Kosmicare project implements crisis intervention in situations related to the use of psychoactive substances at Boom Festival (Portugal). We present evaluation research that aims to contribute to the transformation of the project into an evidence-based intervention model. It relies on harm reduction and risk minimization principles, crisis intervention models, and Grof’s psychedelic psychotherapy approach for crisis intervention in situations related to unsupervised use of psychedelics. Intervention was expected to produce knowledge about the relation between substance use and mental health impact in reducing potential risk related to the use of psychoactive substances and mental illness, as well as an impact upon target population’s views of themselves, their relationship to substance use, and to life events in general. Research includes data on process and outcome indicators through a mixed methods approach, collected next to a sample of n=176 participants. Sample size varied considerably, however, among different research measures. 52% of Kosmicare visitors reported LSD use. Over 40% also presented multiple drug use. Pre-post mental state evaluation showed statistically significant difference (p<.05) confirming crisis resolution. Crisis episodes that presented no resolution were more often related with mental health outburst episodes, with psychoactive substance use or not. Visitors showed high satisfaction with intervention (n=58) and according to follow-up (n=18) this perception was stable over time. Crisis intervention was experienced as very significant. We discuss limitations and implications of evaluating natural setting based interventions, and the relation between psychoactive substance use and psychopathology. Other data on visitor’s profile and vulnerability to crisis showed inconclusive.