Addiction, drug or alcohol.

Repeated use of a psychoactive substance or substances, to the extent that the user (referred to as an addict) is periodically or chronically intoxicated, shows a compulsion to take the preferred substance (or substances), has great difficulty in voluntarily ceasing or modifying substance use, and exhibits determination to obtain psychoactive substances by almost any means.
Typically, tolerance is prominent and a withdrawal syndrome frequently occurs when substance use is interrupted. The life of the addict may be dominated by substance use to the virtual exclusion of all other activities and responsibilities.
The term addiction also conveys the sense that such substance use has a detrimental effect on society, as well as on the individual; when applied to the use of alcohol, it is equivalent to alcoholism.

In the 1960s the World Health Organization recommended that this term be abandoned in favor of dependence, which can exist in various degrees of severity.

World Health Organization. Lexicon of alcohol and drug terms published by the World Health Organization. Internet. Accessed August 20, 2009.