Study Overview: This study is testing a new treatment called COMP360 for people with treatment-resistant depression (TRD). TRD means the depression hasn't improved with other treatments. The study is international and includes people aged 18 and older. Participants will be randomly chosen to receive either COMP360 or a placebo (a pill with no medicine) and will get support from a therapist.

The study has three parts and lasts about 62 weeks. The first part (Part A) is six weeks long and checks how well the treatment works compared to the placebo. Part B lasts 20 weeks and looks at long-term effects and if re-treatment works. Part C is 26 weeks and gives the treatment openly without placebo.

• Participation lasts up to 62 weeks, with several parts and check-ups.
• You must be 18 or older with TRD to join.
• Some mental health conditions and treatments may exclude you from participating.

Note: Discuss with your doctor before deciding to join, as the study involves a long commitment and certain health considerations.