This study is investigating a new treatment for Major Depressive Disorder (MDD) using a compound called CYB003, which is similar to a natural substance found in some mushrooms. The study aims to see if CYB003 works well and is safe for people with MDD, compared to a placebo (a dummy treatment).

**Eligibility:** Participants should be between 18 and 85 years old, have been diagnosed with MDD, and have tried other depression medications without much success. They need to be healthy overall, but those with certain mental health conditions, like schizophrenia, or who have used psychedelics recently, are not eligible.

**Key Points:**

• The study includes several visits and assessments, and participants must follow guidelines like not smoking during certain times.
• Participants need to use effective birth control methods during and after the study.
• There might be risks, such as side effects, but these will be monitored by the study team.

Participants will need to give written consent to join and agree to follow study rules, including recording of sessions.