INTRODUCTION: The intensity of the psychedelic experience has been hypothesized as the main predictor of response to a psychedelic treatment. This study aimed to investigate factors that may be associated with the intensity of mystical experiences during naturalistic psychedelic use. METHODS: The data of this comprehensive sample were aggregated from four previous retrospective surveys, where mystical experience intensity was assessed using the mystical experience questionnaire (MEQ-30). Additional collected data included psychological flexibility levels, intentions regarding psychedelic use, substance used, subjective dosage levels, and socio-demographic information. ANOVA and linear regression were performed to identify predictors of MEQ-30 scores. RESULTS: A total of 1657 participants were included in this study. The significant predictors of the total MEQ score were: the main motive for the psychedelic experience (with a greater impact of spiritual/religious, therapeutic, and self-exploration, compared to recreational), the type of substance used (with a greater impact for Ayahuasca and lysergic acid diethylamide than for psilocybin), the subjective dosage (greater impact of very high, high, and moderate doses, compared to a very low dose), the number of psychedelic sessions, the time elapsed since the experience, and concomitant alcohol use. CONCLUSION: This large sample study highlights significant associations between the intensity of mystical experiences during naturalistic psychedelic use and several key factors: the type of the psychedelic substance used, dosage, and set, particularly participants' intentions. Moreover, results show that concomitant alcohol use is associated with less intense psychedelic experiences, emphasizing the relevance of screening participants for substance use in research settings.