When Addiction and Mental Illness Co-Occur

You may have heard the term “co-occurring disorder” or “dual diagnosis” or “comorbidity”—but what do these constructs mean, exactly?

The term co-occurring disorder refers to the presence of two or more mental health disorders existing simultaneously within the same individual. Common co-occurring disorders are mental illnesses (e.g., depression, anxiety, bipolar, posttraumatic stress disorder) and substance use disorders (SUD). Indeed, statistics reveal that individuals with addiction often have a co-occurring mental health concern (over 60%; Jones & McCance-Katz, 2019), and those with mental health concerns are more likely to use drugs of abuse than those without mental health concerns (SAMHSA, 2020a; SAMHSA, 2020b). In fact, according to 2020 data, about 17 million adults in the United States had both a SUD and a mental illness (SAMHSA, 2021). Co-occurring disorders affect and are affected by one another—these bidirectional conditions are complex, yet they are the norm rather than the exception among those with addiction (SAMHSA, 2020b).

So, why does this overlap exist? There are three possible explanations:

1. Substance use begins after the mental illness as a means of coping with mental health symptoms. In this scenario, the mental health concern precedes the use of substances. The individual may turn to the gratifying properties of drugs of abuse as a way to cope with their distressing mental health symptoms. For example, if an individual has an untreated major depressive disorder, they may find the rewarding properties of drugs of abuse to offer a reprieve from their depressive symptoms. Drugs of abuse are both positively and negatively reinforcing in light of their effects on the reward pathway in the brain. Therefore, alcohol and other drugs are predictable, consistent methods of changing the way a person feels. If a person is in psychological distress, drugs of abuse can offer an escape, albeit temporarily. Although the use of drugs of abuse may have started as a form of "self-medicating" psychiatric symptoms, over time, drug use exacerbates these symptoms, and can itself become another disorder, namely, a SUD (NIDA, 2020; NIMH, n.d.).
2. Mental health concerns emerge after the initiation of substance use. In this situation, an individual begins using drugs of abuse, and later, a mental health concern develops. Depending upon an individual’s genetic makeup and biological vulnerabilities, the use of substances can trigger the onset of a mental illness. For example, research confirms the link between using high-potency cannabis (with higher amounts of the psychoactive ingredient, THC) with psychiatric disorders (Di Forti et al., 2019; Johnson et al., 2021). Thus, for some individuals, the use of a drug of abuse (e.g., methamphetamine, cocaine, cannabis, synthetic drugs) can lead to the onset of mental health symptomology (NIDA, 2020; NIMH, n.d.).
3. A common risk factor predisposes an individual to the independent development of both a mental illness and SUD. In this case, a genetic or environmental factor makes an individual more susceptible to the development of both a SUD and mental health concern. Rather than one disorder preceding the other, the two disorders emerge concurrently. Individuals may have genetic predispositions (for example, deficiencies or dysregulation in reward circuitry or neurotransmission) that make them vulnerable to both addiction and mental illness. Moreover, individuals may have been exposed to adverse childhood experiences, toxic stress, and/or early trauma, which are risk factors for the development of both SUD and mental health concerns (CDC, 2019; Felitti, 1998). Thus, the two disorders can develop from a common risk factor, and subsequently, affect and influence one another as they progress (NIDA, 2020; NIMH, n.d.).

In sum, co-occurring disorders are prevalent, complex, and can develop in different ways. So, what is the solution? The answer: integrated care.

Integrated treatment or integrated care is a type of treatment that simultaneously addresses an individual’s mental health concerns and addiction in the same setting. Thus, treatment providers use assessments, interventions, and treatment plans relevant to both mental illness and SUD (SAMHSA, 2009). Rather than addressing one issue and then the other (e.g., addressing the SUD first, then addressing the mental illness), integrated care treatment providers have training and expertise in both psychiatric and addictive disorders and address them concurrently (SAMHSA, 2009).

Although multifaceted and more intricate, integrated care is quickly becoming the gold standard for the treatment of co-occurring disorders. Recent research confirms that integrated care demonstrates superior treatment outcomes compared to sequential treatment, or the treatment of one disorder at a time (SAMHSA, 2009; SAMHSA, 2020b). As more treatment centers move toward integrated care models, it is likely that individuals with co-occurring disorders will experience larger treatment gains and more positive clinical outcomes.

We should all have basic knowledge about co-occurring disorders so we can recognize and support those around us who struggle with them. Indeed, the more we understand about co-occurring disorders and the benefits of integrated care, the more effectively we can serve the 17 million U.S. adults with a SUD and mental illness.