A1: What is comorbidity?

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Key Points

  • Although other types of ‘comorbidity’ exist, the use of the term in this document refers to the co-occurrence of an AOD use disorder with any other mental health condition.
  • In this document, we use the term comorbid ‘mental health disorder’ when referring to those who have a diagnosable mental health disorder, as defined by the DSM.
  • When using the term ‘mental health condition’, we are referring to both those who have a diagnosable disorder as well as those who display symptoms of disorders while not meeting criteria for a diagnosis of a disorder.
  • There are a number of possible explanations as to why two or more disorders may co-occur. It is most likely, however, that the relationship between comorbid conditions is one of mutual influence.

Use of the term ‘comorbidity’ in these Guidelines refers to the co-occurrence of one or more AOD use disorders with one or more mental health conditions. This phenomenon is often referred to as ‘dual diagnosis’; however, this term is often misleading, as many clients present with a range of co-occurring conditions of varying severity [8]. It should be noted that there are other types of comorbidity. For example, a person may have co-occurring AOD use disorders (i.e., more than one AOD use disorder). Indeed, one of the most common and often overlooked comorbidities in AOD clients is tobacco use (discussed in Chapter B1) [9-12]. Other conditions that are often found to co-occur with AOD use disorders are physical health conditions (e.g., cirrhosis, hepatitis, heart disease, diabetes), intellectual and learning disabilities, cognitive impairment, and chronic pain [13-20]. While there are a number of different types of comorbidity, these Guidelines focus on the co-occurrence of AOD use disorders and mental health conditions.

To be classified as having a mental health disorder, a person must meet a number of diagnostic criteria (see Chapter A4 for a discussion of the classification of mental health disorders) . There are, however, a large number of people who present to AOD treatment who display symptoms of disorders while not meeting criteria for a diagnosis of a disorder. For example, a person may exhibit depressed mood or anxiety without having a diagnosable depressive or anxiety disorder. Although these individuals may not meet full diagnostic criteria according to the classification systems, their symptoms may nonetheless impact significantly on their functioning and treatment outcomes [21]. For example, people who report symptoms of depression but do not meet diagnostic criteria have reduced productivity, increased help-seeking, and an increased risk of attempted suicide [22]. Therefore, rather than viewing mental health as merely the presence or absence of disorder, mental health conditions can be viewed as a continuum ranging from mild symptoms (e.g., mild depression) to severe disorders (e.g., schizophrenia or psychotic/suicidal depression).

In this document we use the term comorbid ‘mental health disorder’ when referring to those who have a diagnosable mental health disorder, as defined by the DSM [23, 24]. When using the term ‘mental health condition’, we are referring to both those who have a diagnosable disorder as well as those who display symptoms of disorders while not meeting criteria for a diagnosis of a disorder.