Abstract. Background. Depression remains one of the most prevalent and disabling psychiatric disorders globally, with growing attention to its interaction with other mental health conditions. The purpose of this study was to conduct a narrative literature review on the impact of comorbid psychiatric disorders on the course and treatment of depression. Methods. The review synthesizes findings from 51 peer-reviewed publications released in 2018-2025, identified through structured searches in major scientific databases, including PubMed, Scopus, Web of Science, and PsycINFO, and supplemented by historically relevant sources. Results. The most clinically significant comorbid psychiatric conditions identified were schizophrenia, attention-deficit hyperactivity disorder (ADHD), and substance use disorders. Schizophrenia frequently co-occurs with depression, particularly in early stages or after psychotic episodes. This comorbidity is linked to poorer prognosis, increased suicidality, and reduced treatment response. Several second-generation antipsychotics such as quetiapine, lurasidone, clozapine demonstrated variable efficacy, though none ensured stable remission across groups. In the context of ADHD, depression follows distinct developmental and clinical trajectories. Early-onset ADHD markedly raises the risk of major depressive episodes, and pharmacological approaches must balance stimulant therapy with mood stabilization. In individuals with both ADHD and depression within bipolar disorder, treatment becomes more complex, requiring caution with antidepressants and use of agents like bupropion or mood stabilizers. Among those with substance use disorders, depression more often becomes chronic and resistant to treatment. Substance use further reduces antidepressant effectiveness and raises relapse risk. Integrated treatment approaches addressing both substance use and depressive symptoms are more effective. Conclusions. The findings emphasize the need for multidimensional diagnostic frameworks and individualized therapeutic strategies when managing depression with psychiatric comorbidity.

Key words: depression, comorbidity, schizophrenia, attention deficit hyperactivity disorder, substance abuse, pharmacotherapy

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