Study drugs
Mescaline hydrochloride (99.3% purity; ReseaChem GmbH, Burgdorf, Switzerland) was administered in opaque capsules that were produced according to Good Manufacturing Practice (GMP) in units that contained 100 mg mescaline. The exact analytically confirmed mescaline hydrochloride content (mean ± SD) was 95.0 ± 0.1 mg (Psilocybin (99.7% purity; ReseaChem GmbH, Burgdorf, Switzerland) was administered in opaque capsules that were produced according to GMP in units that contained 5 mg of psilocybin dihydrate. The exact analytically confirmed psilocybin content was 4.61 ± 0.09 mg (The stability of all formulations was confirmed for the study duration. A double-dummy method was used. Participants 1–16 received seven capsules and one solution in each session: (i) seven placebo capsules and a placebo solution, (ii) four placebo capsules, three 100 mg mescaline capsules, and a placebo solution, (iii) seven placebo capsules and a 100 µg LSD solution, and (iv) three placebo capsules, four 5 mg psilocybin capsules, and a placebo solution. Participants 17–32 received nine capsules and one solution in each session: (i) nine placebo capsules and a placebo solution, (ii) four placebo capsules, five 100 mg mescaline capsules, and a placebo solution, (iii) nine placebo capsules and a 100 µg LSD solution, and (iv) five placebo capsules, four 5 mg psilocybin capsules, and a placebo solution. The participants were asked to guess which substance they had ingested during each study session at t = 3 h as well as after study completion.