The Origins of Co-Occurring Disorders

This blog post is contributed by Izzie Karohl, NREMT, Policy Research Intern

SAMHSA’s 2020 National Survey of Drug Use and Health estimates that 45% of adults with a substance use disorder also have a co-occurring mental health condition. Because people with mental health conditions make up such a large portion of folks who seek substance use disorder treatment, it’s important to understand the complex relationship between the two co-occurring disorders.

This is the first blog post in a series that discusses current approaches to treating co-occurring disorders. But before we talk about treatment, we should start with one, deceptively simple question: why is the rate of co-occurring disorders so high in the first place? This blog post addresses the three most popular theories–the self-medication theory, the gene variants theory, and the kindling effect theory.


Drinking to cope with the distress of an underlying mental health disorder is self-medication. Sometimes, people develop a substance use disorder because the effects of drugs and alcohol alleviate symptoms of PTSD, bipolar disorder, depression, and/or anxiety, just to name a few. Part of recovery is learning how to cope with distress, and this can include symptoms of mental illness. However, it’s important that clinicians identify mental illness as an independent disorder so that the client receives appropriate psychiatric care. 

woman with co-occurring disorders

Gene Variants 

Numerous studies have demonstrated that substance use disorders and mental health disorders run in biological families. The gene variants theory proposes that specific genes linked to substance use disorders are also linked to mental health disorders. For example, you may have heard that people with untreated ADHD are more likely to develop a substance use disorder. The gene variants theory hypothesizes that genetic mutations which result in more hyperactive, impulsive, and reward-driven brains (ADHD) are also mutations that make brains more vulnerable to substance use disorders. Currently, scientists are identifying these specific mutations to create a “genetic risk factor” score that may be able to predict who is more likely to develop co-occurring disorders based on their DNA sequence. 

Kindling Effect 

When trying to start a fire, having one plank of wood only goes so far. But if you add lighter fluid and small sticks around the plank, the likelihood of a blazing flame skyrockets. That’s the basis of the kindling theory. Having an initial mental disorder, whether psychiatric or substance use, changes the neural pathways in the brain: strengthening some, lessening others, and making some more sensitive. These changes add kindling, making it more likely that a future stressor or behavior results in a secondary disorder. Unlike the gene theory that locates risk within one’s DNA, the kindling effect states that the progression of one disease and its changes to the brain are what make it more vulnerable to a second disorder. The kindling effect points to the importance of early intervention to prevent secondary disorder development.

None of these theories are ultimately “right” or “wrong.” Rather, each of them help to explain the various ways co-occurring disorders may develop. It could be that a person is (a) genetically predisposed to both disorders, (b) develops a psychiatric disorder early on and progresses, which increases the vulnerability to developing a substance use disorder and (c) copes with the first disorder by self-medicating. But no matter how a person’s co-occurring disorder came to be, there is effective treatment for both. People can and do recover from co-occurring disorders.

Read more about co-occurring disorders here.