Mental illness is a frequently misunderstood and sometimes devastating disease. So is substance addiction. When you add the two together, the result can be a patient who is both at severe risk and extremely difficult to treat.

This phenomenon, known as dual diagnosis, is becoming increasingly common in the United States. But doctors are just starting to get a handle on how to treat these problematic and vulnerable patients. According to the latest statistics, roughly half of all people with severe mental disorders are also affected by substance abuse. This translates into about seven million people in the United States. The effects are numerous and harsh. Those with a co-occurring disorder are more likely to commit violence and are less likely to take medication. They also tend to be more difficult to treat than those with just a substance abuse or mental disorder alone.

These patients are also at greater risk for a phenomenon known as downward drift. Because of their condition and propensity toward psychosis, they are more likely to live in rough neighborhoods, where crime and diseases such as AIDS are more prevalent.

Many of these patients resist treatment and gaining their trust can be difficult. But mental health experts say there is hope. There is evidence that treating both problems at once can be effective. In fact, studies show patients with a substance abuse disorder are more likely to receive help if they also have a mental illness. Research also shows that when patients with dual diagnosis successfully overcome alcohol abuse, they become much more responsive to treatment.