An agonist is a drug that has affinity for and binds to cell receptors to induce changes in the cell that stimulate physiological activity. The agonist opioid drugs have no clinically relevant ceiling effect for analgesia. As the dose is raised, analgesic effects increase in a linear fashion, until either analgesia is achieved or dose-limiting adverse effects occur. Efficacy is defined by the maximal response induced by administration of the active agent. In practice, this is determined by the degree of analgesia produced following dose escalation through a range limited by the development of adverse effects. Potency, in contrast, reflects the dose-response relationship, and is influenced by pharmacokinetic factors (i.e. how much of the drug enters the body's systemic circulation and then reaches the receptors and by affinity to the drug receptors).
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