Mental illness and drug addiction often occur together. This condition of dual diagnosis presents a challenge to physicians. The patient has two brain diseases that influence one another, and which both need treatment. Some people may begin using drugs of abuse as a form of self-medication. Drugs of abuse may temporarily relieve some of the symptoms associated with mental illness, such as stress, anxiety, social inhibitions, or depression. Addiction and mental illness are both brain diseases. A person vulnerable to one type of brain disease may also be vulnerable to another. Addiction and mental illness involve the same pathways, molecules, and chemicals in the brain, and they share many similarities.