[Prevention of substance abuse: randomised or observational evaluation is absolutely needed]

Prevention programs and interventions in the field of drug abuse are designed to reduce the incidence of onset of substance use, but they can also produce harms. This is the reason why rigorous evaluations are absolutely needed. In Europe, in spite of the large spread of such interventions, they are rarely evaluated with rigorous study designs. For these reasons it is important to promote the evaluation of every program that can be considered of relevance. This would allow the other practitioners to choose the interventions on the base of evidence of effectiveness. But which kind of evaluation prevention programs needed? There is a substantial agreement on the role of the randomized evaluation. Randomization is the only effective way to control for numerous factors that can bias the results. The typical evaluation design associated to most part of intervention programs, the pre-post assessment, cannot produce unbiased estimations. There are however many interventions that cannot be randomised, because they cannot be provided at the individual level (e.g like the price policies for cigarettes). These programs require an observational approach for evaluation, like a cohort or an Interrupted Time Series design. Even if these study designs have a complex methodology, the evaluation is of high relevance in order to allow the development of prevention strategies based on evidence.