Substance use disorder and conduct disorder are two mental health conditions defined in the U.S. according to terms set forth by the American Psychiatric Association. Many people affected by conduct disorder engage in patterns of drug or alcohol intake that put them at risk for developing substance use disorder. Practically speaking, this means that the two conditions appear simultaneously in the same individual with some regularity. In a study published in January 2014 in the journal Drug and Alcohol Dependence, researchers from the University of Colorado looked at what happens to the brain of a teenager affected by both conduct disorder and substance use disorder. Substance Use Disorder Because of their profoundly negative impact on a person\u2019s mental and psychological well-being, all forms of diagnosable substance abuse and substance addiction are classified as mental disorders in the United States. Prior to 2013, the process of diagnosing chemically dependent drug or alcohol addiction was kept separate from the process of diagnosing non-dependent, dysfunctional drug or alcohol abuse. However, current guidelines require doctors to consider both issues together under the heading of substance use disorder. Instead of simply making a general diagnosis of this disorder, doctors must go on to identify the particular substance responsible for a given individual\u2019s problems. They must also use the number of symptoms present in an individual to determine if his or her substance problems qualify as relatively minor or relatively severe. Any diagnosed person must have at least two abuse- or addiction-related symptoms; the maximum number of possible symptoms is 11. Conduct Disorder Conduct disorder is diagnosable only in teenagers or younger children. People with the disorder have persistent problems keeping their emotions and behaviors in line with social and interpersonal expectations. Common examples of conduct disorder\u2019s symptoms, all of which typically feature unusual impulsivity and a disregard for others, include rule-breaking that has no compelling motivation, property vandalization or destruction, arson, malicious or openly violent acts toward humans or animals, truancy prior to the start of adolescence, and use of deceit or falsification to exploit others. Teens and younger children with conduct disorder also often start drinking or using drugs in a quantity and at a rate that set the stage for diagnosable substance abuse or substance addiction (i.e., substance use disorder). The symptoms of conduct disorder are notably worse than the symptoms of another conduct-related childhood condition called oppositional defiant disorder. Combined Brain Effects In the study published in Drug and Alcohol Dependence, the University of Colorado researchers used advanced imaging techniques to examine the brains of teenagers simultaneously diagnosed with both conduct disorder and some form of substance use disorder. Specifically, they assessed the ability of adolescents with this dual diagnosis to properly use parts of the brain that are critical for introspective thinking, the ability to use past events to predict future outcomes and other processes that usually help keep impulsive behaviors in check. The researchers recruited 20 teenage boys diagnosed with substance use disorder and conduct disorder for their project and compared the brain results from this group to the results obtained from another group of 20 teenage boys unaffected by substance use disorder or conduct disorder. After reviewing the testing results of the two groups, the researchers found that, compared to their peers unaffected by conduct disorder or substance use disorder, the teens affected by both conditions have an unusually low level of activity in the brain regions responsible for offsetting impulsive behaviors. The researchers also found that two other significant mental health problems, attention-deficit\/hyperactivity disorder (ADHD) and depression, often produce brain effects that cloud the effects produced by conduct disorder and substance use disorder. However, when steps are taken to account for the impact of ADHD and depression, the brain problems associated with conduct disorder and substance use disorder become quite apparent. Significance and Considerations When two (or more) conditions appear together and produce uniquely damaging outcomes, health professionals refer to those conditions as comorbid. As a result of their findings, the authors of the study published in Drug and Alcohol Dependence concluded that, when they appear together, conduct disorder and substance use disorder have a substantial comorbid effect on normal brain function. They believe that future researchers may be able to use this knowledge to increase their understanding of the role that brain dysfunction plays in each of these conditions.