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What are co-occurring conditions ('comorbidity')?

Page Summary

  • On this website, ‘comorbidity’ refers to the occurrence of a substance use disorder and a mental health disorder, or symptoms of these disorders, at the same time. Another term used for this is ‘co-occurring substance use and mental health disorders’.
  • There are several reasons why substance use and mental health disorders might occur together. Once established, they often interact in ways that maintain one another, making recovery difficult if treatment only focuses on one of these conditions.
  • People with co-occurring substance use (e.g. crystal methamphetamine ‘ice’ dependence) and mental health disorders generally experience poorer overall health and wellbeing compared to those who have just one of these conditions.
  • It is important that treatment services attend to both substance use and mental health problems when discussing treatment with their clients.

What are co-occurring conditions ('comorbidity')?

'Comorbidity' occurs when a person experiences two or more medical conditions at the same time. These co-occurring conditions are referred to as 'comorbid' and often interact with one another over long periods of time. There are many different types of comorbidity that people can experience.

Venn diagram showing comorbidity as overlap of substance use disorder and mental health disorders or symptoms
Here we focus on the type of comorbidity where someone experiences both a substance use disorder (e.g. crystal methamphetamine 'ice' and a mental health disorder (e.g. , or symptoms of these disorders, at the same time.

Watch video: What is comorbidity?

  • How do co-occurring conditions develop?

    There are several possible reasons why two or more disorders may occur at the same time.

    Infographic showing direct impact of substance use (e.g. ice use) on mental health, and vice versa

    One condition may directly cause the other. Someone may become on alcohol and/or other drugs (like ice) because they use them to help cope with mental health symptoms. This is often referred to as “self-medication”. Alternatively, someone’s use of alcohol and/or other drugs may impair the way their brain functions, leading to poor mental health. For example, from ice can leave people feeling depressed due to changes in brain chemicals, and heavy, consistent use of ice can sometimes cause acute reactions. For most people these symptoms may subside after a few hours or a few days, however a small number of people may find these symptoms last much longer (e.g. more than a few weeks) or continue even when they are no longer using ice. This might mean that an underlying psychotic disorder, such as is present.

    A lot of research has been conducted to determine which disorder typically develops first out of the different co-occurring substance use and mental health disorders people experience. The evidence so far is not consistent, and researchers have found differences depending on a person’s gender as well as the specific types of mental health disorder/s people are experiencing.

    Understanding when different substance use and mental health disorders develop and how they interact with one another can help us to understand how best to prevent them from occurring. But sometimes this is tricky to work this out – it can be a bit like the ‘chicken or the egg’ example when trying to work out if a mental health disorder has come first, or whether a substance use disorder has come first. This can also be tricky because often the symptoms of mental and substance use disorders overlap and emerge at similar points in time during adolescence.

    Regardless of which disorder has come first, once these disorders have developed, they most likely maintain and exacerbate one another. This can make it difficult for people to recover if they aren’t able to access treatment for both conditions. For example, someone may use alcohol and/or other drugs to reduce symptoms of yet research shows that repeated use of the substances can lead to increased anxiety for people. Regardless of what order these different disorders develop in, the strategies used to manage them remain the same.

  • How common are co-occurring conditions?

    Substance use disorders and mental health disorders are both common in Australia. National population estimates indicate that one in two Australians will develop a substance use, or mood disorder (e.g. in their lifetime, and one in five meet criteria for a substance use, anxiety or mood disorder annually.

    Infoghraphic showing 1 in 3 individuals with a substance use disorder also have at least one co-occurring anxiety or mood disorder

    The co-occurrence of substance use disorders and mental health disorders is also common. According to the Australian National Survey of Mental Health and Wellbeing approximately 1 in 3 individuals with a substance use disorder also have at least one co-occurring anxiety or mood disorder. Rates are even higher among people who are in alcohol and other drug (AOD) treatment, with up to 3 in 4 clients in Australian AOD treatment services having at least one co-occurring mental health disorder.

    Infographic showing 3 in 4 clients in Australian AOD treatment services have at least one co-occurring mental health disorder

  • What are the harms associated with co-occurring conditions?

  • Do co-occurring conditions affect treatment?

    Research into whether people who have a co-occurring mental health condition respond differently to alcohol and other drug treatment is mixed. Some research suggests people who enter alcohol and other drug (AOD) treatment with a co-occurring mental health disorder do not respond as well to treatment as those without these conditions. Other studies have shown people with a co-occurring mental health disorder benefit as much from AOD treatment as those who do not. Research also suggests, however, that although these people benefit just as much from AOD treatment, they continue to experience more serious substance use problems, as well as poorer physical and mental health and overall functioning compared to others when they exit treatment.

    Female counsellor talking to her client
    On balance, the evidence indicates that having co-occurring conditions is not an insurmountable barrier to treating people with AOD use disorders, however some clients with co-occurring mental health disorders may require additional treatment.  Some interventions have been developed to specifically treat certain comorbidities, however more research is needed before they can be used in practice.

    People experiencing co-occurring substance use and mental health issues should discuss this with their local doctor, AOD counsellor or mental health professional to ensure both areas of concern are addressed in treatment.

  • What mental health problems can co-occur with ice use?

    People who use ice may experience co-occurring mental health problems such as and For more information visit mental health effects of ice. For more information about treatment and support services available visit our get help page.


Information on this webpage has been adapted from the Guidelines on the management of co-occurring alcohol and other drug and mental health conditions in alcohol and other drug treatment settings (3rd edition). Read the full Comorbidity Guidelines, link opens in a new window.

See full reference list below.

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Page last reviewed: Tuesday, 31 January 2023