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Defining addiction

The word addiction is often used to refer to any behaviour that is out of control in some way. People often describe themselves as being addicted, for example, to a TV show or to shopping. The word is also often used to explain the experience of withdrawal when a substance or behaviour is removed or discontinued (e.g., "I must be addicted to coffee – I get a headache when I don't have my cup in the morning"). However, neither enjoyment nor experiencing withdrawal necessarily implies addiction.

Because the term addiction is commonly used in such a vague way, there have been many attempts to define it more clearly. The American Society of Addiction Medicine (2011) provides this definition:

Addiction is a primary, chronic disease of brain reward, motivation, memory and related circuitry. Dysfunction in these circuits leads to characteristic biological, psychological, social and spiritual manifestations. This is reflected in an individual pathologically pursuing reward and/or relief by substance use and other behaviors.

Addiction is characterized by inability to consistently abstain, impairment in behavioral control, craving, diminished recognition of significant problems with one's behaviors and interpersonal relationships, and a dysfunctional emotional response. Like other chronic diseases, addiction often involves cycles of relapse and remission.

Key concepts in addiction

Physical dependence

Physical dependence is an adaptive physiological state that occurs with regular drug use and results in a withdrawal syndrome when drug use is stopped. It is usually associated with increased tolerance. Physical dependence tends to be a characteristic of substance use disorders, but in itself does not imply addiction.


Tolerance is a condition in which higher doses of a drug are required to produce the same effect experienced during initial use. It is often associated with physical dependence.

In the therapeutic use of a drug with psychoactive effects, patients tend to become tolerant to the psychoactive effects (e.g., euphoria, reduced anxiety, sleep inducement), but not as much to the therapeutic effects of pain relief.

For example, a patient who needs higher doses of morphine for chronic pain relief may be seeking the psychoactive effects of the drug to which tolerance has developed. Patients can develop tolerance to a drug without it causing harm or meaning that the person is addicted.

It is sometimes difficult to determine tolerance by a patient's history alone because initial sensitivity to a particular substance varies significantly between individuals.


Withdrawal is a characteristic of substance use disorders, but does not necessarily imply addiction.

With regular use of a substance, biochemical and structural adaptations take place in the brain. Withdrawal is the constellation of symptoms and signs that a person experiences when, after a period of regular use, the quantity of the substance in the brain is reduced. Upon abstinence, central nervous system receptors take days or weeks to normalize.

Symptoms and signs of withdrawal are opposite to the main effect of the drug. Here are specific examples:

  • Sedative withdrawal creates autonomic hyperactivity with dangerous medical complications.
  • Opioid withdrawal is accompanied by anxiety, powerful cravings and flu-like symptoms.
  • Stimulant withdrawal consists of depression, insomnia and cravings.