Substance use among adolescents is a major cause of concern as it can compromise adolescents‘ health, defined in 1948 by the World Health Organization (WHO) as ―a state of complete physical, mental, and social well-being‖ and may hinder adolescents from achieving the developmental transitions they are supposed to accomplish. Further, when individuals initiate substance use as adolescents, addiction is established more easily and quickly (Crews, He, & Hodge,2007; Prokhorov et al., 2006) and individuals remain at greater risk for negative outcomes in the future even if they successfully stop using (Georgiades & Boyle, 2007; Meier et al., 2012; von Sydow, Lieb, Pfister, Höfler, &Wittchen, 2002). With substance use being a preventable behaviour, prevention interventions have been implemented worldwide, mostly focused on demand reduction (Kulis, Nieri, Yabiku, Stromwall, & Marsiglia, 2007) and aimed at achieving some form of abstinence (Midford, 2009). Over recent years, efforts to determine whether prevention interventions are effective have increased due to the growing demand for accountability of interventions in public health (Hillebrand & Burkhart, 2009). Programme evaluation, besides considering the positive and desirable effects from prevention interventions, should also taken into consideration its negative and undesirable effects (i.e. iatrogenic effects) (European Monitoring Centre for Drugs and Drug Addiction (EMCDDA, 2012a). Programme evaluation is the mechanism through which this judgment on efficacy can be made (Midford, 2000), and isan essential tool to enable policy makers and practitioners to decide which projects to fund and whether a particular intervention is worth continuing, adapting, or discarding (EMCDDA, 2012a).
However, in Europe, despite prevention interventions now being systematically monitored by the majority of Member States (EMCDDA, 2009a), prevention effectiveness remains poorly researched (EMCDDA, 2010) and very few prevention interventions have actually been evaluated (EMCDDA, 2012b). Thus, evaluation of prevention interventions is urgently required to increase knowledge about how to enhance their intended effects and decrease the unintended, which in turn will contribute to future prevention interventions‘ success in reducing the prevalence of substance use among adolescents
