An increase in the number of receptors on a cell surface or in their sensitivity to ligands. This is the body’s compensatory response to insufficient signaling—when a signal is chronically low, cells produce more receptors to capture whatever signal is available.

Upregulation has direct clinical implications. When a blocking drug is withdrawn suddenly, the upregulated receptors can produce a rebound effect—an exaggerated response to the body’s normal signaling. This is why abrupt discontinuation of certain medications causes withdrawal symptoms. In peptide contexts, receptor upregulation during off-cycle periods is one proposed rationale for cycling protocols, though direct evidence for this mechanism in most peptide compounds is limited.