"Dual diagnosis is a term that is very easy to define, yet very difficult to treat. When you are suffering from dual diagnosis, what has essentially happened is that you are suffering from both a mental illness problem and a substance abuse problem....[since] one of these is always a mental illness... it is hard to define dual diagnosis without defining the term mental illness as well. A mental illness can be thought of as any illness that can affect what would otherwise be your normal mental abilities. Therefore, something like a concussion would not be considered a mental illness most of the time since it is a symptom that goes away. Something like depression on the other hand would be considered a mental illness because it fits the definition that was given above.
When one is treating dual diagnosis, the main thing to be aware of is that for it to be an effective dual diagnosis treatment, it needs to be something that will work on both the mental illness and the substance abuse problems simultaneously. If you have a mental illness that is treated, the substance abuse could easily bring the mental illness back even after treatment has been successful (i.e. depression resurfacing due to an alcohol addiction). Similarly, if you get great treatment for you substance abuse but leave the mental illness alone, it could easily drive you back to substance abuse later (i.e. bipolar personality shifts recreating the drug addiction that you just got over). Not many clinics actually get the importance of treating dual diagnosis components at the same time, but we certainly do. We understand this because we have seen so many other people mess up at different points. Ultimately, we were the ones that had to clean up the messes left by other clinics, something we really do not enjoy doing. Dual diagnosis absolutely has to be a single shot taking everything out, otherwise you could end up right where you started at the end of the day." (Article taken from Sunset Malibu)