This term refers to any chemical that changes the mental state and that may be used repeatedly for that effect by a person. Drug' now includes alcohol, tobacco, psychoactive pharmaceuticals, illicit drugs and substances such as petrol, some fluorocarbons and anabolic steroids. Such dependent use becomes habitual, stereotyped in pattern, and responsive to external cues and pressures. This pattern is quickly reinstituted after a period of abstinence. Abstinence may be associated with the onset of withdrawal symptoms and the discomfort of withdrawal will become a motivator for renewed drug use.
Psychological dependence describes the behavioural aspects of drug dependence.
Physical dependence exists in a tissue or intact organism when the presence of the drug is necessary for normal functioning. It refers to the tolerance which develops to the effects of drug use and to the withdrawal effects experienced when drug use is stopped. Addiction is a universal term and applies to those who give themselves to a wide variety of behaviours often to the extent that their lives become unmanageable. Addiction to alcohol and drugs is only an example of this pattern of behaviour. However, once an addiction is established that behaviour tends to be relied upon increasingly.