"Individuals with drug use that could be categorised as problematic are generally a hard-to-reach population, making it difficult to obtain a picture of population size and development. Sweden also lacks a well-established definition of problematic or harmful drug use. In order to reach a more accurate picture of the Problem Drug Use (PDU) population size, as well as their living conditions, three nationwide studies were conducted in 1979, 1992 and 1998. In these case-finding studies, data was collected from professionals who met drug users in their daily work in e.g. the social services, healthcare, the police, the correctional system, customs and various treatment centres, including NGOs. Within a given period of time, the professionals reported clients or patients that either injected drugs at some point in the past 12 months or used illicit drugs daily or on an almost daily basis in the past four weeks. Those meeting these criteria where classified as problematic drug users. Estimates were obtained through capture-recapture calculations (Olsson et al., 2001).
"The population of problematic drug users in Sweden was estimated at approximately 15,000 in 1979, approximately 19,000 in 1992 and around 26,000 in 1998. This means an increase in nominal figures. However, a per capita figure would be more accurate since the general population increased during the same time period. In 1979, there were 1.8 PDUs per 1,000 inhabitants according to the above estimates. In 1998, this figure increased to 2.9 per 1000. Please note that the above figures refer to all ages."

Source

Swedish National Institute of Public Health. "2012 National Report (2011 data) To the EMCDDA by the Reitox National Focal Point: Sweden: New Development, Trends and in-depth information on selected issues." Östersund: Swedish National Institute of Public Health, 2012, p. 46.
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