Addiction is a chronic but treatable brain disorder in which people lose the ability to control their need for alcohol or other drugs. It is characterized by the repetitive, compulsive use of a substance that occurs despite negative consequences to the user. Both psychological dependence and physical dependence coexist in people who are strongly addicted to something. This definition of addiction applies to the compulsive, repetitive use of alcohol, nicotine, opiate drugs like heroin, as well as cocaine and other stimulants. Some people also engage in activities such as overeating, gambling, and sex to the point that there are negative consequences for themselves and their families. These behaviors resemble drug-seeking behaviors in an addicted person, and more and more research shows that the same neural circuits may be involved.

Addiction can cause a person to abandon his job, his family, and his life or to ignore the most basic, life-sustaining impulses to eat and reproduce. While personal characteristics, including a lack of morals, having different brain chemistry, or experiencing mental illness or extreme trauma are all factors that influence addiction, something more primal is at work. Addiction is so powerful because it mobilizes basic brain functions that are designed to guarantee the survival of the species. Since the neural mechanisms by which addictive drugs act exist in every brain, potentially any human being could become a drug addict.

Addictive drugs (stimulants, opiates, alcohol, nicotine, etc.) are reinforcers, or pleasure pursuing drugs, and can actually substitute for food, sex, or other primary reinforcers. This explains why rapid injection of heroin or methamphetamine produces a "rush" of pure pleasure that most users compare to the pleasure of an orgasm. Enjoying the rush of pleasure from a drug is only part of addiction. Once the body adapts to the drug and physical dependence develops, a daily cycle of drug taking, pleasure, gradual waning of drug effect, and the onset of withdrawal symptoms emerges. Avoiding the unpleasant feelings of withdrawal and satisfying the desire for more drugs can be strong motives for drug taking.