The objectives were to analyze the knowledge about overdose prevention, the use of naloxone, and the number of fatal overdoses after the implementation of Systematic Training in Overdose Prevention (STOOP) program. We conducted a quasi-experimental study, and held face-to-face interviews before (n = 725) and after (n = 722) implementation of systematic training in two different samples of people who injected opioids attending harm reduction centers. We asked participants to list the main causes of overdose and the main actions that should be taken when witnessing an overdose. We created two dependent variables, the number of (a) correct and (b) incorrect answers. The main independent variable was Study Group: Intervention Group (IG), Comparison Group (CG), Pre-Intervention Group With Sporadic Training in Overdose Prevention (PREIGS), or Pre-Intervention Group Without Training in Overdose Prevention (PREIGW). The relationship between the dependent and independent variables was assessed using a multivariate Poisson regression analysis. Finally, we conducted an interrupted time series analysis of monthly fatal overdoses before and after the implementation of systematic program during the period 2006-2015. Knowledge of overdose prevention increased after implementing systematic training program. Compared to the PREIGW, the IG gave more correct answers (IRR = 1.40;95%CI:1.33-1.47), and fewer incorrect answers (IRR = 0.33;95%CI:0.25-0.44). Forty percent of people who injected opioids who received a naloxone kit had used the kit in response to an overdose they witnessed. These courses increase knowledge of overdose prevention in people who use opioids, give them the necessary skills to use naloxone, and slightly diminish the number of fatal opioid overdoses in the city of Barcelona.