Comorbidity

« Back to Glossary Index

Comorbidity refers to the presence of two or more diagnosable medical or psychiatric conditions in the same individual, often associated with greater clinical complexity and increased treatment burden. Traditionally, comorbidity has been understood to mean the co-occurrence of distinct, independent disorders — attention-deficit/hyperactivity disorder (ADHD) alongside autism, or bipolar disorder alongside substance use disorder — each with its own underlying pathology, interacting to shape the overall course of illness. This model remains useful in many areas of medicine, particularly when conditions arise from clearly separate disease processes. However, in psychiatry, this definition is increasingly under revision. Many commonly observed “comorbidities” may instead reflect shared underlying systems — such as dysregulation in stress-response, inflammation, reward processing, or circadian rhythm — expressing themselves through different but overlapping symptom patterns. In this sense, what appears as multiple co-occurring disorders may, in some cases, represent different facets of a single, broader vulnerability rather than truly independent conditions. This distinction doesn’t eliminate the concept of comorbidity but refines it, helping to differentiate between co-occurrence due to coincidence and co-occurrence due to connection — a shift with important implications for both understanding and treatment. In sum:

• “Pure” (or system-based) comorbidity, where multiple diagnoses arise from shared underlying mechanisms

• Traditional comorbidity, where separate conditions co-exist and interact