is used most frequently is the benchmark method.

The latest estimation based on population surveys from 2006 shows that the number of problem drug users ranges between 100 000 and 125 000 individuals. Among this group, the number of problem opiate users has been estimated to range between 25 000 and 29 000 (Institute of Psychiatry and Neurology 2007).