Substance dependence is classified as a chronic, complex disease involving compulsive use of psychoactive substances despite experiencing adverse consequences. From a biological perspective, the condition involves profound neuroadaptation, particularly affecting the brain's reward circuitry. These alterations are marked by changes in neurotransmitter systems, which govern pleasure, motivation, and reinforcement. Over time, repeated exposure to a substance can lead to a blunting of natural reward responses and the development of tolerance, requiring increasing doses to achieve desired effects. Physiologically, the reliance on the substance disrupts the brain's equilibrium, leading to withdrawal symptoms when the substance is removed, reflecting the body’s attempt to re-regulate core homeostatic systems.