Addiction is a persistent, compulsive dependence on a behavior or substance. The term has been partially replaced by the word dependence for substance abuse. Addiction has been extended, however, to include mood-altering behaviors or activities. There are mainly two types of addictions: substance addictions (for example, alcoholism, drug abuse, and smoking); and process addictions (for example, gambling, spending, shopping, eating, and activity). Some defining characteristics of addiction include impaired control over the substance/behavior, preoccupation with the substance/behavior, continued use despite consequences, and denial. Addiction can also be viewed as a continued involvement with a substance or activity despite the negative consequences associated with it. Pleasure, enjoyment or relief from actual or perceived ailments would have originally been sought; however, over a period of time involvement with the substance or activity is needed to feel normal. The Tenth Revision of the International Classification of Diseases and Health Problems (ICD-10) defines the dependence syndrome as being a cluster of physiological, behavioural, and cognitive phenomena in which the use of a substance or a class of substances takes on a much higher priority for a given individual than other behaviours that once had greater value. A central descriptive characteristic of the dependence syndrome is the desire (often strong, sometimes overpowering) to take the drugs (which may or not have been medically prescribed), alcohol, or tobacco. There may be evidence that return to substance use after a period of abstinence leads to a more rapid reappearance of other features of the syndrome than occurs with nondependent individuals.In 1964 a WHO Expert Committee introduced the term dependence to replace the terms addiction and habituation. The term can be used generally with reference to the whole range of drugs, Drug dependence, chemical dependence, substance use dependence, or with specific reference to a particular drug or class of drugs (e.g. alcohol dependence, opioid dependence). While ICD-10 describes dependence in terms applicable across drug classes, there are differences in the characteristic dependence symptoms for different drugs.Behavioral addictions are patterns of behavior, which follow a cycle similar to that of substance dependence.This begins with the individual experiencing pleasure in association with a behavior and seeking that behavior out, initially as a way of enhancing their experience of life, and later, as a way of coping with stress. The process of seeking out and engaging in the behavior becomes more frequent and ritualized, until it becomes a significant part of the person's daily life. When the person is addicted, they experience urges or cravings to engage in the behavior, which intensify until the person carries out the behavior again, usually feeling relief and elation.Causes and symptomsAddiction to substances results from the interaction of several factors:Drug chemistrySome substances are more addictive than others, either because they produce a rapid and intense change in mood; or because they produce painful withdrawal symptoms when stopped suddenly.Genetic factorSome people appear to be more vulnerable to addiction because their body chemistry increases their sensitivity to drugs. Some forms of substance abuse and dependence seem to run in families; and this may be the result of a genetic predisposition, environmental influences, or a combination of both.