Tachyphylaxis.

A rapid loss of response to a compound after repeated exposure.

Tachyphylaxis is a rapid decline in response to a compound following repeated or continuous exposure, often developing within hours or days. The usual mechanisms are receptor desensitization, where the receptor is chemically modified so it no longer couples efficiently to its signalling machinery, receptor internalization, where the cell withdraws receptors from its surface, or depletion of a downstream store that the response depends on. It is distinguished from tolerance mainly by speed, tolerance describing the slower adaptation that develops over weeks. The concept is directly relevant to peptides because so many of them act on receptor systems that are naturally stimulated in pulses rather than continuously, and those systems are built to adapt. It is one of the strongest arguments made for pulsatile rather than continuous stimulation of the growth-hormone axis, and the GnRH example shows the extreme version, where continuous stimulation of a pulsatile system reverses the outcome entirely. The practical implication is that a diminishing effect over time is often a predictable property of the receptor system rather than a sign of a degraded product or an inadequate amount, and that the pattern of exposure can matter as much as the compound itself.