The presence of depression in people with anxiety disorders increases the severity of the illnesses, the likelihood of alcohol or substance abuse, and the risk of suicide. It also reduces the chances that treatment will succeed, unless both disorders are fully treated.
In other cases, people may have one of these disorders first, recover from it, and then develop the other. Anxiety can also be a symptom of depressive disorders, and depression can be a symptom of anxiety disorders.
Anxiety and depression are much more closely linked than was once thought. Many scientists now believe that anxiety and depression are different expressions of a single, shared underlying biological problem. They point out, for example, that the same kinds of abnormalities in neurotransmitters (chemical messengers in the brain) that promote depression can also trigger anxiety. Researchers have also found that the brain structures that react to perceived threats are hypersensitive in some people who have either depression or anxiety disorders, or both.