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

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.

Quasi-experimental design. Participants: Over 1,529 students belonging to 12 high schools in Madrid (M=15.36 years, DT=1.13, range: 13-18, 51.8% boys) Experimental group: N=790, intervention period October-December 2018. Control group: N=469. The Student's t test was used for the repeated means test. When the difference between the means was significant, the effect size was calculated using Cohen's d index.

Alcohol: consumption (t=2.08, p<0.05, d=0.07), intention (t=3.01, p<0.01, d=0.12), risk perception (t=-502, p<0.001, d=0.20), attitudes (t=5.77, p<0.001, d=0.21), subjective norm (t=2.17, p<0.05, d=0.08). Cannabis: intention (t=2.98, p<0.01, d=0.11), risk perception (t=-3.99, p<0.001, d=0.14), attitudes (t=6.19, p<0.001, d=0.23).

Students who received the intervention reduced the favourable attitudes and intention of alcohol and cannabis consumption and alcohol consumption, and increased the risk perception on both substances, with significant differences, but low effect sizes. Related to control group and regarding cannabis there were no significant differences. The program has been shown to be potentially effective, but would benefit from a review for fit to its theoretical framework.