Dual coding is a term that indicates that a patient has been diagnosed as having co-existing mental health and substance abuse disorders. In the modern world this is very common place. Frequently, it is a lot like the question of the chicken and the egg as to which one preceded the other: many people use drugs/alcohol to self-medicate pre-existing problems, however drug/alcohol use can also create mental health problems. When administering treatment a therapist must try to determine which problem predates the other in order to create an appropriate treatment plan that adequately addresses both problems.
For instance, a depressed person may start using alcohol or drugs to help themselves feel better due to an underlying psychological condition. Or a person who develops a substance abuse problem may start feeling depressed, anxious, aggressive, or paranoid as a side-effect of long-term and detrimental drug use and/or addiction.