MK-677

Thoroughbred Labs MK-677 is a 60-capsule pack of ibutamoren (MK-677), a ghrelin receptor agonist and growth hormone secretagogue, made by Thoroughbred Labs and stocked by Free Muscle in the Muscle Market range. The manufacturer prints no per-capsule strength on the label, so we list it as not stated rather than guess. It is sold only for laboratory research, to adults aged 18 and over.

About MK-677

Ibutamoren (MK-677, L-163,191, sometimes sold under the name Nutrobal) is a small-molecule, non-peptide agonist of the growth hormone secretagogue receptor, GHS-R1a, the receptor for the hunger hormone ghrelin. Because it is orally available and mimics ghrelin at the pituitary, it stimulates the release of the body's own growth hormone rather than supplying the hormone directly. PubChem records the free base under CAS number 159634-47-6, with formula C27H36N4O5S, molecular weight 528.7 and InChIKey UMUPQWIGCOZEOY-JOCHJYFZSA-N.

Merck studied MK-677 in the 1990s and 2000s. Chapman and colleagues gave it by mouth once a day to 32 healthy volunteers aged 64 to 81, over study periods of two and four weeks. Growth hormone pulses grew taller without becoming more frequent, and serum IGF-1 climbed into the range seen in young adults; fasting glucose also rose, prolactin went up by 23 per cent while staying within the normal range, and cortisol did not change (Chapman et al., 1996).

The longest trial randomised 65 healthy adults aged 60 to 81 to MK-677 or placebo for up to two years, with its main end points assessed at one year (Nass et al., 2008). Fat-free mass rose by an average of 1.1 kg on MK-677 and fell by 0.5 kg on placebo, yet strength and function did not improve. On the other side of the ledger, body weight rose by 2.7 kg on MK-677 and 0.8 kg on placebo, fasting glucose climbed by an average of 0.3 mmol/L, insulin sensitivity fell and cortisol rose. Increased appetite, which eased after a few months, and transient mild leg swelling and muscle pain were the commonest side effects. MK-677 has never been licensed as a medicine. None of this research constitutes a claim about the product on this page.

This listing

• Compound: MK-677 (Ibutamoren)
• Brand: Thoroughbred Labs
• Form: capsules
• Pack size: 60 capsules
• Strength per capsule: not stated by the manufacturer on the label
• Total per pack: cannot be calculated without a stated strength
• Intended use: laboratory research only, not for human consumption
• Sold to adults aged 18 and over

With no strength on the label there is no total per pack and no price per milligram, so this pack cannot be set against brands that print a figure. The one document that could supply a measured content is a batch certificate, where Thoroughbred Labs provides one. The method for packs that do state a strength is explained in price per milligram.

MK-677 in the UK: buying, legal status and what to check

Of the UK searches for this compound, "MK 677 UK legal" is a question about the law, while "MK677 UK buy" and "MK677 UK supplier" come down to what a listing lets you verify.

The legal position

Ibutamoren has never received an MHRA marketing authorisation, so in the UK it is not a licensed medicine. Nor is it an authorised food supplement, so nobody may lawfully sell it for people to consume. It is not a controlled drug. Free Muscle therefore sells it only as a laboratory research compound, to buyers aged 18 or over. WADA prohibits it at all times under section S2, and each buyer is responsible for the rules that bind them, an anti-doping code included. More detail is on our MK-677 legal status in the UK page.

What a UK listing should let you check

1. Named businesses. Thoroughbred Labs manufactures the capsules; FreeMuscle LTD, a UK company based in Scunthorpe, sells and ships them.
2. A strength and a count. The count is 60 capsules; the strength is missing because the manufacturer does not state it.
3. A lot number tied to a certificate. We show a certificate only where Thoroughbred Labs supplies one for the batch we hold; email us to ask whether one exists for the current stock.
4. No directions for use. A research compound cannot also be sold for consumption, so no quantities or promised outcomes appear here.
5. A parcel you can follow. Each UK order goes out free, tracked and in plain packaging, and anything ordered before 2pm on a weekday is processed that working day; EU orders arrive in 3-7 working days.
6. A published returns policy. A sealed pack can be sent back within 30 days; our delivery and guarantee pages give the conditions.

Against that list, this pack's weak point is the missing strength. Where a stated figure matters, the MK-677 collection includes packs that print one, such as iMuscle's 10mg capsules and Free Muscle's own 10mg, 12.5mg and 25mg strengths, which Janoshik Analytical assays batch by batch. Forum threads and reviews can report how a parcel arrived, never what a capsule held. Our UK supplier checklist applies the same tests to any shop.

Capsules compared with other forms

MK-677 is searched for as tablets, capsules, liquid and even as a "peptide". It is not a peptide but a synthetic small molecule designed to mimic growth hormone releasing peptides at their receptor while staying active by mouth; both trials above gave it orally, and neither used an injectable form. The digestive breakdown that limits swallowed peptides (see do oral peptides work) is therefore not the issue. What changes between forms is where the stated amount can drift and what a certificate can vouch for.

Salt form matters too. Ibutamoren is often made as its mesylate (methanesulfonate) salt, which is heavier than the free base, so a milligram of salt holds less than a milligram of ibutamoren; a label or certificate should say which it means. None of this suggests one form performs differently; it concerns only what can be verified.

MK-677 compared with ipamorelin

"Ibutamoren vs ipamorelin" is the comparison most searched for this compound. Both activate GHS-R1a and prompt the pituitary to release its own growth hormone, but they are different kinds of molecule with unequal research records.

The word that separates them is "selective". In the 1998 paper introducing ipamorelin, Novo Nordisk researchers found that in conscious pigs it released growth hormone about as potently as the older peptide GHRP-6 but, unlike GHRP-6 and GHRP-2, did not raise ACTH or cortisol above the levels seen with GHRH, even at amounts far beyond those needed for a growth hormone response (Raun et al., 1998). For MK-677 the comparable data are human: cortisol held steady over four weeks but rose over twelve months. Species, designs and durations differ, so these results cannot show which is more selective in people, and we know of no trial giving both to the same participants.

Ipamorelin is a peptide, so absorption by mouth is an open question for it in a way it is not for MK-677; its human studies are summarised in the ipamorelin research guide. Regulatory status is identical: both are unlicensed, sold here only for laboratory research to adults, and listed in WADA section S2. The ipamorelin we stock is on the ipamorelin collection page, and MK-677 vs ipamorelin vs HGH adds somatropin to the comparison. None of this recommends either compound.

Reading the certificate of analysis for this product

Thoroughbred Labs, not Free Muscle, is the source of any certificate of analysis (COA) for this product. For third-party brands we display a COA only when the manufacturer has supplied one for the batch actually in our stock, and otherwise say none is available rather than show a generic document; email us before ordering to ask what exists for the current batch. Janoshik Analytical's independent batch testing covers Free Muscle-brand compounds only, not Thoroughbred Labs stock. Where a certificate exists, read it field by field.

For this pack the content row matters most: purity without milligrams per capsule confirms the substance but not the amount. See how to read a COA and our lab tests page, and contact us with the lot number if a certificate does not match your pack.

Detection, sport and testing

WADA names ibutamoren among the growth hormone secretagogues in section S2 of the Prohibited List, banned in and out of competition. Accredited laboratories screen urine for it and its metabolites, and under strict liability an athlete answers for whatever is found in their sample, however it got there.

How long it stays detectable is less settled. We have found no verified human study that sets a urine detection window. Hair has been studied: in a forensic report, hair collected four weeks after a single oral exposure in one male volunteer tested positive for ibutamoren, using a method with a limit of detection of 0.1 pg/mg (Kintz and Gheddar, 2026). The authors wanted reference data because anti-doping authorities have urged caution over hair results in Court of Arbitration for Sport hearings. One volunteer and one exposure cannot define a window, and the finding concerns hair rather than the urine samples most tests rely on. For urine the accurate statement is: detectable, window not established. Anyone bound by an anti-doping code should treat any contact with MK-677 as a risk of an adverse finding.

Storage, stability and handling

To our knowledge nobody has published a stability study of ibutamoren in capsule form, so no shelf life is quoted here. Work to any date Thoroughbred Labs prints on the pack; a certificate describes a batch only as it was when tested.

• Original container. The lot number on the manufacturer's pack is what ties the capsules to a certificate. Decanting them breaks that link, and so does mixing capsules from two packs.
• Cool, dry and dark. Keep the pack away from radiators, sunny windowsills and steamy rooms, as capsule shells absorb moisture and can soften and stick together.
• Sealed. Close the lid fully after each opening, leaving any desiccant sachet in place.
• Checked. Set aside discoloured, clumped or cracked capsules and report the lot number to us.
• Recorded and handled with care. Log the dates of receipt and first opening, wear nitrile gloves, keep the material away from food and out of reach of children and pets, and send surplus for chemical waste disposal.

Keep the seal intact until needed, because the 30-day return option covers unopened items only.

Glossary

• Ibutamoren: the International Nonproprietary Name for MK-677, whose Merck codes include L-163,191 and MK-0677.
• Growth hormone secretagogue: the class name for compounds, ibutamoren among them, that raise growth hormone by acting on the pituitary instead of replacing the hormone.
• GHS-R1a: growth hormone secretagogue receptor type 1a, the receptor ghrelin binds and the one both MK-677 and ipamorelin activate.
• IGF-1: insulin-like growth factor 1, made largely in the liver in response to growth hormone; trials use its level as a sign that the growth hormone axis has been stimulated.
• Fat-free mass: everything except fat, including water and organs, so a rise is not the same as a muscle gain.
• Insulin sensitivity: how strongly tissues respond to insulin; when it falls, blood glucose tends to rise, as in the MK-677 trials.
• Free base and mesylate: the neutral molecule and its heavier methanesulfonate salt; milligram figures for one do not transfer to the other.
• Enantiomer: either of two mirror-image versions of one molecule, identical in mass and separable only by chiral analysis.
• Limit of detection: the lowest concentration at which a test can confirm a substance is present.

Related

• Every MK-677 brand we stock, side by side on strength and batch testing
• The Thoroughbred Labs range
• Related collections: Ipamorelin, MK-777 (Acetamoren) and HGH (Somatropin)
• MK-677 (ibutamoren): what the research says and MK-677 by brand
• Delivery and returns

Research use only: Thoroughbred Labs MK-677 is neither a medicine nor a food supplement, has not been evaluated by the MHRA, EMA or FDA, and is not for human consumption. We sell it only to adults aged 18 and over, and nothing on this page is medical, dosing or usage advice.

References

1. Chapman IM, et al. Stimulation of the growth hormone (GH)-insulin-like growth factor I axis by daily oral administration of a GH secretogogue (MK-677) in healthy elderly subjects. J Clin Endocrinol Metab. 1996;81(12):4249-57. PMID: 8954023.
2. Nass R, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med. 2008;149(9):601-11. PMID: 18981485.
3. Raun K, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-61. PMID: 9849822.
4. Kintz P, Gheddar L. Knowing the minimal detectable dose can facilitate the interpretation of a hair test result: II. Case example with ibutamoren (MK-677), a growth hormone secretagogue. Clin Chim Acta. 2026;578:120578. PMID: 40882886.