Dependency describes a compulsion to continue taking a drug in order to feel good or to avoid feeling bad. When this is done to avoid physical discomfort or withdrawal, it is known as physical dependence; when it has a psychological aspect (the need for stimulation or pleasure, or to escape reality) then it is known as psychological dependence.

Physical dependence is when someone has taken drugs in quantity for a time and comes to rely on the use of them in order to feel well and for their body to function ‘normally’. It usually happens when the body has built up a tolerance to the drug and in its absence, physical withdrawal symptoms appear. It mainly happens with depressant drugs like alcohol, barbiturates, heroin or tranquillisers. However, the deep depressions and even suicidal feelings that can follow cocaine and ecstasy use could be counted as physical dependence, because users will take more of the drug to escape these feelings.

Psychological dependence is when the user experiences an overwhelming desire to continue with the drug experience. This can be because of the pleasurable effects and the desire to keep experiencing them. It can, however, also represent some sort of psychological crutch. The drug experience can become a way of blocking out reality, making life bearable, and a way of facing the world. Without the crutch life seems worthless. It can happen with any drug or any activity which takes over a person’s life including eating, sex, work, or jogging.

From NIDA glossary: A condition that can occur with the regular use of illicit or some prescription drugs, even if taken as prescribed. Dependence is characterized by withdrawal symptoms when drug use is stopped. A person can be dependent on a substance without being addicted, but dependence sometimes leads to addiction.

ICD 11 6C40.2 Alcohol dependence: Alcohol dependence is a disorder of regulation of alcohol use arising from repeated or continuous use of alcohol. The characteristic feature is a strong internal drive to use alcohol, which is manifested by impaired ability to control use, increasing priority given to use over other activities and persistence of use despite harm or negative consequences. These experiences are often accompanied by a subjective sensation of urge or craving to use alcohol. Physiological features of dependence may also be present, including tolerance to the effects of alcohol, withdrawal symptoms following cessation or reduction in use of alcohol, or repeated use of alcohol or pharmacologically similar substances to prevent or alleviate withdrawal symptoms. The features of dependence are usually evident over a period of at least 12 months but the diagnosis may be made if alcohol use is continuous (daily or almost daily) for at least 3 months.

The concept of alcoholism (no longer used in ICD): The belief that alcoholism is a condition of primary biological causation and predictable natural history, conforming to accepted definitions of a disease. The lay perspective of Alcoholics Anonymous (1939)-that alcoholism, characterized by the individual’s loss of control over drinking and thus over his or her life, was a "sickness"-was carried into the scholarly literature in the 1950s in the form of the disease concept of alcoholism. The concept was rooted in 19th-century medical and lay conceptions of inebriety as a disease. In 1977, a WHO Group of Investigators* responding to the loose and varying usage of alcoholism, proposed substituting the term alcohol dependence syndrome in psychiatric nosology (From the WHO Lexicon of alcohol and drug terms).

Dependence: A condition that can occur with the regular use of illicit or some prescription drugs, even if taken as prescribed. Dependence is characterized by withdrawal symptoms when drug use is stopped. A person can be dependent on a substance without being addicted, but dependence sometimes leads to addiction (NIDA glossary).