As a recovering alcoholic with bipolar disorder, I’m always looking for information on how the two are related, which illness is responsible for what behavior, and how their treatments might collaborate or clash.
One of the best overviews I’ve read on the relationships between addiction and mental illness is written by J. Raymond DePaulo, Jr. a professor of psychiatry and Director of the Affective Disorders Clinic at the Johns Hopkins University School of Medicine, where I was evaluated in March of 2006. Following is an excerpt from chapter 10 of his book, “Understanding Depression,” on destructive behaviors.

Nothing makes the job of a psychiatrist treating depression and manic depression harder than alcohol and drugs. The most difficult treatment situations that I have ever seen patients and families confront, since I started my training in psychiatry twenty-seven years ago, occur when the patients’ illnesses are complicated by what we call addictive behaviors. While I have seen many successful outcomes, none were easy to achieve.

Let me be precise about what I mean by the word behavior. Depression and manic depression are diseases, not behaviors. They are, however, associated with certain types of behaviors. We’d say that seeking help is a good behavior and that the most destructive behavior of all associated with depressive illness is suicide. Alcohol abuse and dependence, drug dependence, anorexia nervosa, pathological gambling, and repetitive self-injury are all negative behaviors. That is, they are activities defined in terms of their goals or their consequences. Addictive behavior, we would say, is “abnormally” driven.
Depressive illness sometimes seems to make some people more prone to destructive behaviors; at the same time destructive behaviors generally tend to make depression and manic depression worse. …There’s a greater risk of abusing alcohol or drugs by people who have depression of moderate severity than for patients whose depression is quite severe. People with illness of moderate severity, after all, can still move around. As a result, they still have enough strength and initiative to seek out a number of “remedies” that actually make their depression worse and more difficult to treat. To compound the situation further, the remedies can become very big problems in their own right.

A number of such depressive-related destructive behaviors, when combined with depressive illness, can wreak havoc. Some of this behavior probably would never have occurred if the person hadn’t been depressed at the time. But many patterns of behavior as they relate to depression and to mania are common and do involve choices, at least when they start. While they are linked to the illness, they are not integral to it. That is to say, only the patient can stop them.
The affected person makes this difficult decision and if things go well—he or she gets support like from Alcoholics Anonymous (AA), and treatment—the person becomes sober and is well at least “one day at a time.” By taking responsibility for his or her own recovery, the individual with depression can level the playing field. That leaves only the depression demon to face down, which is hard enough. Ironically, the most severe depressions can actually stop someone’s drinking because he or she lacks the energy or the drive for anything. That’s one of the few positive things in the relationship between depression and alcohol. Unfortunately, the effect is often temporary since many of these people often go back to drinking when they recover. As I said, there are also many different kinds of destructive behaviors that can come into play and make treating the depression difficult: smoking, gambling, anorexia or bulimia nervosa.
But there is no question that the most common destructive behavior affecting depressed patients, baring suicide, is alcohol or any substance abuse.