Double-Blind Comparison of the Two Hallucinogens Dextromethorphan and Psilocybin: Experience-Dependent and Enduring Psychological Effects in Healthy Volunteers.

Rationale: N-methyl-D-aspartate receptor-mediated dissociatives and serotonergic hallucinogens are being increasingly used in therapeutic interventions that involve nonordinary states of consciousness and may represent a unique mental health paradigm wherein pharmacologically induced experiences are conducive to psychological well-being. Objective: The aim of this study was to further understand how the phenomenological and health-promoting effects of high-dose dextromethorphan (DXM) compared to psilocybin in the same participants when administered under experimental conditions that are typical of therapeutic psychedelic trials. Methods: Single, acute oral doses of DXM (400 mg/70 kg), psilocybin (10, 20, 30 mg/70 kg), and inactive placebo were administered under double-blind and psychologically supportive conditions to 20 healthy participants with histories of hallucinogen use. Ratings of personal meaning, spiritual significance, psychological challenge, and psychological insight attributed to acute drug experiences were assessed 7 h (at session end) and 1 week after each drug administration. Persisting psychological effects were assessed 1 week after each drug administration. Results: High-dose DXM and psilocybin produced similar increases over placebo in ratings of drug experience that was predictive of psychological benefit at 1 week, even when expectancy effects were minimized. These effects tended to favor psilocybin in a dose-dependent manner and were limited by poor physical tolerability for DXM. Conclusions: This analysis suggests the utility of exploring clinical applications of dissociatives that occur within the supportive contexts that are characteristic of psychedelic research and that prioritize the optimization of psychologically valuable drug experiences. This study was registered with ClinicalTrials.gov (NCT02033707).