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The section “Definitions” updated to reflect recent research. Bibliography expanded and updated.

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Abstract and Keywords

This entry defines comorbidity and similar terms used in various fields of practice. It addresses the prevalence of comorbidity, suggests explanations for comorbidity, and discusses integrated treatment for comorbid conditions and the importance of the concept of comorbidity in social work practice.

Keywords: coexisting disorders, comorbidity, co-occurring disorders, cognitive disabilities, developmental disabilities, dual diagnoses, intellectual disabilities, medical illness, mental disabilities, mental disorders, mental illness, intellectual disability, physical disabilities, substance-use disorders


Comorbidity has been defined as the simultaneous presence of two or more illnesses, diseases, disorders, or disabilities. These conditions may be related to one another or unrelated, and they may be physical (medical), mental (psychiatric, including substance-use disorders), and/or cognitive (intellectual). Comorbidity is also referred to as dual diagnosis or dual diagnoses, coexisting disorders, and co-occurring disorders. These terms and their definitions may vary by field of practice. For example, in the developmental-disabilities field, the term dual diagnosis is often used to mean that a person has an intellectual disability and a mental disorder or disorders (see, for example, McGilvery & Sweetland, 2011). Mental health and substance-abuse professionals use several terms interchangeably when referring to the condition of having one or more mental disorders or one or more mental disorders and one or more substance-use disorders (DiNitto & Webb, 2012). The National Institute of Mental Health (NIMH) often uses the term comorbidity, while another U.S. government agency, the Substance Abuse and Mental Health Services Administration (SAMHSA), often uses the term co-occurring disorders (Center for Substance Abuse Treatment, 2005).

In the United States, the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM) is generally used as the basis for diagnosing mental, substance-use, and cognitive disorders. The forthcoming DSM-5 will continue to allow for recording an individual’s co-occurring mental (including substance-use) disorders and related or relevant medical conditions, with notations for psychosocial and contextual factors and functional level (American Psychiatric Association, 2012). The American Association on Mental Retardation also recommends a multidimensional diagnostic, classification, and assessment system (Schalock et al., 2010).

Frequency or Prevalence

The National Comorbidity Survey Replication (NCS-R) funded by NIMH is a major effort to determine the incidence, prevalence, and co-occurrence of mental disorders (including substance-use disorders) in the United States. According to the NCS-R, 26% of the noninstitutionalized population had at least one mental disorder in the last 12 months; 55% of those identified as having mental disorders had a single diagnosis, 22% had two diagnoses, and 23% had three or more diagnoses (Kessler, Chiu, Demler, Merikangas, & Walters, 2005). Those with two or more diagnoses represent nearly 12% of the noninstitutionalized population. This 12% figure may underestimate the total population prevalence of co-occurring mental disorders for various reasons; for example, the institutionalized population most likely has a higher incidence of co-occurring disorders, and community residents may be reluctant to report symptoms of mental illness (Kessler et al., 2005).

Whitaker and Read’s (2006) analysis of the literature suggests that children with intellectual disability and adults with severe intellectual disability have higher rates of psychiatric (mental) disorders than the general population (they found no convincing evidence that adults with mild intellectual disability have higher rates of psychiatric disorders). The literature also suggests that many mental and physical (medical) disorders are closely related (see, for example, Carney, Jones, & Woolson, 2006; Sederer, Silver, McVeigh, & Levy, 2006).

Etiological Explanations of Comorbidity

Information about the nature and extent of co-occurring conditions is useful for prevention and treatment, but in many cases, the reasons that some people develop multiple disorders are not clear. For example, Khantzian (1997) has suggested that people with mental disorders abuse alcohol or other drugs in an attempt to alleviate distressing symptoms of these disorders, “primarily in regulating affects, self-esteem, relationships, and self-care,” but he notes that evidence has not consistently supported this self-medication hypothesis (p. 231). Other theories suggest that mental and substance disorders may have a common etiology (such as genetic factors), that the relationship between disorders is reciprocal or bidirectional, or that one type of disorder increases risk for the other (Mueser, Drake, & Wallach, 1998), though mental disorders often precede substance-use disorders rather than vice versa (Kessler et al., 1996). Mueser et al. (1998) also find support for the theory that people with severe mental illness use alcohol and drugs for the same reasons that many others do—in an attempt to alleviate dysphoria (distressing mental states).

For people with intellectual disability, hypotheses are that brain pathology, “poor understanding of one’s own emotions or poor self-esteem,” may be predisposing factors for psychiatric disorders (Whitaker & Read, 2006, p. 342). The relationships among co-occurring medical and mental illnesses may also be explained in a number of ways. For example, those with some mental illnesses, such as depressive disorders, are more likely to develop particular medical illnesses, such as heart disease, and those with medical illnesses, such as diabetes, are more likely to be depressed (Sederer et al., 2006). Additional explanations for the relationships between mental and medical illnesses are that alcohol and drug disorders and smoking, which can lead to serious health problems, are more common among those with mental illness; those with mental illness may have greater difficulty adhering to medical-treatment regimens, increasing the likelihood of medical problems; and indications are that “novel antipsychotic medications are associated with complications such as obesity, high blood glucose levels, and diabetes” (Sederer et al., 2006, para. 4).


Recovery from or management of multiple illnesses or disorders may be more difficult than recovery from or management of a single disorder (for example, Drake & Mueser, 2000). Separate and distinct groups of providers generally treat mental disorders, substance-use disorders, intellectual disability, and medical illnesses. Individuals with co-occurring disorders (or their representatives) may find it difficult to negotiate multiple service providers due to their mental, intellectual, and/or physical disabilities or disorders. Treatment providers from different fields of practice may also have different treatment approaches or philosophies. For example, treatment of mental and medical illnesses and intellectual disability has historically been expert based (physicians or other professionals prescribe a treatment or service plan), while approaches to addressing substance-use disorders evolved from the mutual-help movement Alcoholics Anonymous. To reduce the difficulties clients face in negotiating multiple social service and health care systems that may embrace different and even conflicting philosophies of service provision, social workers have supported the concept of integrated treatment. Integrated treatment combines services for co-occurring disorders in a unified plan with services often provided by a single treatment team (for example, DiNitto & Webb, 2012).

Controlled studies have not consistently shown that integrated treatment for people with mental and substance-use disorders produces superior outcomes compared to standard approaches (treatment as usual) or other comparison treatments (see Cleary, Hunt, Matheson, Siegfried, & Walter, 2008; DiNitto & Webb, 2012). Nevertheless, integrated treatment, sometimes provided through case-management models, can make it easier for these clients to access assistance for the multiple problems they face (DiNitto & Webb, 2012). An integrated, or multidimensional and multidisciplinary, treatment approach has also been advocated for people with intellectual disability and mental health problems (see, for example, Dosen, 2007), and calls to integrate treatment for medical and mental illnesses have increased (see, for example, Amiel & Pincus, 2011; Sederer et al., 2006). Integrated treatment often requires that service-delivery systems as well as individual treatment providers change the way services are delivered (DiNitto & Webb, 2012; Sederer et al., 2006). One way this is being done is through “medical homes” where patients are able to access care for medical and mental illnesses through a single point of care, often a primary health care provider (see, for example, Amiel & Pincus, 2011).

Future Considerations

Given social work’s focus on clients’ biopsychosocial well-being, social workers are concerned about preventing, recognizing, and addressing co-occurring disorders. The concept of integrated treatment for co-occurring disorders is highly consonant with social work practice and speaks to the continuing need to serve clients in a holistic fashion. The future for treatment of comorbidity (co-occurring disorders) suggests that social workers will work more frequently in integrated-services systems that can address the complexities of mental, substance-use, cognitive, and health disorders. Public and private medical (including mental health) insurance must also respond to clients’ or patients’ multiple needs. Research is now focusing on identifying effective health care systems and practices that can help clients with multiple diagnoses. Social work education must respond with courses and field placements, including interprofessional education collaborations, to better prepare graduates with the technological advances needed to assist clients with co-occurring disorders.


American Psychiatric Association. (2012, 1 December). American Psychiatric Association Board of Trustees approves DSM-5. Arlington, VA: Author.Find this resource:

    Amiel, J. M., & Pincus, H. A. (2011). The medical home: New opportunities for psychiatric services in the United States. Current Opinion in Psychiatry, 24(6), 562-568.Find this resource:

      Carney, C. P., Jones, J., & Woolson, R. F. (2006). Medical comorbidity in women and men with schizophrenia: A population-based controlled study. Journal of General Internal Medicine, 21, 1133–1137.Find this resource:

        Center for Substance Abuse Treatment. (2005). Substance abuse treatment for persons with co-occurring disorders (Treatment Improvement Protocol (TIP) Series 42. DHHS Publication No. (SMA) 05-3992). Rockville, MD: Substance Abuse and Mental Health Services Administration.Find this resource:

          Cleary, M., Hunt, G. E., Matheson, S., Siegfried, N., & Walter, G. (2008). Psychosocial treatment programs for people with both severe mental illness and substance misuse. Schizophrenia Bulletin, 34(2), 226–228.Find this resource:

            DiNitto, D. M., & Webb, D. K. (2012). Substance use disorders and co-occurring disabilities. In C. A. McNeece & D. M. DiNitto, Chemical dependency: A systems approach (4th ed., pp. 354–406). Boston: Pearson.Find this resource:

              Dosen, A. (2007). Integrative treatment in persons with intellectual disability and mental health problems. Journal of Intellectual Disability Research, 51(1), 66–74.Find this resource:

                Drake, R. E., & Mueser, K. T. (2000). Psychosocial approaches to dual diagnosis. Schizophrenia Bulletin, 26, 105–118.Find this resource:

                  Kessler, R. C., Chiu, W. T., Demler, O., Merikangas, K. R., & Walters, E. E. (2005). Prevalence, severity, and comorbidity of 12-month DSM-IV disorders in the National Comorbidity Survey Replication. Archives of General Psychiatry, 62, 617–627.Find this resource:

                    Kessler, R. C., Nelson, C. B., McGonagle, K. A., Edlund, M. J., Frank, R. G., & Leaf, P. J. (1996). The epidemiology of co-occurring addictive and mental disorders: Implications for prevention and service utilization. American Journal of Orthopsychiatry, 66(1), 17–31.Find this resource:

                      Khantzian, E. J. (1997). The self-medication hypothesis of substance use disorders: A reconsideration and recent applications. Harvard Review of Psychiatry, 4, 231–244.Find this resource:

                        McGilvery, S., & Sweetland, D. (2011). Intellectual disability and mental health: A training manual in dual diagnosis. Kingston, NY: NADD Press.Find this resource:

                          Mueser, K. T., Drake, R. E., & Wallach, M. A. (1998). Dual diagnosis: A review of etiological theories. Addictive Behaviors, 23, 717–734.Find this resource:

                            Schalock, R., L., Borthwick-Duffy, S. A., Bradley, V. J., Buntinx, W. H. E., Coulter, D. L., Craig, E. M. et al. (2010). Intellectual disability: Definition, classification, and systems of supports (11th ed.). Washington, DC: American Association on Intellectual and Developmental Disabilities.Find this resource:

                              Sederer, L. I., Silver, L., McVeigh, K. H., & Levy, J. (2006, April). Integrating care for medical and mental illness. Preventing Chronic Disease: Public Health Research, Practice, and Policy, 3(2). Retrieved April 25, 2013, from this resource:

                                Whitaker, S., & Read, S. (2006). The prevalence of psychiatric disorders among people with intellectual disabilities: An analysis of the literature. Journal of Applied Research in Intellectual Disabilities, 19, 330–345.Find this resource:

                                  Further Reading

                                  Dual Recovery Anonymous:

                                  Journal of Dual Diagnosis:

                                  NADD, an Association for persons with developmental disabilities and mental health needs:

                                  National Comorbidity Survey and the National Comorbidity Survey Replication:

                                  Substance Abuse and Mental Health Services Administration: