Acute Effects of 3,4-methylenedioxymethamphetamine (MDMA) With and Without a Booster Dose

Inclusion Criteria: 1. Good understanding of the German language. 2. Understanding the procedures and the risks that are associated with the study. 3. Participants must be willing to adhere to the protocol and sign the consent form. 4. Participants must be willing to refrain from taking illicit psychoactive substances during the study. 5. Participants must be willing to drink only alcohol-free liquids and no coffee, black or green tea, or energy drinks after midnight of the evening before the study session, as well as during the study day. 6. Participants must be willing not to drive a traffic vehicle or to operate machines within 48h after substance administration. 7. Willing to use effective birth control throughout study participation. 8. Body mass index between 18-29 kg/m2. Exclusion Criteria: 1. Relevant chronic or acute medical condition. 2. Current or previous major psychiatric disorder. 3. Psychotic 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. 4. Hypertension (SBP\>140/90 mmHg) or hypotension (SBP\<85 mmHg). 5. Previous MDMA use more than 20 times or any time within the previous month. 6. Pregnant or nursing women. 7. Participation in another clinical trial (currently or within the last 30 days). 8. Use of medications that may interfere with the effects of the study medications. 9. Tobacco smoking (\>10 cigarettes/day). 10. Consumption of alcoholic drinks (\>15 drinks/week).