The program under evaluation, which we will refer to as "Program B," is a peer-led prevention intervention targeting tobacco, alcohol, and cannabis use, designed and implemented by a prevention organization. This program involves a group of 11th-grade high school students (youth facilitators) receiving training (28 hours) provided by the prevention organization, with the goal of delivering an addiction awareness session (2 hours) to their 10th-grade peers. The training for the peer leaders is based on an addiction prevention awareness program (developing knowledge about substances, observing a court hearing, conducting research, participating in a roundtable discussion, giving presentations, and simulating a session). Following their training, the awareness session they deliver to their peers involves screening short films followed by a discussion on the various options presented in the films. During these discussions, the youth ambassadors provide information on substances, contexts, the risks of use, as well as available resources and support. The evaluation consists of a quasi-experimental controlled before-and-after study, with measurements at T0 (1 month before the intervention) and T1 (1 month after the intervention). The study was conducted among 313 young people, with an average age of 15.9 years, enrolled in 10th grade at three high schools in a French department. The analysis compares the "Intervention" group (n=115), comprising two high schools that received Program "B," to the "Control" group (n=198), comprising one high school that received no structured prevention program. Data are collected via a paper questionnaire. The primary indicators are tobacco, alcohol, and cannabis use, as well as binge drinking behaviors. The secondary indicators are the intermediate variables directly targeted by the program: intention to use, knowledge and attitudes toward substances, resistance to peer pressure, perceived peer use, and school climate. The effectiveness evaluation is supplemented by a process evaluation using the following indicators: dosage and target achievement, fidelity, facilitator participation levels, opinions on youth peer leaders, and program satisfaction. The results show no effect of the intervention on substance use (tobacco, alcohol, cannabis), a counterproductive effect of the intervention on the intention to consume alcohol, and a positive effect of the intervention on the perceived ability to resist peer pressure regarding alcohol consumption. The process evaluation highlights several weaknesses in the program’s implementation: low credibility of the youth peer leaders among their peers, an excessive training workload, variability in the content delivered, and significant inequality in access to prevention between youth peer leaders and the youth they target in terms of the amount of prevention provided. Based on the analyzed data and despite the evaluation’s limitations, the results suggest that this program should not be recommended in its current form. Within the framework of peer-to-peer prevention, proposals consistent with the relevant scientific literature are formulated to make major modifications to the intervention.