GHRH plus GHRP receptor synergy
Endocrine pharmacology studies report that simultaneous stimulation of GHRH receptors and ghrelin-mimetic GHS-R1a receptors produces growth-hormone release greater than the sum of either pathway alone, while ipamorelin's selectivity avoids the cortisol and prolactin elevation seen with earlier GHRPs.
In plain English
The combination engages two different switches for the same hormone system. Ipamorelin is favored in research because it is unusually selective.