Schizophrenia is a mental disorder characterized variously by hallucinations, delusions, disorganized thinking and behavior, and flat or inappropriate affect. Symptoms develop gradually and typically begin during young adulthood and are never resolved.
Alzheimer's disease is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events.
Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that each last from days to weeks.
Depression is a mental state of low mood and aversion to activity. It affects about 3.5% of the global population, or about 280 million people worldwide, as of 2020. Depression affects a person's thoughts, behavior, feelings, and sense of well-being.
Panic disorder is a mental and behavioral disorder, specifically an anxiety disorder characterized by reoccurring unexpected panic attacks.
Symptoms: Sudden periods of intense fear, palpitations, sweating, shaking, shortness of breath, numbness
Medication: Antidepressants, benzodiazepines, beta blockers
Treatment: Counselling, medications
Diagnostic method: Based on symptoms after ruling out other potential causes
Symptoms: Hallucinations, delusions, disorganized thinking and behavior, flat or inappropriate affect
Management: Counseling, life skills training
Medication: Antipsychotics
Causes: Environmental and genetic factors
Deaths: ~17,000
Diagnostic method: Based on observed behavior, reported experiences, and reports of others familiar with the person
Duration: Chronic
Mental health
In psychiatry, psychology, and mental health counseling, comorbidity refers to the presence of more than one diagnosis occurring in an individual at the same time. However, in psychiatric classification, comorbidity does not necessarily imply the presence of multiple diseases, but instead can reflect current inability to supply a single diagnosis accounting for all symptoms.[10] On the DSM Axis I, major depressive disorder is a very common comorbid disorder. The Axis II personality disorders are often criticized because their comorbidity rates are excessively high, approaching 60% in some cases. Critics[who?] assert this indicates these categories of mental illness are too imprecisely distinguished to be usefully valid for diagnostic purposes, impacting treatment and resource allocation.[citation needed] Symptom overlap is a key component against DSM classification and serves as a note towards redefining criteria in disorders that the root cause may not be understood thoroughly. Regardless of criticisms, it stands that, annually[where?], up to 45% of mental health patients fit the criteria for a comorbid diagnosis. A comorbid diagnosis is associated with more severe symptomatic expression and greater chance of dismal prognosis.[11] Certain diagnoses such as ADHD, autism, OCD, and mood disorders have higher rates of co-occurring or being prevalent in separate diagnoses. "Comorbidity in OCD is the rule rather than the exception" with OCD diagnoses facing a lifetime rate of 90%.[12] With overlapping symptoms comes overlap in treatment as well, CBT for example is common for both ADHD and OCD with pediatric onset and can be effective for both in a comorbid diagnosis.[13] OCD and eating disorders have a high rate of occurrence, it is estimated that 20-60% of patients with an eating disorder have OCD.[14] More often, comorbidity complicates and can prevent treatment efficacy on a varying scale depending on the circumstances.

The term 'comorbidity' was introduced in medicine by Feinstein (1970) to describe cases in which a 'distinct additional clinical entity' occurred before or during treatment for the 'index disease', the original or primary diagnosis. Since the terms were coined, meta studies have shown that criteria used to determine the index disease were flawed and subjective, and moreover, trying to identify an index disease as the cause of the others can be counterproductive to understanding and treating interdependent conditions. In response, 'multimorbidity' was introduced to describe concurrent conditions without relativity to or implied dependency on another disease, so that the complex interactions to emerge naturally under analysis of the system as a whole.[15]

Although the term 'comorbidity' has recently become very fashionable in psychiatry, its use to indicate the concomitance of two or more psychiatric diagnoses is said to be incorrect because in most cases it is unclear whether the concomitant diagnoses actually reflect the presence of distinct clinical entities or refer to multiple manifestations of a single clinical entity. It has been argued that because "'the use of imprecise language may lead to correspondingly imprecise thinking', this usage of the term 'comorbidity' should probably be avoided".[16]

Due to its artifactual nature, psychiatric comorbidity has been considered as a Kuhnian anomaly leading the DSM to a scientific crisis[17] and a comprehensive review on the matter considers comorbidity as an epistemological challenge to modern psychiatry.[18] The Hierarchical Taxonomy of Psychopathology is a leading alternative classification system that addresses these concerns about comorbidity.