Longitudinal trajectories of major depressive disorder provide further clinical perspectives for precision psychiatry.

Major depressive disorder (MDD) is a common and debilitating disorder. MDD was considered to be a relatively benign disorder compared to other major psychiatric disorders. However, longitudinal research revealed heterogeneous and frequently chronic course. Episodic or chronic progress reframes MDD as a disorder with a variable and enduring course. The recognition of MDD's heterogeneity has prompted a methodological evolution in psychiatric research, moving away from group-level averages toward more sophisticated, person-centered analytical techniques. In addition to inherent heterogeneity, biological, psychosocial, and environmental factors are also influential on the clinical manifestations of MDD. The course of major depressive disorder is not only heterogeneous among individuals but also varies systematically across different populations. Multiple models have consistently identified several distinct trajectory classes across diverse populations and study designs, providing a more nuanced map of how MDD unfolds over a lifetime. The concept of MDD subtyping within precision psychiatry directly addresses its inherent heterogeneity. If different subtypes of MDD possess distinct neurobiological underpinnings, then identifying these specific subtypes through biomarkers may lead to highly targeted and significantly more effective treatments. Longitudinal perspectives are indispensable for an accurate understanding of prognosis, the development of preventive strategies, and the implementation of effective long-term management plans that can alter the illness's trajectory. It was aimed to review the MDD trajectories, treatments, and symptom clusters from population and life span-based perspectives in this paper.