Most scientists believe that mental illnesses result from problems with the communication between neurons in the brain For example, the level of the neurotransmitter serotonin is lower in individuals who have depression.
Substance Abuse and Brain Functioning
Addiction is defined as a chronic, relapsing disorder characterized by compulsive drug seeking and use despite adverse consequences. It is considered a brain disorder, because it involves functional changes to brain circuits involved in reward, stress, and self-control.
Brain and Disorders