Role of the Serotonin 2A Receptor in Psilocybin-induced Altered States of Consciousness

Inclusion Criteria: 1. Age between 25 and 75 years. 2. Sufficient understanding of the German language. 3. Understanding the procedures and the risks that are associated with the study. 4. Participants must be willing to adhere to the protocol and sign the consent form. 5. Participants must be willing to refrain from taking illicit psychoactive substances during the study (not including cannabis). 6. Participants must be willing not to drive a traffic vehicle or to operate machines within 48 h after substance administration. 7. Women of childbearing potential must be willing to use effective birth-control throughout study participation Exclusion Criteria: 1. Chronic or acute medical condition, including a history of seizures. 2. Body mass index 18-29.9 kg/m2 3. Current or previous major psychiatric disorder (e.g. psychotic disorders, mania / hypomania, anxiety disorders). 4. Psychotic or bipolar disorder in first-degree relatives, not including psychotic disorders secondary to an apparent medical reason, e.g., brain injury, dementia, or lesions of the brain. 5. Hypertension (SBP\>140/90 mmHg) or hypotension (SBP\<85 mmHg) 6. Psychedelic substance use (with the exception of cannabis) more than 20 times or any time within the previous two months 7. Pregnant or nursing women. 8. Participation in another clinical trial (currently or within the last 30 days). 9. Use of medications that may interfere with the effects of the study medications (any psychiatric medications and any medication with known to interact with the study substances). 10. Tobacco smoking (\>10 cigarettes/day). 11. Consumption of alcoholic drinks (\>15 drinks / week). 12. Body weight \< 45 kg.