Cannabis use disorder is a new mental health term used to identify people seriously affected by issues related to cannabis (marijuana or hashish) abuse or addiction. It belongs to a range of abuse- and addiction-related problems called substance use disorders. According to the results of a study published in November 2013 in the journal Drug and Alcohol Dependence, teenagers affected by cannabis use disorder experience clearly observable changes in two key brain functions. These changes apparently persist over time and can increase future risks for ongoing cannabis addiction. Cannabis Dependence and Addiction Physical dependence is the medical term for the development of a central nervous system-based reliance on a given substance\u2019s effects. Addiction adds to dependence by fostering substance cravings and reorienting a person\u2019s behaviors and lifestyle toward obtaining and using the substance in question. Large segments of the U.S. population view cannabis in general (and marijuana in particular) as a relatively harmless substance that\u2019s safe for both recreational use and medical application. However, while cannabis use typically presents lower levels of risk for dependence and addiction than a number of other well-known substances of abuse, it still poses substantial danger. According to figures compiled by the National Institute on Drug Abuse, roughly 9 percent of all marijuana users will develop drug dependence. Dependence rates for marijuana rise to about 17 percent in teenagers and up to 50 percent in people who use the drug every day. Cannabis Use Disorder In the U.S., the American Psychiatric Association defines all forms of substance use disorder. In order to qualify for a diagnosis, any affected individual must have multiple symptoms related to non-addicted abuse of the substance in question or physical addiction to the substance in question. These symptoms must substantially impair the individual\u2019s mental equilibrium and\/or substantially impair participation in certain vital aspects of the individual\u2019s social, personal or work routines. Cannabis use disorder is diagnosed when these criteria apply to the use of marijuana, hashish or a hashish derivative called hashish oil. Like all other substance use disorders, Cannabis Use Disorder can vary in its impact from mild to severe, with corresponding degrees of impairment in mental well-being or the ability to participate in routine activities. Brain Effects In the study published in Drug and Alcohol Dependence, a team of American and Chinese researchers examined the brain changes associated with the presence of cannabis use disorder in teenagers. This examination involved 15 teenage boys diagnosed with cannabis use disorder but no longer using cannabis; 23 teenage boys with no history of cannabis use disorder but with a history of other mental health problems; and 18 teenage boys with no history of cannabis use disorder or any separate mental health issues. The researchers studied these participants\u2019 brain functions through an imaging procedure called functional magnetic resonance imaging (fMRI). When they assessed the results of the fMRI exams, the researchers found that the teenagers with a history of cannabis use disorder have two key brain alterations when compared to teenagers with mental health issues other than cannabis use disorder or to mentally healthy teenagers. First, the cannabis-use-disorder-affected teens undergo a brain change that results in an unusually strong tendency to make high-risk choices when confronted with situations that produce mental uncertainty. Cannabis-use-disorder-affected teens also undergo a brain change that results in an impaired ability to properly judge the rewards that stem from high-risk decisions. Reward processing problems are worse in teens with a longstanding cannabis use disorder diagnosis and in cannabis-use-disorder-affected adolescents who also used other types of drugs in the past. Significance and Considerations The authors of the study published in Drug and Alcohol Dependence note that they specifically looked at the brain changes in drug-abstinent teenagers previously diagnosed with cannabis use disorder, not the brain changes in teens currently using marijuana or hashish. The fact that these currently drug-free teens have ongoing brain function disruptions strongly indicates that the damaging effects of cannabis abuse and cannabis addiction linger for extended periods of time. In addition, the authors believe that persistent problems with critical decision-making and the ability to judge the rewards of prior decisions may predispose abstinent teens already diagnosed with cannabis use disorder to a return to active cannabis use in the future. The same problems may also lead to continuing issues with cannabis addiction once a return to cannabis use is underway.