A Clinical Trial of Add-on Oral Slow-release Ketamine Treatment in Major Depression

* The goal of this clinical trial is to explore if the treatment with ketamine tablets in addition to standard antidepressant therapy can reduce depressive symptoms in adults with Major Depressive Disorder. The main question it aims to answer is: Does adjunctive ketamine therapy reduce depressive symptoms after one week of treatment compared to baseline, measured by the Montgomery-Åsberg Depression Rating Scale (MADRS)? * Participants will start ketamine treatment together with a new standard antidepressant. During the treatment week, patients will receive four doses of Ketamine Hydrochloride Prolonged-Release Tablets (240 mg) at the clinic. They will fill in different questionnaires and rating scales during screening, treatment and follow-up, and will leave blood samples at five of the visits to monitor side effects and identify possible biomarkers. After a week, the ketamine treatment is finished while the standard antidepressant therapy continues. The participation in this trial is completed after three aditional weeks of follow-up.