Addiction is defined as a chronic, relapsing brain disease that is characterized by compulsive drug seeking and use, despite harmful consequences.It is considered a brain disease because drugs change the brain; they change its structure and how it works.ddiction occurs when a person cannot control the impulse to use drugs even when there are negative consequences—the defining characteristic of addiction. These behavioral changes are also accompanied by changes in brain functioning, especially in the brain’s natural inhibition and reward centers.