Double-blind comparison of the two hallucinogens psilocybin and dextromethorphan: similarities and differences in subjective experiences
Although psilocybin and dextromethorphan (DXM) are hallucinogens, they have different receptor mechanisms of action and have not been directly compared.
This study compared subjective, behavioral, and physiological effects of psilocybin and dextromethorphan under conditions that minimized expectancy effects.
Single, acute oral doses of psilocybin (10, 20, 30 mg/70 kg), DXM (400 mg/70 kg), and placebo were administered under double-blind conditions to 20 healthy participants with histories of hallucinogen use. Instructions to participants and staff minimized expectancy effects. Various subjective, behavioral, and physiological effects were assessed after drug administration.
High doses of both drugs produced similar increases in participant ratings of peak overall drug effect strength, with similar times to maximal effect and time-course. Psilocybin produced orderly dose-related increases on most participant-rated subjective measures previously shown sensitive to hallucinogens. DXM produced increases on most of these same measures. However, the high dose of psilocybin produced significantly greater and more diverse visual effects than DXM including greater movement and more frequent, brighter, distinctive, and complex (including textured and kaleidoscopic) images and visions. Compared to DXM, psilocybin also produced significantly greater mystical-type and psychologically insightful experiences and greater absorption in music. In contrast, DXM produced larger effects than psilocybin on measures of disembodiment, nausea/emesis, and light-headedness. Both drugs increased systolic blood pressure, heart rate, and pupil dilation and decreased psychomotor performance and balance.
Psilocybin and DXM produced similar profiles of subjective experiences, with psilocybin producing relatively greater visual, mystical-type, insightful, and musical experiences, and DXM producing greater disembodiment.
KeywordsPsilocybin Dextromethorphan Hallucinogen Psychedelic Subjective experience Mystical experience Insightful experience Humans
We thank Mary Cosimano, M.S.W, Taylor Marcus, Darrick May, M.D., and five other staff members for their roles as session monitors, Dr. Annie Umbricht for medical management, Frederick Barrett, Ph.D., for contributing to the study design, Lisa Schade for technical assistance, Linda Felch for statistical assistance, and the pharmacy and medical staff. We also thank David Nichols, Ph.D., for synthesizing the psilocybin and Michelle Rudek, Pharm.D., Ph.D., and Nichole Anders of the Analytical Pharmacology Core for analyzing dextromethorphan metabolism. The study was conducted in compliance with US laws.
Compliance with ethical standards
The study was approved by the Institutional Review Board of the Johns Hopkins University School of Medicine. Participants gave their written informed consent before beginning the study procedures and were paid for their participation.
Conflict of interest
Dr. Carbonaro is an employee of the U.S. Food and Drug Administration (FDA); however, the views presented in this article do not necessarily reflect those of the FDA and no official support or endorsement of this article by the FDA is intended or should be inferred. Roland Griffiths is on the Board of Directors of the Heffter Research Institute.
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