Epidemiological indicators represent important fragments of information about drug situation in a defined population and/or area. Professionals and their institutions involved in collecting of appropriate data sometimes have a tendency to overestimate their “own” data and underestimate data of other institutions or data collected according to different methods. In the case of long-term duration of such unscientific approach, it necessarily makes the situation less clear and practically impossible to evaluate. In the course of 1990’s, European drug epidemiology defined “five harmonized indicators”. They represent a minimum set of data that allow for qualified assessment of the level and severity of drug use at the international, national, regional or local levels.