Psilocybin Efficacy With or Without Pimavanserin Pretreatment

Inclusion Criteria: * Adults aged 21 to 65 years old * Fluency in English * At least high school level of education * Right-handed * Female participants of childbearing potential must agree to use a highly effective method of contraception throughout the study. Highly effective methods are defined as those that, when used consistently and correctly (alone or in combination), are associated with a failure rate of less than 1% per year. Condoms alone are not considered highly effective. Abstinence is not considered a highly effective method. However, at the investigator's discretion, abstinence may be accepted if, based on the investigator's judgment and knowledge of the participant's usual and preferred lifestyle, it can be reasonably expected to result in 100% effectiveness. * Agree to abstain from any psychoactive drugs on the day prior to and the day of the drug administration session as demonstrated by a negative toxicology report. * Agree to consume approximately the same amount of caffeine-containing beverage (e.g., coffee, tea) that he/she consumes on a usual morning, before arriving at the research unit on the mornings of drug session days. If the participant does not routinely consume caffeinated beverages, he/she must agree not to do so on session days. * Agree not to take any PRN medications on the mornings of drug sessions * Agree that for one week before the drug session, he/she will refrain from taking any nonprescription medication, nutritional or herbal supple…