The program under evaluation, which we will refer to as "Program A," is an intervention aimed at preventing alcohol and cannabis use, designed and implemented by a prevention organization. This program involves CFA (Apprentice Training Center) staff receiving a two-day training session, with the goal of delivering the intervention to each class of apprentices. The intervention consists of two 3-hour modules delivered three weeks apart. The modules are structured into different segments focusing primarily on knowledge and attitudes toward substances, the development of psychosocial skills, and the reduction of substance use. They rely on the use of video clips and group activities (role-playing, self-assessment, slogan creation, etc.). The evaluation consists of a quasi-experimental controlled before-and-after study, with measurements at T0 (1 month before the intervention), T1 (1 month after the intervention), and T2 (3 months after the intervention). The study was conducted among 343 young apprentices, with an average age of 16.7 years, enrolled in the first year of the CAP (Certificate of Professional Competence) program at six CFA (Apprenticeship Training Centers) specializing in construction and public works in the same French region. The analysis compares the "Intervention" group (n=175), consisting of classes that received Program "A," to the "Control" group (n=168), consisting of classes that did not receive any structured prevention program. Data were collected via a paper questionnaire. The primary indicators are initiation and use of alcohol and cannabis, as well as drunkenness behaviors; the secondary indicators are the intermediate variables targeted by the program: intention to use, knowledge and attitudes toward substances, psychosocial skills, perceived peer use, school climate, and substance-related behavioral problems. The effectiveness evaluation is supplemented by a process evaluation with the following indicators: program dosage and target achievement, fidelity, facilitator participation, facilitation techniques, program satisfaction, and exposure to other prevention interventions. The results show counterproductive effects of the intervention on alcohol use (increased intention to consume alcohol), drunkenness (increased proportion of youth exhibiting drunkenness behavior), and cannabis use (reduced proportion of youth attempting to reduce cannabis use). The process evaluation highlights several weaknesses in the program regarding program dosage, content, and activities that need to be adapted to the target audience. Based on the analyzed data and despite the evaluation’s limitations, the results suggest that implementing this program in its current form is not recommended. Proposals consistent with the scientific literature are formulated to make major modifications to the intervention.