In medical terminology, addiction is a state in which the body relies on a substance for normal functioning and develops physical dependence, as in drug addiction. When the drug or substance on which someone is dependent is suddenly removed, it will cause withdrawal, a characteristic set of signs and symptoms. Addiction is generally associated with increased drug tolerance. In physiological terms, addiction is not necessarily associated with substance abuse since this form of addiction can result from using medication as prescribed by a doctor.

However, common usage of the term addiction has spread to include psychological dependence. In this context, the term is used in drug addiction and substance abuse problems, but also refers to behaviours that are not generally recognised by the medical community as problems of addiction, such as compulsive overeating.

The term addiction is also sometimes applied to compulsions that are not substance-related, such as problem gambling and computer addiction. In these kinds of common usages, the term addiction is used to describe a recurring compulsion by an individual to engage in some specific activity, despite harmful consequences to the individual's health, mental state or social life.

History of addiction models

The term 'addiction' appeared as far back as at least 1599, when Shakespeare used it in the first scene of Henry V; however it wasn't until 1906, in reference to opium, that it began to be used regularly. There is also an isolated usage, recorded from 1779, referring to tobacco. The first use of the adjective 'addict' (with the meaning of 'delivered, devoted') was in 1529 and comes from the Latin addictus, pp. of addicere ('deliver, yield, devote,' from ad-, 'to' + dicere, 'say, declare').

Prior to the latter half of the 20th Century, addiction was primarily a pharmacological term that referred to the process of developing drug tolerance so that more of a drug was required, more frequently, for the same effect to occur. However, with the founding of Alcoholics Anonymous in 1938, the allergy concept eventually morphed into the disease-model of addiction was proposed, based on the work of Dr. William Duncan Silkworth, and began to gather support in the professional community, amongst medical and social services workers, and amongst addicts themselves. The disease-model concept led to a definition of addiction based on the continued use of alcohol or drugs despite negative consequences for the user. This latter definition is now thought of as a disease state by the medical community. Morse and Flavin summarise the disease-model definition of addiction commonly utilized by treatment centers and substance abuse counselors:

Addiction is a primary, progressive, chronic disease with genetic, psychosocial, and environmental factors influencing its development and manifestations. The disease is often progressive and fatal. It is characterized by impaired control over use of the substance, preoccupation with the substance, use of the substance despite adverse consequences, and distortions in thinking.