Addiction is characterized as a chronic, relapsing brain disease and a behavioral condition marked by compulsive substance use or engagement despite harmful consequences. It involves complex neurobiological changes, particularly within brain reward pathways, where repeated exposure to psychoactive substances or behaviors modifies the systems responsible for pleasure and motivation. The underlying mechanism involves alterations in neurotransmitter function, notably dopamine, leading to the establishment of powerful salience associations. These changes result in a cycle where the substance or behavior itself becomes inextricably linked to the reward system, leading to tolerance, withdrawal symptoms upon cessation, and a marked loss of control over usage patterns.