Ipamorelin vs MK-677: Same Ghrelin Receptor, Not the Same Molecule

Ipamorelin is a five-amino-acid growth hormone-releasing peptide. MK-677, also called ibutamoren or MK-0677, is a Merck spiropiperidine that was built to copy GHRP signaling by mouth. Both turn on the ghrelin receptor GHS-R1a. They are not the same class of molecule, and neither is an FDA-approved growth hormone drug.

TL;DR

• Ipamorelin is a peptide. Novo Nordisk published Aib-His-D-2-Nal-D-Phe-Lys-NH2 in 1998 as a selective GHRP (Raun et al., 1998).
• MK-677 is not a peptide. Patchett’s group at Merck made L-163,191 (MK-0677) as an oral GHRP mimetic so it would survive the gut (Patchett et al., 1995).
• Same receptor, different clock. Human ipamorelin IV work showed a terminal half-life of about 2 hours and one GH pulse. Oral MK-677 was dosed once daily and raised 24-hour GH profiles.
• The longest MK-677 body-composition trial did not make people stronger. Nass 2008 added fat-free mass in older adults without improving strength or function. Glucose and HbA1c rose.
• FDA put ibutamoren mesylate in compounding Category 2 after a hip-fracture trial stopped on a CHF signal. WADA lists both under S2.2.4.

Educational only. Not medical advice, not a stack, and not a reason to treat a research chemical as GH.

Is MK-677 a peptide?

No. A peptide is a short amino-acid chain, and ipamorelin is one. MK-677 is not.

Patchett, Nargund, Tata, and colleagues described L-163,191 (MK-0677) in PNAS as a potent, orally active growth hormone secretagogue from a new structural class. Merck’s point was oral bioavailability. A GHRP pentapeptide does not survive digestion, so they built a small molecule that still talked to the GHRP receptor (Patchett et al., 1995).

Gym language still files MK-677 next to “peptides.” That is a catalog habit. See Peptides 101.

What is ipamorelin doing on that receptor?

It is a GHRP, not a GHRH analog. Raun’s group cut the central Ala-Trp of GHRP-1 and kept GH release. In rat pituitary cells the EC50 was 1.3 nmol/L, next to 2.2 nmol/L for GHRP-6 (Raun et al., 1998).

The target is GHS-R1a, the ghrelin receptor on somatotrophs. That path is separate from GHRH analogs such as sermorelin and CJC-1295. Boards pair them because two receptors feed one pituitary, which is a mechanism claim, not a protocol.

In conscious swine, GHRP-2 and GHRP-6 raised ACTH and cortisol. Ipamorelin did not, even at doses more than 200-fold its GH ED50. That swine profile is why it was called the first selective GHS, not a human recomp trial. The standalone write-up is Ipamorelin: the selective GH secretagogue.

How long does each GH pulse last?

Gobburu, Agerso, Jusko, and Ynddal infused ipamorelin over 15 minutes in healthy men. Terminal half-life was about 2 hours. GH peaked at 0.67 hours, then a single pulse fell away (Gobburu et al., 1999).

MK-677 was built for once-daily oral use. Copinschi compared bedtime placebo, 5 mg, and 25 mg for 7 days in nine healthy young men, then drew 24-hour GH and cortisol profiles plus morning IGF-I and IGFBP-3 (Copinschi et al., 1996).

Chapman then gave daily oral MK-677 to healthy older adults. GH, IGF-I, and IGFBP-3 rose. Cortisol did not change, and prolactin rose 23% while staying in the normal range (Chapman et al., 1996).

Short pulse versus all-day receptor occupancy is the real comparison. That is why appetite, edema, and glucose show up more often in the oral-mimetic literature.

What did the longest MK-677 trial actually measure?

Nass, Thorner, and colleagues ran a two-year randomized, placebo-controlled, modified-crossover trial of 25 mg oral MK-677 daily in 65 healthy adults aged 60 to 81 (Nass et al., 2008).

Lean mass moved. Performance did not. Nass reported a 1.1 kg fat-free-mass gain versus a 0.5 kg loss on placebo. Strength and physical function did not improve.

Fasting glucose, HbA1c, appetite, and body weight increased. That is what a 24-hour ghrelin-receptor agonist is built to do. The cohort was healthy older adults, not contest prep, and it remains the best human body-composition dataset on MK-677.

Ipamorelin has no matching two-year lean-mass study. Its later human trial was postoperative ileus, and it missed the meal-tolerance endpoint.

Why did FDA flag ibutamoren for compounding?

Adunsky’s phase IIb hip-fracture study randomized older patients to MK-0677 25 mg or placebo. IGF-1 rose and stair-climbing power did not. The trial terminated early because of a potential congestive-heart-failure signal (Adunsky et al., 2011).

FDA cites that paper on its compounding safety list. Ibutamoren mesylate is Category 2 under 503A and 503B, for that CHF risk. Ipamorelin acetate is separately on the 503B Category 2 list for immunogenicity, unnatural amino acids, and serious events in the IV ileus study, including death, with causality not established for every event (FDA list).

Category 2 means FDA sees significant safety risk in compounding that bulk substance. It is not an approval. Licensed peptide products live in channels the provider directory and price index compare.

Does WADA treat them the same?

Yes, as GH secretagogues. The 2026 Prohibited List, section S2.2.4, names ibutamoren (MK-677) and ipamorelin (WADA Prohibited List). A pill does not create a chemistry exception for tested athletes.

A research vial is still a COA problem: identity, purity, and endotoxin. Reconstitution math lives in the reconstitution guide and the calculator, not a dose chart. Clinician voices are on Experts.

Bottom line: Ipamorelin is a short GHRP. MK-677 is Merck’s oral non-peptide ghrelin mimetic. They share GHS-R1a, they do not share chemistry or a human performance trial, and neither is an approved GH drug.

Educational only. Not medical advice. Not a recommendation to use ipamorelin, MK-677, or any unapproved secretagogue.