Comorbidity describes two or more disorders or illnesses occurring in the same person. They can occur at the same time or one after the other. Comorbidity also implies interactions between the illnesses that can worsen the course of both. Dual diagnosis is a term used to describe a person who is diagnosed with a mental illness and a substance use disorder at the same time. Injection drug use contributes to a significant proportion of cumulative HIV/AIDS cases in the United States. The term triple diagnosis is used to describe persons with a dual diagnosis plus HIV/AIDS at the same time.1
Drug addiction is a chronic mental illness. Addiction changes the brain structure and function in significant ways, changing a person’s physiological needs and desires and replacing them with new priorities associated with seeking and using the drug. This results in compulsive behaviors that diminish the ability to control impulses, despite the adverse consequences, and are similar to hallmarks of other mental illnesses. Many people who have a substance use disorder also develop other mental illnesses, just as many people who are diagnosed with mental illness are often diagnosed with a substance use disorder. For example, about half of people who experience a mental illness will also experience a substance use disorder at some point in their lives and vice versa. Few studies have been done on comorbidity in children, but those that have been conducted suggest that youth with substance use disorders also have high rates of co-occurring mental illness, such as depression and anxiety.
Although substance use disorders commonly co-occur with other mental illnesses, this does not mean that one caused the other, even if one appeared first. In fact, establishing which came first or why can be difficult. Three main pathways can contribute to the comorbidity between substance use disorders and mental illnesses:
Common risk factors can contribute to both mental illness and substance use disorders. Research suggests that genetic factors can contribute to the risk of developing both a substance use disorder and a mental illness. For example, some people have a specific gene that can make them at increased risk of mental illness as an adult, if they frequently used marijuana as a child. A gene can also influence how a person responds to a drug – whether or not using the drug makes them feel good. Environmental factors, such as stress or trauma (transgenerational trauma), can cause genetic changes that are passed down through generations and may contribute to the development of mental illnesses or a substance use disorder. Adverse childhood experiences (childhood trauma including physical, emotional, and sexual abuse) is associated with increased risk of substance use disorders and other negative mental health effects in adulthood.2,3
Mental illnesses can contribute to drug use and substance use disorders. Some mental health conditions have been identified as risk factors for developing a substance use disorder. For example, some research suggests that people with mental illness may use drugs or alcohol as a form of self-medication. Although some drugs may help alleviate mental illness symptoms, sometimes the drugs can also make the symptoms worse. Additionally, when a person develops a mental illness, brain changes may enhance the rewarding effects of substances, predisposing the person to continue using the substance.
Substance use and addiction can contribute to the development of other mental illnesses. Substance use may change the brain in ways that make a person more likely to develop a mental illness. The high rate of comorbidity between substance use disorders and other mental illnesses calls for a comprehensive approach that identifies and evaluates both. Accordingly, anyone seeking help for either substance use, misuse, or addiction or another mental disorder should be evaluated for both and treated accordingly. Several behavioral therapies have shown promise for treating co-occurring mental disorders. These approaches can be tailored to patients according to age, the specific substance abused, and other factors. They can be used alone or in combinations with medications. Effective medications exist for treating opioid, alcohol, and nicotine addiction and for alleviating the symptoms of many other mental disorders, yet most have not been well studied in comorbid populations. Some medications may benefit multiple problems. For example, bupropion is approved for treating both depression and nicotine dependence. Community health workers have a critical role to play not only in preventing substance use and abuse but also providing essential information about available counseling and therapeutic resources for persons who use drugs.2,3