Science is working to understand the basis for the fact that mental illnesses and substance abuse disorders tend to occur in clusters.  One theory is that individuals might use illicit drugs or alcohol in an attempt to medicate an underlying mental illness like anxiety or depression.  Other researchers point to the fact that individuals who are likely to misuse substances are also inclined to engage in high-risk behaviors or to be impulsive.  And still other research puts stress as the underlying causal factor in both disorders.  Recent studies indicate that for adolescents who later develop substance-use disorders, 80% had a preexisting mental illness.

Individuals with co-occurring illnesses are a distinct group with specialized treatment requirements.  Because of the differing perspectives on these groups of illnesses, neither the mental health nor the substance abuse systems of care are prepared to respond to these combined conditions.  Individuals with co-occurring illnesses often fall between the cracks in these systems resulting in the over utilization of police and medical crisis services, homeless shelters and correctional facilities.   There is growing recognition that individuals treated in either the mental health or substance abuse systems are likely to have elements of both types of illness.