# What Is Ibutamoren (MK-677)? Non-Peptide Ghrelin-Receptor Agonist: Structure, Mechanism, Evidence > MK-677 is a small molecule, not a peptide: orally active, active for about 24 hours, and the source of both its research appeal and its interpretive difficulties. Web page: https://peptidemedixeu.com/en/learn/what-is-ibutamoren-mk-677/ · Peptide Medix EU · 6 min read · Updated: 2026-01-16 [Ibutamoren](https://peptidemedixeu.com/en/products/ibutamoren-mk-677/) — listed as MK-677 or MK-0677 — is not a peptide. It is a spiropiperidine-based small molecule weighing 528.66 g/mol that binds and activates GHS-R1a, the very receptor the growth hormone releasing peptides act on. Two characteristics set it apart from every injectable secretagogue in this catalogue: it works when taken orally, because a small synthetic molecule withstands the gut and first-pass metabolism in a way a five-residue peptide cannot, and its reported half-life is roughly 24 hours rather than the few minutes typical of peptide secretagogues. Those two facts account both for its attraction as a research compound and for the interpretive complications it creates. Peptide Medix EU offers Ibutamoren (MK-677) Capsules in bottles of 30 at 12.5 mg and 25 mg, within the [GHRPs](https://peptidemedixeu.com/en/glossary/ghrp/) and secretagogues range and the oral capsules format. ## Ibutamoren (MK-677) defined Ibutamoren is a growth hormone secretagogue — that is, a compound that prompts the body to release its own growth hormone rather than supplying the hormone directly. Whereas the GHRP family recreates a peptide pharmacophore, ibutamoren is a peptidomimetic: a small molecule identified through structure-based optimisation that fills the same receptor pocket while bearing no structural likeness to ghrelin or to the GHRPs. It is identified by CAS 159752-10-0, formula C27H36N4O5S and a molecular weight of 528.66 g/mol, and also appears as MK-0677 and L-163,191, usually supplied as the mesylate salt. Being a small molecule rather than a peptide, it is stable as a solid at room temperature, needs no reconstitution and requires no cold chain — a practical distinction from everything else in this product family. ## Discovery and structure Ibutamoren emerged from Merck's secretagogue programme in the mid-1990s, which aimed to translate peptide GHRP pharmacology into an orally available drug candidate. The chemical series behind it used a spiropiperidine core carrying sulfonamide and indoline groups; the resulting compound bound the receptor with high affinity, was absorbed orally and lasted long enough for once-daily dosing. The receptor itself was cloned in 1996 with this compound class serving as the probe, and ghrelin, the natural ligand, was not identified until 1999 — so the synthetic mimic preceded the hormone it imitates, an unusual order of events in pharmacology. ## Mechanism of action Ibutamoren acts as an agonist at the ghrelin receptor GHS-R1a, a Gq-coupled receptor found on pituitary somatotrophs and in the hypothalamic arcuate nucleus. Activation stimulates phospholipase C, generates inositol trisphosphate and raises intracellular calcium, which in pituitary preparations accompanies release of secretory granules. Since the same receptor carries [ghrelin's appetite signal](https://peptidemedixeu.com/en/learn/research-catalog-obesity/), agonism in animal and human studies comes with greater food intake — an on-target result of the same receptor, not an incidental observation. For study design, the pharmacokinetics are the most consequential feature. Natural growth hormone secretion is pulsatile, and peptide secretagogues generate brief pulses that largely preserve that rhythm. A compound lasting 24 hours instead produces almost continuous receptor occupancy. Published data indicate that ibutamoren increases pulse amplitude and 24-hour output while keeping pulsatility intact, yet continuous GHS-R1a occupancy still raises the prospect of receptor desensitisation, which any reading of long-term data must confront. Its selectivity is incomplete. Human studies reported small simultaneous rises in cortisol and prolactin — less pronounced than with the older GHRPs, but not absent as they are with ipamorelin. ## Research applications ### Long-term studies in older adults The most frequently cited human study is a two-year randomised, placebo-controlled trial in healthy older adults published in 2008. It found lasting increases in growth hormone and IGF-1 along with a gain in fat-free mass, and also reported higher fasting glucose, markers of insulin resistance and increased appetite. It remains the single most informative account of what prolonged GHS-R1a agonism does in people over time. ### Metabolic and glycaemic findings The glucose finding recurs throughout the literature rather than standing alone. Growth hormone is counter-regulatory, so sustained elevation would be expected to lower insulin sensitivity, and studies in older and obese populations reported effects in the same direction. This is both the main reason the compound interests metabolic researchers and the main reason its development stopped. ### Bone, muscle and recovery studies Trials assessed bone mineral density markers and functional recovery following hip fracture, while separate work examined nitrogen balance during caloric restriction. Outcomes were mixed, with biochemical changes proving more consistent than functional ones. ### Sleep architecture As growth hormone release is tied to [slow-wave sleep](https://peptidemedixeu.com/en/learn/peptides-for-sleep-research-overview/), several studies used polysomnography and reported changes in slow-wave and REM sleep measures in both younger and older participants. ### Development status Despite a substantial clinical dataset, ibutamoren was never approved for any indication and its development was abandoned. It is best regarded as a thoroughly characterised investigational compound with published human pharmacology — not an unstudied research chemical, and equally not an approved medicine. ## Available formats The [capsule presentation](https://peptidemedixeu.com/en/learn/oral-peptides-capsules-troches/) eliminates the reconstitution and cold-chain variables that dominate handling of lyophilized peptides, which is why oral small molecules appeal in study designs where preparation error is a worry. The general trade-offs of oral formats are discussed in our guide to oral peptides, capsules and [troches](https://peptidemedixeu.com/en/glossary/sublingual-troche/). ## Handling and storage at the bench There is nothing to reconstitute — this is a finished capsule product, not lyophilized powder. Keep bottles at room temperature, sealed with the desiccant in place, away from light, heat and humidity. Moisture is the practical hazard for a capsule product: gelatin shells soften and stick together once the desiccant is spent or the bottle has been left open, and clumped capsules are a quantification problem, not just an aesthetic one. If a laboratory needs the compound dissolved for an [in-vitro assay](https://peptidemedixeu.com/en/glossary/in-vitro/), capsules are the wrong source material; free compound is needed, since capsule excipients will interfere with a cell-based readout. Ibutamoren dissolves poorly in water and is usually taken into DMSO for culture work, then diluted so the final solvent level remains within what the cell system tolerates. General storage principles appear in how to store peptides. ## Purity and reading the certificate of analysis Certificates for small molecules differ from peptide certificates. HPLC purity is still the headline figure, but identity is established by a [mass spectrometry result](https://peptidemedixeu.com/en/glossary/mass-spectrometry/) matching 528.66 g/mol — and for a molecule this size, an NMR spectrum is a far stronger identity check than mass alone, because it separates structural isomers sharing the same formula. The salt form should be stated, since the mesylate salt and the free base differ in mass and hence in how much compound a nominal 25 mg capsule actually contains. For a capsule product in particular, content uniformity is the key specification: the label value is a claim about every capsule in the bottle, not merely an average. Our guide to reading a certificate of analysis explains the general structure of these documents. ## Regulatory position Ibutamoren holds no marketing authorisation as a medicine in any jurisdiction. Clinical development finished without approval, so no reference-listed product or approved labelling exists, and it is not a food supplement — US regulators have taken the view that it does not qualify as a dietary ingredient. WADA names ibutamoren explicitly on its prohibited list under growth hormone secretagogues, banned both in and out of competition. Material supplied here is for laboratory research only and not for human consumption. ## Related compounds and further reading Its injectable counterparts at the same receptor are Ipamorelin, the selective pentapeptide, and blends combining a secretagogue with a GHRH analogue such as CJC-1295 + Ipamorelin. When a study calls for the peptide comparator rather than the oral small molecule, start with our ipamorelin explainer; for broader orientation, see the muscle growth research overview. ## Frequently asked questions ### Is Ibutamoren a peptide? No. It is a non-peptide spiropiperidine small molecule of 528.66 g/mol that activates the same receptor targeted by the growth hormone releasing peptides. It bears no structural resemblance to ghrelin or the GHRPs, which is precisely why it is orally active and stable at room temperature. ### Which receptor does MK-677 activate? GHS-R1a, the ghrelin receptor — a Gq-coupled receptor on pituitary somatotrophs and in the hypothalamic arcuate nucleus. Activation triggers phospholipase C signalling and raises intracellular calcium. Because the same receptor carries ghrelin's appetite signal, the increased food intake seen in animals and participants is an on-target effect. ### Why is the 24-hour half-life important for experimental design? Natural growth hormone secretion is pulsatile, and peptide secretagogues produce short pulses that largely preserve that rhythm, whereas a 24-hour half-life gives nearly continuous receptor occupancy. Published data report that pulsatility is maintained, but continuous GHS-R1a occupancy raises desensitisation questions that any long-term interpretation has to address. ### What did the two-year human trial find? In healthy older adults, a randomised placebo-controlled trial reported sustained rises in growth hormone and IGF-1 and an increase in fat-free mass, together with higher fasting glucose, markers of insulin resistance and greater appetite. It is the most informative published account of prolonged GHS-R1a agonism. ### Has MK-677 ever been approved? No. Despite an extensive clinical dataset it was never approved for any indication and development was halted. It is not a food supplement either — US regulators consider that it does not qualify as a dietary ingredient — and WADA names it explicitly on the prohibited list. ### How should Ibutamoren be stored? At room temperature in the sealed bottle with the desiccant left in, shielded from light, heat and humidity. No reconstitution or cold chain is needed. Moisture is the practical risk: gelatin shells soften and clump once the desiccant is exhausted, which turns into a quantification problem. ### Are capsules suitable for in-vitro experiments? Not as they are. Capsules contain excipients that interfere with cell-based readouts, so free compound is needed instead. Ibutamoren is poorly soluble in water and is normally dissolved in DMSO for culture work, then diluted so the final solvent concentration stays within the cell system's tolerance. --- For laboratory research use only. This page is provided for scientific and educational information. Materials referenced here are sold strictly for in-vitro laboratory research by qualified professionals — not for human or veterinary use, and nothing on this page is medical advice.