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It's tough to understand why people become addicted to drugs. Addiction is a chronic disease characterized by compulsion or uncontrollable use, and usually drug seeking, despite harmful results.

We know that addiction is a disease of the brain.

The human brain is the most complex organ in the body—you need it to drive a car, to enjoy a meal, to breathe, to create an artistic masterpiece, and to enjoy everyday activities. In brief, the brain regulates your body’s basic functions; enables you to interpret and respond to everything you experience; and shapes your thoughts, emotions, and behavior. Drugs, however, can alter important brain areas that are necessary for life-sustaining functions and can drive the compulsive drug use that marks addiction. Understanding the science of addiction first requires a general understanding of how the brain works.

Over the last several decades, research on substances of abuse has vastly improved our understanding of human behavior and physiology and the nature of substance abuse and dependence. Basic neurobiological research has enhanced our understanding of the biological and genetic causes of addiction. These discoveries have helped establish addiction as a biological brain disease that is chronic and relapsing in nature. By mapping the neural pathways of pleasure and pain through the human brain, investigators are beginning to understand how abused psychoactive substances, including stimulants, interact with various cells and chemicals in the brain.

Addiction is a complex phenomenon with important psychological and social causes and consequences. However, at its core, it involves a biological process: the effects of repeated exposure to a biological agent (a substance) on a biological substrate (the brain) over time. Ultimately, adaptations that substance exposure elicits in individual neurons alter the functioning of those neurons, which in turn alters the functioning of the neural circuits in which those neurons operate. This eventually leads to the complex behaviors (e.g., dependence, tolerance, sensitization, craving) that characterize addiction.

View More: Understanding Drug Use and Addiction: National Institute for Drug Abuse