Sarah Jenks:
People with mental illnesses, such as depression or schizophrenia, have also been found to frequently suffer from addictions. Studies have shown that damaged amygdalas lead to impaired behavior and heightened drug responses. Therefore rather than writing off an addiction as a bad coping mechanism for those with a mental illness, people should treat it as another symptom perhaps of a neurological disease.
http://health.msn.com/health-topics/addiction/articlepage.aspx?cp-documentid=100185428