Substance use disorder is characterized as a chronic, relapsing brain disorder defined by a compulsive pattern of substance seeking and use, despite harmful consequences. It involves significant changes in brain circuitry, particularly those associated with reward, memory, and impulse control. The physiological basis of the disorder is linked to the way certain substances hijack the brain's natural dopamine reward pathways, leading to altered neural plasticity and tolerance. Over time, repeated exposure can cause neuroadaptation, making the brain rely on the substance to achieve normal function. Treatment typically focuses on managing withdrawal symptoms, identifying triggers, and employing behavioral therapies to restore functional connectivity and establish healthier coping mechanisms.