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Hexarelin is one of several synthetic met-enkephalin analogs that include unnatural D-amino acids. They were developed for their growth hormone (GH) releasing activity, then called GH secretatogues. They lack opioid activity but are potent stimulators of GH release. These secretatogues are distinct from the growth hormone releasing hormone (GHRH or GHRF) in that they share no sequence relation and derive their function through action at a completely different receptor, the ghrelin receptor.