The purpose of this research was to understand the reasons that led a group of young university students into the use of legal and illegal psychoactive drugs. This is a qualitative case study research, with a phenomenological design, with the participation of 32 students. The research techniques used were participant observation, non-participant observation, and focus groups. For data analysis we applied moment distinction, phenomenological reductions; and open, axial, and selective codification criteria. The main results show that the most used substances are alcohol, tobacco, and marihuana; regarding motivations, these are associated to family and academic settings, relationship with peers, personal decision, micro-traffic of illegal substances, and commerce of legal substances. Drug use turns into a network of social determinations around which the University could open a critical and self-reflective debate, focused on the subject and not on the substances.