Study procedures
The study comprised a screening visit, five 25-h test sessions, and an end-of-study visit. The sessions were conducted in a calm hospital room. One participant and one or two investigators were present during each test session. The sessions began at 8:00 AM. A urine sample was taken to verify abstinence from drugs of abuse, and a urine pregnancy test was performed in women before each session. The participants then received a standardized breakfast (two croissants) and underwent baseline measurements. Mescaline, LSD, psilocybin, or placebo was administered at 9:00 AM. Outcome measures were repeatedly assessed for 24 h. From t = 2 h to t = 3 h, participants underwent a functional neuroimaging scan; its results are subject to a separate publication. The participants remained under constant supervision during the acute effect phase and an investigator spent the night in the room next to the participants. The participants were sent home the next day at approximately 9:15 AM.