Background: The aim of the study was to address the lack of evaluations of school-based substance use prevention programs and to conduct a quasi-experimental evaluation of the alcohol use part of the Triad intervention. Methods: Eleven Swedish intervention schools (285 pupils) and three control schools (159 pupils) participated in the evaluation. Baseline measurements were conducted in 2011 before the alcohol part in the prevention program was implemented in the intervention schools (school year 6, ages 12-13). We estimated an Intention-To-Treat (ITT) Difference-in-Difference (DD) model to analyze the effectiveness of the intervention on subsequent alcohol use measured in grades 7, 8 and 9. Results: The main results show no effect on the likelihood of drinking alcohol or drinking to intoxication. Conclusions: The lack of positive effects highlights the need for policy-makers and public health officials need to carefully consider and evaluate prevention programs in order to ensure that they are worthwhile from school, health, and societal perspectives.