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. These brain changes can be long lasting, and can lead to the harmful behaviours seen in people who abuse drugs.
In a true disease, some part of the body is in a state of physiological malfunctioning and this causes the undesirable symptoms. Though, in addiction there is no physiological malfunction.