A number of individuals occasionally use or abuse alcohol or drugs without becoming addicted, but for many abuse continues despite repeated attempts to return to more social or controlled use and leads to addiction. Addiction is characterized by the repeated, compulsive seeking or use of a substance despite adverse social, psychological and/or physical consequences.
A wide range of substances, both legal and illegal, can be abused addictively. Addiction is often, but not always, accompanied by physical dependence, a withdrawal syndrome and tolerance. Physical dependence is defined as a physiological state of adaptation to a substance such that the absence of the substance produces symptoms of withdrawal. A withdrawal syndrome consists of a predictable group of signs and symptoms that result from abrupt removal of, or rapid decrease in the regular dosage of, a psychoactive substance. Tolerance is a state in which a drug produces a diminishing biologic or behavioral response, which means higher doses are needed to produce the same effect that the user experienced initially.