ADD is often accompanied by a coexisting condition. On the other hand, often ADD and these other conditions have similar symptoms and, therefore, are easily misdiagnosed. Of course, this creates a plethora of problems as these disorders, when separate, must be treated differently. A misdiagnosis will lead to ineffective treatment. As coexisting conditions, these are especially of concern, because they increase the severity of many of the symptoms of ADD, as well as the negative impact on those that suffer them.

One such condition is Oppositional Defiant Disorder or Conduct Disorder. Children with this disorder tend to be very aggressive and defiant. They usually lose their tempers very easily and challenge authority figures. This is the most easily diagnosable behaviors, as teachers and parents become aware of this conduct quickly and seek treatment almost immediately. This particular disorder is common in children that are the hyperactive/impulsive type of ADD and occurs in 35% of children with ADD. This coexisting condition makes it much more likely that these sufferers will get in legal trouble as they age due to their authoritative defiance.

Many children that have ADD also suffer from mood disorders and/or depression. The complication associated with this coexisting condition is the increased likelihood of suicide common during teen years. 18% of ADD children also suffer from some type of mood disorder, and is it more likely to occur within the inattentive type of ADD. This is often not as easily diagnosed as compared to other conditions. Children with mood disorders as a coexisting condition must be treated with different medication than typical ADD treatment; therefore, such therapy may be more difficult.

Also a concern is anxiety disorders among ADD sufferers, which constitutes roughly 25% of those with ADD. Traits common of Anxiety Disorders are fear, panic, and worry, which may necessitate medication and/or counseling. As with mood disorders, combined treatment make therapy more complex and arduous.

Learning Disabilities are also conditions common with children that have ADD. While ADD, in itself, is not a learning disability, it can make it extremely difficult for children to perform well in school. If coupled with a learning disability, it makes it considerably more challenging for children to grasp academic concepts and utilize such knowledge.

For the most part, coexisting conditions make treatment more difficult and the side effects of both disorders more severe. The typical signs and symptoms of ADD are difficult enough to deal with, but with more intense severity, it often makes side effects deadly or incredibly negatively life altering.