Tachyphylaxis

Tachyphylaxis is defined by speed: the response falls measurably within a short series of doses, sometimes between the first and the second. Several mechanisms produce the same picture. A releasable pool can be depleted, so an agent acting indirectly runs out of substrate. A receptor can be phosphorylated and uncoupled within minutes. A counter-regulatory loop can oppose the effect at the level of the whole system rather than the receptor.

Indirect sympathomimetics such as ephedrine are the classical demonstration, losing effect as they exhaust the noradrenaline stores they act by releasing. Organic nitrates lose vasodilator effect over hours of continuous exposure, which is why nitrate-free intervals are built into their use. In the growth hormone axis, closely spaced or continuous stimulation with releasing peptides attenuates the GH response compared with well-separated exposures.

Naming the mechanism matters because it dictates the fix. Depletion recovers when stores are replenished, a matter of synthesis and reuptake. Receptor uncoupling recovers on dephosphorylation once agonist clears. Counter-regulation persists as long as the perturbation does. Each implies a different interval before the response returns, and only receptor-level loss is addressed by spacing doses further apart.

The reasoning error is treating any fading effect as receptor burnout, a phrase with no mechanism attached, and escalating the dose. If the cause is depletion or feedback, more drug accelerates the loss rather than reversing it. The complementary error is on the study side: a trial short enough to capture only the first exposures reports an effect size the regimen will not sustain.