The hidden population in the context of drugs is the population of problem drug users who for a number of reasons were not entered in drug treatment records, police statistics or others and due to their lifestyle are frequently not covered by such measurements as drug prevalence estimates. Estimating the size of this population is a challenge for sociologists and statisticians.

The basic problem in estimating hidden populations is the precise definition of this group. Estimates describes in this section of the website are based on the national definition of problem drug use, which is understood as long-term use (of illicit substances) in the course of which the user develops serious problems (National Report 2006). The Polish definition covers problem users of opioids, amphetamines, cocaine (though registered occasionally), cannabis and other drugs.

The national definition is not identical the definition of the European Monitoring Centre for Drugs and Drug Addiction which defines problem drug use as injecting drug use or long duration/regular use of opioids, cocaine and/or amphetamines.

The definition clearly defines characteristics of the population under study and has impact on the study results. In practice we often face a dilemma: on the one hand we would like to adopt the best, most comprehensive and precise definition of problem drug use in an operating sense of the word, on the other hand, however, we need to consider limitation resulting from the availability of data sources and application of specific methods. Most definitions adopted are not ideal. They are the outcome of a broad compromise.