Acceptance and Committment Therapy Combined With Ketamine in Difficult-to-Treat Depression.

Inclusion Criteria: To participate in this study, individuals must be eligible for and enrolled in the Ketamine Intervention Program (KIP) at UBC Hospital, an ongoing clinical program for difficult-to-treat depression, which has the following inclusion criteria: 1. Be of age 18 or older. 2. Have a DSM-5-TR diagnosis of major depressive disorder (MDD) or bipolar disorder (BD), in a current major depressive episode. 3. If diagnosed with MDD, have failed adequate trials of at least 2 antidepressants and 1 adjunctive medication(s); if BD, having failed adequate trials of at least 3 agents for bipolar depression. 4. Have a referral from a community psychiatrist who agrees to follow the patient throughout the entire trial. 5. Have the ability to provide informed consent. Exclusion Criteria: Individuals will not be eligible for the K-ACT study if they are deemed ineligible for enrollment with the KIP at UBC Hospital due to meeting any of the following existing exclusion criteria: 1. They are experiencing active psychosis. 2. They have: any of history of ketamine use disorder; a history of alcohol or other substance use disorder in the past 6 months; dementia or delirium; unstable medical conditions; uncontrolled hypertension; a history of seizures in the past 6 months; a history of elevated intracranial pressure/aneurysmal disease/arteriovenous malformation/intracranial hemorrhage. 3. They are currently pregnant or breastfeeding. 4. They have an allergy to ketamine. Exclusion criteria specific to the K-ACT study include: 1. Currently undergoing ACT. 2. Past ACT within last 12 months. 3. Currently undergoing non-ACT structured psychotherapy (e.g., cognitive-behavioural therapy).