COMORBIDITY

MDI can occur with other psychiatric or medical illnesses. The co-occurrence of two or more diseases is called Comorbidity. The most common diagnoses associated with MDI are other psychiatric conditions including Substance Abuse/Dependence (40%) and Anxiety Disorders (40%). -Brady

Approximately 20% of patients diagnosed with BPD suffer from Obsessive Compulsive Disorder, another 20% from Panic Disorder.
-McElroy

In children Attention, Conduct, Oppositional-defiant and Anxiety Disorders are common. In adolescents, substance abuse, Conduct and Anxiety Disorders are most prevalent. Eating Disorders are also quite common. -Johnson

The occurrence of a medical or psychiatric condition together with MDI can complicate treatment (i.e., OCD requiring antidepressant treatment or Multiple Sclerosis requiring treatment with steroids).