A rapid decrease in the response to a drug after repeated administration. Unlike gradual tolerance (which develops over weeks), tachyphylaxis can occur within hours to days. The effect is often dramatic: a dose that produced a strong response on day one may produce almost no response by day three.

In the peptide space, tachyphylaxis is a practical concern for several compound classes. GHRP-6 and other growth hormone secretagogues can show diminished GH release with continuous use, which is why many protocols include off-days or cycling periods. Melanotan II users report decreasing tanning response over extended use. Understanding tachyphylaxis is essential for interpreting both clinical trial results and community dosing reports—a protocol that works initially may not sustain its effect.