The primary objective of this study is to assess the long-term efficacy of psilocybin with respect to use of new antidepressant treatment, hospitalisations for depression, suicidality, and depressive severity rated using the Montgomery and Asberg Depression Rating Scale (MADRS) over a total of 52 weeks (compared across the 1 mg, 10 mg and 25 mg psilocybin groups from COMP 001).

In this present study (COMP 004), the aim is to follow up participants from COMP 001 and COMP 003 in a long-term follow up study, with both remote and digital assessments, to explore the long term efficacy and safety of the three different doses of psilocybin (1 mg, 10 mg, and 25 mg) administered to patients with TRD as a monotherapy in COMP 001 and 25 mg psilocybin administered as an adjunct to an SSRI in COMP 003. Patients previously treated in COMP001 will be followed for approximately 40 weeks and patients previosuly treated in COMP003 will be followed for approximately 49 weeks giving a total follow up period of 52 weeks from psilocybin dosing.

Inclusion Criteria:

Signed ICF Each participant having completed the final study visit of either COMP 001 or COMP 003 Ability to complete all protocol required assessment tools (including having access to the internet in order to complete the digital assessments) without any assistance or alteration to the copyrighted assessments, and to comply with all study visits

Exclusion Criteria:

Subject has any condition, for which in the opinion of the investigator, participation would not be in the interest of the subject eg participation could compromise the wellbeing of the participant or prevent, limit, or confound the protocol-specified assessments

TRD patients who completed COMP001 or COMP003