MK-677 is sold by SARMs Store strictly as a research compound. Nothing on this page is medical advice or an instruction for human use.
MK-677, also written ibutamoren or ibutamoren mesylate and originally coded L-163,191 by Merck, is the most searched compound on this site and one of the most studied. It is not a SARM at all, although it is almost always grouped with them. It is an orally active growth hormone secretagogue: a small molecule that mimics the hormone ghrelin and prompts the pituitary to release growth hormone. This guide brings together what the peer-reviewed literature has actually shown, what it has not, how the UK treats the compound in 2026, and how to judge whether a supplier’s product is what the label says.
What MK-677 is
Ibutamoren is a non-peptide agonist of the growth hormone secretagogue receptor (GHS-R1a), the same receptor ghrelin binds to. Because it is a small molecule rather than a peptide it survives digestion, which is why it was developed as a once-daily oral alternative to injected growth hormone in the 1990s. Its half-life is around 24 hours, and in the clinical studies a single daily dose produced a sustained rise in both growth hormone pulses and, downstream, insulin-like growth factor 1 (IGF-1).
That distinction matters for anyone reading about it. Unlike RAD-140 or Ostarine, MK-677 does not touch the androgen receptor. It does not suppress testosterone production and the research does not describe the androgenic effects associated with true SARMs. What it does is raise circulating growth hormone and IGF-1, and every documented benefit and side effect flows from that.
MK-677 and ibutamoren: one compound, several names
MK-677, MK677, MK 677 and MK-0677 are all the same compound as ibutamoren. MK-677 is Merck's development code, ibutamoren is the international non-proprietary name, ibutamoren mesylate is the salt form used in the clinical trials, and L-163,191 is the earlier laboratory code under which Patchett and colleagues first described it in 1995 (Proceedings of the National Academy of Sciences). Nutrobal appears as a trade name on some products. Anyone comparing "MK-677 vs ibutamoren" is comparing a compound with itself. What can differ between two products is the salt form, the amount per capsule and whether the batch has actually been tested.
What the clinical research found
MK-677 is unusual among compounds sold as research chemicals in that it went through real human trials, mostly in the late 1990s and 2000s.
Older adults, two-year study. The best known trial (Nass and colleagues, Annals of Internal Medicine, 2008) gave 65 healthy adults aged 60 to 81 either 25 mg of MK-677 or placebo daily for one year, with a second year of open-label treatment. IGF-1 rose to the levels typical of young adults. Fat-free mass increased by about 1.1 kg relative to placebo and limb lean mass increased, while fat mass did not change significantly. The authors concluded it “restores” the growth hormone profile of youth but noted that strength and function did not improve in step with body composition.
Hip fracture recovery. A 24-week study in older patients recovering from hip fracture found faster improvement in gait speed and stair climbing on MK-677, but it was stopped early because of a higher rate of congestive heart failure in the treated group, which is a reminder that raising growth hormone is not risk-free in frail populations.
Obesity. A two-month study in obese men (Svensson and colleagues, 1998) recorded increased fat-free mass and basal metabolic rate, but also increased fasting glucose and reduced insulin sensitivity.
Sleep. A small crossover study in young and older adults found MK-677 increased stage IV (deep) sleep and REM sleep, which is the origin of the “better sleep” claim seen across forums.
Bone. Studies in older women showed increased markers of bone turnover and modest gains in bone mineral density over 12 months.
What the literature does not contain is any trial in healthy young men training for muscle gain, which is the group most interested in it. Anecdotal reports are not evidence, and this site does not repeat them.
Side effects reported in studies
The trials are consistent on the profile. The most common reports were increased appetite (expected, since it acts on the ghrelin receptor), transient lower-limb oedema, muscle pain, and lethargy in the first weeks. The metabolically important findings were raised fasting glucose and reduced insulin sensitivity, which persisted while the compound was taken. The hip-fracture trial’s cardiac-failure signal in frail elderly patients, and the general caution that raised IGF-1 is undesirable in anyone with a history of cancer, are the two findings that responsible discussion of MK-677 should always include.
MK-677 versus growth hormone and peptides
MK-677 is often compared with injected somatropin and with peptide secretagogues such as CJC-1295 and ipamorelin. The research-relevant differences are simple. MK-677 is oral and long-acting; peptides are injected and short-acting. MK-677 raises growth hormone through the body’s own pulsatile release, so peak levels are lower than with injected hormone but the profile is closer to natural physiology. It is also far cheaper per month of study material, which is most of the reason for its popularity.
| Compound | What it is | Receptor | Form in the studies | Highest evidence |
|---|---|---|---|---|
| MK-677 (ibutamoren) | Small molecule | Ghrelin receptor (GHS-R1a) | Oral, once daily | Human trials up to two years; never licensed |
| Ipamorelin | Pentapeptide | Ghrelin receptor (GHS-R1a) | Injected | Phase I; phase II in post-operative ileus, stopped |
| CJC-1295 | Modified GHRH analogue | GHRH receptor | Injected, long acting | Phase I and II pharmacology studies |
| Sermorelin | First 29 amino acids of GHRH | GHRH receptor | Injected | Once licensed in the US as Geref; withdrawn 2008 |
| Tesamorelin | Modified GHRH analogue | GHRH receptor | Injected | Two phase III trials; US approval 2010 for one indication |
MK-677 and ipamorelin act on the same receptor as ghrelin; CJC-1295, sermorelin and tesamorelin act on the receptor for growth hormone releasing hormone. That is why they are sometimes studied in pairs: the two receptor types prompt growth hormone release by different routes.
Capsules, tablets and liquid
MK-677 is sold as capsules, tablets and liquids. The compound is the same in each; what differs is how reliably the stated amount is still there at the end of the bottle. A capsule or tablet is a fixed unit made once under manufacturing control. A liquid depends on the solvent, on how well the compound stays dissolved, and on how accurately each measure is drawn, and retail research liquids rarely carry any stability data. Our capsules or liquid guide sets out the stability and testing differences in detail. Dexterz Labs MK-677 is supplied as capsules only.
Legal status in the UK, 2026
MK-677 is not a controlled substance under the Misuse of Drugs Act 1971 and it is not listed as a Class C anabolic steroid. It is, however, an unlicensed medicine in the eyes of the MHRA if it is sold or advertised for human use, which is why legitimate UK suppliers sell it strictly for laboratory research and why product pages carry the notice you will see on ours. For athletes it is prohibited at all times under the WADA list (section S2, growth hormone secretagogues), and it has been detected in anti-doping tests since 2015. Import for personal use is a grey area that depends on customs classification and destination; SARMs Store ships from the UK and restricts destinations where seizure rates are high.
How to check a supplier
Three things separate a genuine research supplier from the rest. First, a batch-specific certificate of analysis from an independent laboratory, showing identity by HPLC or LC-MS and a purity figure, not a generic PDF. Second, transparent labelling: compound name, amount per capsule, capsule count, batch number, and a research-only notice. Third, a track record you can verify, such as reviews on the product page that are collected through a verified-purchase system. Dexterz Labs MK-677 is manufactured in the UK to 10 mg per capsule, 90 capsules. Every batch is independently tested and its certificate of analysis is published on the product page and the Lab Testing page.
Frequently asked questions
Is MK-677 a SARM? No. It is a growth hormone secretagogue. It is grouped with SARMs because it is sold by the same suppliers and studied by the same community.
Does MK-677 suppress testosterone? The published studies did not report suppression of testosterone or LH, because it does not act on the androgen receptor.
How long does MK-677 stay in the system? Its half-life is roughly 24 hours, and IGF-1 stays elevated for as long as daily administration continues in the studies, returning to baseline within weeks of stopping.
Why does MK-677 increase hunger? It activates the ghrelin receptor. Ghrelin is the hormone that signals hunger, so increased appetite is expected and was recorded in every trial.
Does MK-677 affect blood sugar? Yes. Reduced insulin sensitivity and increased fasting glucose were consistent findings and are the main reason the compound was never approved.
Is MK-677 legal in the UK? It is not a controlled drug. It is legal to buy and possess as a research compound, and illegal to sell or advertise for human consumption.
Is MK-677 the same as ibutamoren? Yes. MK-677 is Merck's development code and ibutamoren is the generic name for the same molecule. Ibutamoren mesylate is the salt form used in the trials.
Is MK-677 a steroid? No. It is not a steroid, not a SARM and not a hormone. It is a small molecule that acts on the ghrelin receptor.
Is MK-677 the same as growth hormone? No. It contains no growth hormone. In the studies it prompted the pituitary gland to release more of the body's own growth hormone.
Is MK-677 available on prescription in the UK? No. It has never been licensed as a medicine in the UK or anywhere else, so it cannot be prescribed.
Related reading
- Ostarine (MK-2866): the complete research guide
- RAD-140 (Testolone): what the studies show
- Are SARMs legal in the UK? 2026 update
- Third-party testing: how to read a certificate of analysis
- Browse the range: SARMs UK
Research product: Dexterz Labs Ibutamoren MK-677 10 mg, 90 capsules
On this site MK-677 is also one of four compounds in the Dexterz Labs Mass Series, alongside LGD-4033, RAD-140 and YK-11 in a single capsule. No published study has tested MK-677 in combination with any of them.
Related peptides in the research
MK-677 is usually read alongside the peptide growth hormone secretagogues it is compared with in the literature: CJC-1295 with Ipamorelin, Ipamorelin and Sermorelin. Guides: CJC-1295 and Ipamorelin and Sermorelin. For how a small-molecule secretagogue and a peptide differ in mechanism, evidence and storage, read SARMs vs peptides. You can buy research peptides UK from the Origin Peptides range.
Sources
Primary papers and official guidance cited in this guide, linked so each one can be checked directly.
- Nass R et al., Ann Intern Med 2008;149:601-611. PubMed 18981485
- Svensson J et al., J Clin Endocrinol Metab 1998;83:362-369. PubMed 9467542
- Murphy MG et al., J Clin Endocrinol Metab 2001;86:1116-1125. PubMed 11238495
- Copinschi G et al., Neuroendocrinology 1997;66:278-286. PubMed 9349662
- Adunsky A et al., Arch Gerontol Geriatr 2011;53:183-189. PubMed 21067829
- WADA Prohibited List 2026.
- Patchett AA et al., Proc Natl Acad Sci USA 1995;92:7001-7005. PubMed 7624358