The exploration of classic psychedelics as potential treatments for psychiatric disorders has gained momentum, yet the understanding of how these substances interact with other psychoactive drugs, like cannabis, is still in its infancy. A study involving 321 participants sought to shed light on the subjective experiences induced by the concomitant use of psychedelics and varying doses of cannabis.Through online surveys administered 7 days before and 1 day after a planned psychedelic experience, researchers collected data on demographics, setting variables, and responses to five validated questionnaires designed to measure aspects of the psychedelic experience. These included the Mystical Experience Questionnaire (MEQ), visual subscales of the Altered States of Consciousness Questionnaire (ASC-Vis), the Challenging Experience Questionnaire (CEQ), the Ego Dissolution Inventory (EDI), and the Emotional Breakthrough Inventory (EBI). Participants were categorized based on their reported cannabis use: none, low, medium, or high dose, in conjunction with a serotonergic psychedelic.The findings suggest a significant interaction between cannabis and psychedelics on the intensity of the psychedelic experience. A linear relationship was observed between the cannabis dose and the scores on the MEQ, ASC-Vis, and EDI, indicating that higher cannabis doses were associated with more intense mystical experiences, visual alterations, and feelings of ego dissolution. Interestingly, the relationship between cannabis dose and CEQ scores, which measure challenging experiences, followed a quadratic trend, hinting at a more complex interaction that might not linearly escalate with higher cannabis doses. No significant correlation was found between the cannabis dose and EBI scores, suggesting that the emotional breakthrough component of psychedelic experiences remains unaffected by cannabis use.These findings point to a nuanced interaction between cannabis and psychedelics, affecting the acute subjective experiences of users. However, the study's design constraints limit the ability to definitively conclude the causality direction and the clinical implications of these interactions. This underscores the need for further research into how psychedelics and cannabis interact and the potential implications for therapeutic settings, emphasizing the complexity and individual variability of combining psychoactive substances.