Addiction is a condition that results when a person ingests a substance (e.g., alcohol, cocaine, nicotine) or engages in an activity (e.g., gambling, sex, shopping) that can be pleasurable but the continued use/act of which becomes compulsive and interferes with ordinary life responsibilities, such as work, relationships, or health. Users may not be aware that their behavior is out of control and causing problems for themselves and others.

The word addiction is used in several different ways. One definition describes physical addiction. This is a biological state in which the body adapts to the presence of a drug so that drug no longer has the same effect, otherwise known as a tolerance. Another form of physical addiction is the phenomenon of overreaction by the brain to drugs (or to cues associated with the drugs). An alcoholic walking into a bar, for instance, will feel an extra pull to have a drink because of these cues.

However, most addictive behavior is not related to either physical tolerance or exposure to cues. People compulsively use drugs, gamble, or shop nearly always in reaction to being emotionally stressed, whether or not they have a physical addiction. Since these psychologically based addictions are not based on drug or brain effects, they can account for why people frequently switch addictive actions from one drug to a completely different kind of drug, or even to a non-drug behavior. The focus of the addiction isn't what matters; it's the need to take action under certain kinds of stress. Treating this kind of addiction requires an understanding of how it works psychologically.

When referring to any kind of addiction, it is important to recognize that its cause is not simply a search for pleasure and that addiction has nothing to do with one's morality or strength of character. Experts debate whether addiction is a "disease" or a true mental illness, whether drug dependence and addiction mean the same thing, and many other aspects of addiction. Such debates are not likely to be resolved soon. But the lack of resolution does not preclude effective treatment.