According to a report published in the Journal of the American Medical Association, 37% of those who are addicted to alcohol and 53% of those with drug addictions also have at least one serious mental illness. Of all people diagnosed as mentally ill, 29% have substance addictions. When a person has both a substance addiction and a mental illness, it is referred to as a “co-occurring disorder” and it is important to treat both issues. Many people with untreated mental illness use substances as an attempt to self-medicate.

In the homeless population, the amount of co-occurring disorders is greater, and it is especially high in Santa Monica. A study conducted by the Urban Institute in 2006 reported that 38% of the homeless population in Santa Monica in 2006 had a mental illness. 14% suffered from mental illness only, but 24% had co-occurring substance abuse issues. An addition 56% of the Santa Monica homeless population had substance addictions without mental illness. Only 6% of the homeless had neither.