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Same Prevention, Different Effects? Effect modification in an alcohol misuse prevention project among high-school juniors

Background: The 'Healthy School and Drugs' (HSD) project is the most widely disseminated substance misuse prevention project among high-school juniors (aged 12-15) in the Netherlands. Its aim is a more controlled use of tobacco, alcohol and cannabis. Previous studies showed that the HSD project is effective in reducing alcohol use prevalence. However, what is not known is whether this intervention creates differential effects in various segments of the student population. Aim: To identify factors that modify the effect of the intervention with regard to alcohol-use prevalence. Method: In a prospective quasi-experiment six experimental schools participated in the HSD project and four control schools did not. During four consecutive years 1720 students reported on their substance use. In addition, a number of putative effect modifiers (e.g. age, gender) were registered. Results: Multivariate logistic regression analysis showed that the effect of the intervention on alcohol use was less favourable in students who disliked school. A positive trend, bordering on significance, was found in those who perceived drinking as unhealthy. Two adverse effects showed p -values under 0.10: being a girl in the third grade of the high-school and having parents who forbid teenage drinking. Discussion: Knowledge of modifiers can be used to tailor the intervention to be more effective for specific population segments that were previously unaffected or adversely affected by the intervention.