Abuse, drug:

Any use of an illegal drug, or the intentional self-administration of a medication for a non-medical
purpose such as altering one’s state of consciousness, e.g., “getting high.” (Canadian Guideline for
Safe and Effective Use of Opioids for Chronic Non-cancer Pain 2010)

Abuse:

Self-administration of any drug in a culturally disapproved manner that causes adverse consequences.

Addiction:

A behavioral pattern of drug abuse characterized by overwhelming involvement with the use of a drug (compulsive use) despite harm, the securing of its supply and a high tendency to relapse after discontinuation.  Addiction to a drug may or may not be associated with dependence on the drug. A hallmark of addiction is the euphoria (or high) experienced by the user.  The stronger and faster the euphoria from a drug via a certain route is experienced, the more addictive potential it has.

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Agonist:

A substance that acts at a neuronal receptor to produce effects similar to those of a reference psychoactive substance, e.g. methadone is an agonist at the opioid receptors.

Drugs that interact with receptor sites to cause the same effect that natural chemicals would case at these sites. Karch, A, M. (2008). Focus on nursing in pharmacology. (4th ed.). Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins.

Antagonist:

Drugs that interact with receptor sites to cause the same effect that natural chemicals would case at these sites. Karch, A, M. (2008). Focus on nursing in pharmacology. (4th ed.). Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins.

A substance that counteracts the effects of a reference psychoactive substance by inhibiting or reversing its effects at a neuronal receptor site, e.g. naltrexone acts as an antagonist at the opioid receptor. (Adopted Canadian Society of Addiction Medicine October 14, 1999)