Background/Aims: Polydrug use is a complicated phenomenon that is measured in a wide variety of different ways. Using Finland as an example, we aimed to demonstrate how the prevalence and prevalence trends of concurrent polydrug use (CPU) varied in the general population based on the different measurements used. Methods: Population-based Drug Surveys conducted every 4 years during 1998-2014 were used. CPU was measured with different measurements: strict, medial and loose definition of CPU, which were based on different combinations of alcohol, illicit drugs, pharmaceutical drugs and cigarettes used during the last 12 months/30 days. Logistic regression was used to estimate the p values for assessing trends. Results: Depending on the measurements used, the prevalence of CPU in 2014 varied between 2.0 and 18.7%. Different definitions also produced contradictory trends of CPU: there was a modest increase in prevalence if it was measured with a medial (p < 0.001) or strict (p = 0.054) definition, but when measured with the loose definition (only measure that included smoking), there was a decrease in prevalence (p < 0.001). Conclusions: The prevalence of CPU varies greatly depending on the measurement used, as does the course of the prevalence trends. The concept of simultaneous polydrug use may capture the phenomenon better compared to the concept of CPU.