Ipamorelin and hexarelin usually turn up in the same catalog section, labeled for research use only, described in nearly identical language. One is a pentapeptide. The other is a hexapeptide. Five residues against six — and if that were the whole story, you'd reasonably expect two near-twins separated by one extra link in the chain.
The structure-activity literature says otherwise. The residue count is real, but it's the smallest meaningful difference between these two compounds. The larger ones sit in which residues occupy the variable end of each chain, how many receptors each is documented to engage, and how cleanly each moves growth hormone without moving anything else. This article walks both sequences end to end and gets to the part that actually separates them: hexarelin has a well-characterized second binding partner that ipamorelin is not reported to have.
What a Growth Hormone Secretagogue Actually Is
Start with the category. A growth hormone secretagogue (GHS) prompts the pituitary to release growth hormone the body already made, rather than supplying it from outside. The compound is a signal, not a payload.
Two separate doorways into the same room
Growth hormone releasing hormone (GHRH) acts at its own receptor. The growth hormone releasing peptide family — the GHRPs — does not. These compounds derive their function through activation of a completely different receptor, originally named the growth hormone secretagogue receptor (GHSR). The receptor was characterized before anyone knew what normally bound to it. Its natural ligand turned out to be ghrelin.
That history leaves the family with an odd description. Every peptide in it is a synthetic ghrelin mimetic, yet none share sequence similarity with ghrelin. They aren't ghrelin fragments or analogues in any structural sense — they're unrelated molecules that happen to fit the same receptor pocket, a situation with close parallels elsewhere in peptide pharmacology, as our structural comparison of two closely related receptors illustrates from the receptor side.
Where the family came from
GHRP-6, carrying the developmental code SKF-110679, is the parent hexapeptide of the series, engineered from synthetic met-enkephalin analogues. Several numbered siblings followed. Ipamorelin descends from GHRP-1; hexarelin descends from GHRP-6. Same family tree, different branches — which is the first clue that the two compounds you came here to compare were never designed as variants of each other.
Reading the Two Sequences, Residue by Residue
Here's what the two molecules are actually made of, written out, with a note on what each substitution appears to be doing.
Hexarelin: His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH₂
Hexarelin — formally examorelin — is a hexapeptide with the sequence His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH₂. Molecular formula C₄₇H₅₈N₁₂O₆, molar mass 887.06 g/mol, CAS number 140703-51-1. Mediolanum Farmaceutici developed it, building directly from GHRP-6 by methylating the tryptophan at position two.
Note what that leaves in the chain: two tryptophan residues, at positions two and four. Tryptophan is the most photolabile of the common amino acids, so this is a practical handling consideration, not an academic one — our explainer on how tryptophan residues drive photodegradation covers why amber vials and dark storage are standard for sequences like this.
Ipamorelin: Aib-His-D-2-Nal-D-Phe-Lys-NH₂
Ipamorelin is a pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH₂, derived from GHRP-1. Molecular formula C₃₈H₄₉N₉O₅, molar mass 711.87 g/mol, CAS number 170851-70-4, original development code NNC 26-0161 from Novo Nordisk.
Two substitutions are worth sitting with. The chain opens with Aib — alpha-aminoisobutyric acid — which isn't one of the twenty proteinogenic amino acids at all. It carries two methyl groups on its alpha carbon, sharply constraining how freely the backbone can rotate there. Peptide chemists reach for Aib precisely when they want a locked conformation rather than a floppy one. At position three sits D-2-naphthylalanine, a bulky bicyclic aromatic side chain standing in where the hexapeptides carry tryptophan.
And ipamorelin contains no tryptophan anywhere in the sequence.
What the side-by-side actually shows
Roughly 175 g/mol separates the two molecules, which tracks the extra residue well enough. But reading that gap as "one more amino acid" misses the point. The conserved portion of both chains is the C-terminal end; the variable portion is the N-terminal end — and ipamorelin's isn't a truncated hexapeptide, it's a rebuilt recognition motif using residues the hexapeptide doesn't contain. That pattern of a fixed tail and an engineered head recurs across the whole GHRP series.
The Shared Pharmacophore and the Point of Unnatural Residues
A few features show up in every member of this family, and each is there for an identifiable chemical reason rather than historical accident.