About half of all people diagnosed with this disorders also suffer from depression. Depression and anxiety overlap so much that increasingly, clinical psychologists and researchers see anxiety and depression as facets of the same illness.

Most people have feelings of sadness at some point in their lives. The loss of a job, death of a loved one, or other disappointments, causes many of the symptoms of depression. Similarly, most people experience the latter at some point in their lives when confronted with stressful situations. These feelings are normal, up to a point.

When feelings of overwhelming sadness or hopelessness, accompanied by fatigue, and changes of appetite and sleep patterns persist for more than a few weeks, the person may be diagnosed with clinical depression. When feelings of anxiety are out of proportion to the stresses being experienced, or a person has panic attacks several times over a period of weeks, they may be suffering from an anxiety disorder. Anxiety and depression are so often intertwined that one must be treated before the other can be dealt with.

Just as there is a lot of overlap between depression and anxiety, there is also a lot of overlap in treatment for the two disorders. Anxiety disorders are often treated with antidepressant medications. Some forms of psychotherapy, especially cognitive and behavioral therapy, have been very successful in treating both conditions.

At times, it may be necessary to treat the depression first. In one form of therapy, called exposure therapy, the patient is exposed to gradually increasing stress factors to help them learn to cope with anxiety. Depression can drain a person of energy and leave them unable to cooperate, so sometimes it must be dealt with first for the anxiety therapy to be effective.

Sometimes, a patient's depression must be treated before the disorder of the other condition can be dealt with. Other times, it may work the other way around. Anxiety usually presents itself at an earlier age than depression, typically during childhood or adolescence.

In these cases, successful treatment of the latter may prevent depression from developing when the patient reaches adulthood. Depression and anxiety are such frequent fellow travelers that often, successful treatment of one often includes treating the other.