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What Is Comorbidity?

Comorbidity is when an individual has two or more distinct illnesses at the same time. The ailments could be physical or mental. For example, a person might suffer from depression and multiple sclerosis or anxiety and an eating disorder.

Disease overlap is common: Historically, 80 percent of Medicare spending covers patients with four or more conditions. Multiple disorders can exacerbate one another and make treatment a longer process or more difficult to achieve.

Two related terms — co-occurring disorders and dual diagnosis — refer specifically to the presence of a substance use disorder and another mental health condition, such as schizophrenia or depression.

There is debate in the field as to the definition of comorbidity, such as whether the term encompasses overlap between two conditions of any kind or between one medical and one psychiatric, as well as the relationship between the two, such as whether to distinguish between primary and secondary conditions.

Continuing to research the relationship between different diseases is critical to develop the most effective treatment approaches. On an individual level, disclosing multiple disorders to a doctor is key to achieving successful care.

What Does "Co-Occurring Disorders" Mean?

The term co-occurring disorders refers to when an individual has a substance abuse disorder and another mental illness at the same time. The term dual diagnosis describes the same condition.

This combination is widespread: Half of the people who experience a mental illness will also be diagnosed with a substance use disorder at some point in their lives, according to the National Institute on Drug Abuse. The same goes for those first diagnosed with a substance use disorder and later a mental illness. In 2018, 9.2 million Americans had both a substance use disorder and mental illness, according to the National Survey on Drug Use and Health.

The two categories often overlap for a few reasons. The same genetic or environmental factors could increase the risk of developing both disorders. There’s also evidence that the development of some mental disorders can render the individual more vulnerable to a substance use disorder and vice versa.

Treatment should center around both illnesses, rather than treating one in isolation. Successful treatment may leverage both medications, such as buprenorphine, and behavioral therapies, such as cognitive-behavioral therapy or dialectical behavior therapy.

What Are Common Illnesses that Co-Occur?

Many different mental health disorders can co-occur. One common example is depression and anxiety, although some argue that the two have similar roots and thus do not constitute distinct disorders. Depression commonly overlaps with other disorders as well, such as bipolar disorder and ADHD.

The most common comorbidities that appear in cancer survivors are hypertension, hyperlipidemia, osteoarthritis, hypothyroidism, diabetes mellitus, and coronary artery disease.

For cardiovascular disease, the common comorbid illnesses are cancer, diabetes, back and neck problems, osteoarthritis, chronic obstructive pulmonary disease, and low vision.

In multiple sclerosis, those conditions are depression, anxiety, hypertension, hyperlipidemia, and chronic lung disease.

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