Addiction:
Addiction is generally recognized to be a state of intense
dependence (upon a drug) that is
characterized by
uncontrollable drug-seeking behavior,
tolerance for the drug's
pleasure-giving effects, and
withdrawal manifestations when the drug is withheld. These features constitute
physical dependence-- a state of physical incorporation of the drug into the fundamental biochemistry of specific
brain activities.
--From The Essential Guide to Prescription Drugs