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This paper presents new methods for synthesizing results from subgroup and moderation analyses across different randomized trials. We demonstrate that such a synthesis generally results in additional power to detect significant moderation findings above what one would find in a single trial. Three general methods for conducting synthesis analyses are discussed, with two methods, integrative data analysis, and parallel analyses, sharing a large advantage over traditional methods available in meta-analysis. We present a broad class of analytic models to examine moderation effects across trials that can be used to assess their overall effect and explain sources of heterogeneity, and present ways to disentangle differences across trials due to individual differences, contextual level differences, intervention, and trial design.

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In contrast to the achievements with other substances, it has not yet been possible to reduce the levels of cannabis use, the most used illegal substance among Spanish adolescents. The objective of this paper consists of updating levels of use (incorporating ages 12 and 13), estimating high-risk use and analyzing possible associated variables. For this purpose, a correlational method was used consisting of the administration of a survey to compulsory secondary school and high school students from the autonomous community of Galicia (Spain) in 2016. Results obtained from a sample of 3,882 Galician adolescents aged 12 to 18 (M = 14.52; SD = 1.72) reveal that the percentage of adolescents currently using tobacco and cannabis is higher than that of those using tobacco alone (12.7% vs 10.5%). This implies not only a higher probability of using other illegal substances, but also of developing rather high-risk use pattern, binge drinking or even experiencing problematic Internet use or cyberbullying. From a preventive perspective, the results reveal that personal variables such as self-esteem, assertiveness, social skills or impulsiveness have really weak explanatory power compared with other variables related to the setting of rules and limits by parents. One of the main conclusions of this paper is the need to adopt a comprehensive prevention approach.

The EU-Dap study aimed to develop and evaluate a school-based curriculum for the prevention of substance use among young people. The school curriculum, "Unplugged," is based on social influence approach and addresses social and personal skills, knowledge, and normative beliefs. It consists of 12 one-hour interactive sessions delivered by teachers. Its effectiveness was evaluated through a randomized trial involving 7,079 pupils of seven European countries. Unplugged was effective in reducing cigarette smoking, episodes of drunkenness, and the use of cannabis at short term. This association, however, was confined to boys, with age and self-esteem as possible explanations of this difference. Beneficial effects associated with the program persisted at fifteen-month follow-up for drunkenness, alcohol-related problems, and cannabis use, and were stronger among adolescents in schools of average low socioeconomic level. These results are of scientific importance and may inform the adoption of effective public health interventions at population level.

Unplugged is a school-based prevention programme designed and tested in the EU-Dap trial. The programme consists of 12 units delivered by class teachers to adolescents 12-14 years old. It is a strongly interactive programme including a training of personal and social skills with a specific focus on normative beliefs. The aim of this work is to define the theoretical model of the program, the contribution of the theories to the units, and the targeted mediators. The programme integrates several theories: Social Learning, Social Norms, Health Belief, theory of Reasoned Action-Attitude, and Problem Behaviour theory. Every theory contributes to the development of the units' contents, with specific weights. Knowledge, risk perception, attitudes towards drugs, normative beliefs, critical and creative thinking, relationship skills, communication skills, assertiveness, refusal skills, ability to manage emotions and to cope with stress, empathy, problem solving and decision making skills are the targeted mediators of the program.

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