The study assesses the safety and preliminary effectiveness of MDMA-assisted cognitive behavioral therapy in participants diagnosed with obsessive-compulsive disorder (OCD).

Obsessive-Compulsive Disorder (OCD) is a chronic and disabling disorder that costs the economy over $2 billion annually and represents a significant public health problem. The proposed projects tests whether 3,4-Methylenedioxymethamphetamine (MDMA) reduces OCD symptoms.

Inclusion Criteria:

1. At least 18 years old
2. Fluent in speaking and reading the predominantly used or recognized language of the study site
3. Able to swallow pills
4. Meet the criteria for OCD diagnosis
5. YBOCS total score of at least 16
6. Not on psychotropic medications 1 month prior to study enrollment
7. Able to tolerate a treatment-free period
8. Able to tolerate study procedures
9. Failed at least 1 prior trial of standard first-line OCD treatment
10. Agree to the following lifestyle modifications: comply with requirements for fasting and refraining from certain medications prior to the Experimental Session, not enroll in any other interventional clinical trials during the duration of the study, and commit to medication dosing, therapy, and study procedures.

Exclusion Criteria:

1. Pregnant or nursing, or able to become pregnant and are not practicing an effective means of birth control
2. Weigh less than 48 kilograms (kgs)
3. Any current problem which, in the opinion of the investigator or study physician, might interfere with participation