[Evaluation of the effectiveness of a school-based alcohol and cannabis prevention programme among adolescents.].

Objective: Alcohol and cannabis consumption in young people aged 14 to 18 years show high and stable prevalence and incidence. The Instituto de Adicciones de Madrid Salud (IAMS) within its prevention strategy aimed at adolescents and young people in Madrid, implements an universal 3-sessions school-based prevention program. To evaluate the effectiveness of a program aimed at alcohol and cannabis from the Theory of Planned Behavior (TPB, Ajzen, 1991), we analysed the changes in consumption and intention of consumption, attitudes and subjective norm as well as in the risk perception.