ABSTRACT
A substance is dened as an opioid at the clinical level if it has a capability to produce effects that are produced by one or more of the poppy-derived opiates, and in particular morphine-like effects (or to block them if the substance is an antagonist). Since morphine mimics the pharmacology of endogenous compounds (such as endorphins and enkephalins), morphine-like effects can encompass a large variety of physiological processes. A more modern denition of opioid involves the molecular mechanism through which morphine produces its effect-activation of seven-transmembrane G-protein-coupled receptors (7-TM-GPCRs). Substances are said to be opioids if they possess an afnity for opioid receptors (this has an advantage in that it automatically includes antagonists).
