An agonist activates a receptor and stimulates a response. When given frequently over time, the body may:

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When a receptor is activated frequently over time, the body can downregulate the numbers of that specific receptor. This is a compensatory mechanism in response to prolonged agonist exposure that helps to maintain homeostasis and prevent overstimulation of the target tissue. Downregulation can result in decreased sensitivity to the agonist and may require higher doses to achieve the same response. Upregulation (A) typically occurs in response to chronic antagonist exposure, and results in increased numbers of receptors to maintain function. Partial agonists (B) do not block the receptor, but rather are less efficacious at stimulating a response compared to full agonists. Altering the drug's metabolism (C) may occur with repeated dosing, but is not the primary mechanism underlying changes in receptor expression.

An Agonist Activates a Receptor and Stimulates a Response