Categorization of psychoactive substances into "hard drugs" and "soft drugs": a critical review of terminology used in current scientific literature

Background: Precise terminology and definitions are important components of scientific language. Although the terms hard drugs and soft drugs are used widely by professionals, neither the International Classification of Diseases nor the Diagnostic and Statistical Manual classify psychoactive substances into the categories hard and soft.Objectives: To analyze the occurrence of the terms hard drugs and soft drugs in recent scientific literature and to establish the degree of consensus in labeling psychoactive substances as hard or soft.Methods: A critical review of scientific papers listed in PubMed and Scopus between 2011 and 2015. Three hundred thirty-four articles were initially identified as potentially relevant for review, 132 of which were included in the final analysis.Results: One hundred twenty-four articles used the term hard drugs and 84.7% provided examples of substances considered hard. Forty-four articles used the term soft drugs and 90.9% provided examples of substances considered soft. Citations of relevant articles supporting categorization as hard or soft were not given in 90% of the articles. The authors often provided no or only very sparse information on their reasons for considering specific drugs as hard or soft.Conclusions: Although it initially appeared that there is substantial agreement as to which psychoactive substances should be regarded as hard and soft, closer inspection shows that the dividing line is blurred without clear criteria for categorization. At this time, it remains uncertain whether these terms should persist in the scientific literature. We therefore recommend these terms should be avoided or, if used, be clearly and precisely defined.