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Intravenous ketamine has emerged as a rapid-acting antidepressant intervention that has shown promising effects in patients with treatment-resistant depression, although evidence remains heterogeneous and less well established across diagnostic subtypes. Nevertheless, real-world data comparing diagnostic subtypes and sex-related differences remain limited. This observational study examined the antidepressant efficacy, tolerability, and dissociative effects of intravenous ketamine in patients with treatment-resistant unipolar depression (TRD) and treatment-resistant bipolar depression (TRBD) in a naturalistic clinical setting.

Ninety-seven adult patients meeting DSM-5 criteria for unipolar or bipolar depression received racemic intravenous ketamine at a university psychiatric center. Depressive symptoms and global clinical severity were assessed at baseline and up to three months using the Montgomery-Åsberg Depression Rating Scale (MADRS) and the Clinical Global Impression-Severity scale (CGI-S). Dissociative symptoms were measured with the Clinician-Administered Dissociative States Scale (CADSS). Longitudinal outcomes were analyzed using mixed-effects models.

Both groups experienced significant reductions in depressive symptoms over time. Patients with bipolar depression demonstrated a faster and greater improvement than those with unipolar depression, with differences emerging from week 2 and persisting through three months. CGI-S scores showed a comparable pattern. The dissociative symptoms showed no change throughout the study and they appeared at the same level in all diagnostic categories. Sex-stratified analyses revealed no differences in antidepressant efficacy, although women with unipolar depression exhibited higher dissociative symptoms at three months.

Intravenous ketamine is effective in real-world treatment-resistant depression, with superior and more sustained antidepressant effects in bipolar compared with unipolar depression.