MK-677 (ibutamoren): the oral growth hormone secretagogue Merck abandoned, and what its trials show

MK-677, also sold as ibutamoren or ibutamoren mesylate, is a synthetic, orally active growth hormone secretagogue. It was designed at Merck as a non-peptide molecule that would do what the injected growth hormone releasing peptides do, but survive digestion and work as a once-daily tablet ^[1][2]. In the trial literature it appears under the codes MK-677, MK-0677 and L-163,191; the name Nutrobal is used by some sellers and has no regulatory meaning ^[10].

It matters for a buyer to understand that MK-677 is not a peptide. Compounds like ipamorelin, GHRP-2, GHRP-6 and hexarelin are short chains of amino acids that act at the same ghrelin receptor but must be injected. MK-677 is a spiroindoline small molecule, chemically closer to an ordinary tablet drug than to any of those. PepFinder lists it under the heading of other compounds sold by peptide stores, alongside YK-11 and other non-peptides, because that is where people actually encounter it: on the same shop pages as research peptides, often in capsule or liquid form. Our guide to what research peptides are explains the category and why it now includes products that are not peptides at all.

Anyone asking what is MK-677 and whether it works has an unusual amount of human data to draw on, because Merck ran a proper clinical programme. That programme is also the reason the answer is not a simple yes: MK-677 reliably raises growth hormone and IGF-1, produced some modest changes in body composition, failed in the two disease trials that would have led to a licence, and was discontinued. This guide sets out that record, the MK-677 benefits and MK-677 side effects reported in each trial, and how it is sold today.

Ghrelin is a hormone made mainly in the stomach that signals hunger and, separately, stimulates the pituitary gland to release growth hormone. MK-677 is a ghrelin mimetic: it binds and activates the growth hormone secretagogue receptor (GHS-R1a), the same receptor ghrelin uses, so the pituitary releases more growth hormone in larger pulses, and the liver responds by making more insulin-like growth factor 1 (IGF-1) ^[1][2]. Because it is taken by mouth and acts for many hours, one daily dose was enough to sustain the effect in every trial that measured it ^[1][8].

Two consequences follow from that mechanism, and both show up in the trial results. First, MK-677 does not add growth hormone from outside in the way that injected hGH does; it amplifies the body's own pulsatile release, which stays under the normal feedback control of somatostatin and IGF-1. Second, because ghrelin is the hunger hormone, a drug that mimics it makes people hungrier. Increased appetite is one of the most consistent findings in the human studies and is not a side effect in the accidental sense; it is the receptor doing its job ^[8][11].

The same receptor is targeted by the growth hormone releasing peptides, and a different one, the GHRH receptor, by sermorelin, CJC-1295 and tesamorelin. Combining a ghrelin-receptor agonist with a GHRH analogue is the logic behind stacks such as CJC-1295 and ipamorelin; no published trial has tested MK-677 in such a combination.

The pharmacology is well established. In healthy young men, seven days of oral MK-677 increased 24-hour growth hormone secretion and raised IGF-1, in a study that also tracked adrenocortical hormones ^[2]. In healthy elderly subjects, daily oral doses raised IGF-1 towards the levels seen in young adults, with a sustained rather than tolerance-limited response over the weeks studied ^[1]. These two papers, both from 1996, are the basis of every later claim that MK-677 restores a youthful growth hormone axis, and on that narrow point they hold up.

Whether raising GH and IGF-1 in this way does anything useful is the harder question, and Merck tested it across several settings. In healthy volunteers placed on a calorie-restricted diet, two weeks of MK-677 reversed the nitrogen loss that dieting causes, which is a marker of protein sparing and the basis of the muscle-preserving claims ^[4]. In obese men treated for two months, MK-677 increased growth hormone secretion, fat-free mass and energy expenditure; the same paper reported no meaningful loss of body fat over that period ^[5]. In healthy and functionally impaired elderly adults, and in postmenopausal women with osteoporosis, MK-677 increased biochemical markers of bone turnover, meaning both formation and resorption rose ^[6][7]. The osteoporosis trial combined it with alendronate and did not produce the bone density gains that would have justified further development for bone ^[7].

The best-known study is the one-year trial in healthy older adults published in Annals of Internal Medicine in 2008. Compared with placebo, 25 mg of MK-677 a day increased fat-free mass and raised IGF-1, but did not improve strength or function, and it raised fasting glucose and reduced insulin sensitivity, with more reports of increased appetite, oedema and muscle pain in the treated group ^[8]. The authors framed it as a proof of concept that an oral ghrelin mimetic can change body composition, not as evidence of clinical benefit.

Sleep is a smaller but often-cited strand: a polysomnography study in young and older men reported that a week or more of oral MK-677 improved sleep quality, with more deep sleep and REM sleep reported ^[3]. It was a short study in a small group and has not been replicated at scale. A 2018 review of the safety and efficacy of growth hormone secretagogues, which covers MK-677 alongside the injectable peptides, concluded that the class reliably raises GH and IGF-1 but that long-term safety data are lacking and clinical outcome data are thin ^[11].

MK-677 has more randomised human data than almost any product on a peptide store's shelves. The early trials were short and small: seven days in healthy young men ^[2], two to four weeks in healthy elderly subjects ^[1], two weeks of diet-induced catabolism in volunteers ^[4], two months in obese men ^[5], nine weeks in elderly adults for bone markers ^[6], and a year of sleep and bone work in postmenopausal women ^[7]. All of them measured hormones and surrogate markers rather than outcomes a patient would notice.

The one-year trial in 65 healthy older adults was the first to run long enough to matter ^[8]. Its results are worth stating precisely, because they are quoted selectively. Fat-free mass rose by around a kilogram relative to placebo and IGF-1 rose into the range of young adults, which is the MK-677 results sellers describe. In the same participants, fasting glucose rose, insulin sensitivity fell, and the treated group reported more appetite increase, more oedema and more muscle pain; strength and physical function did not improve ^[8]. A reader looking for MK-677 before and after evidence will find a modest lean-mass gain paired with a measurable move towards insulin resistance, in a population that was healthy to begin with.

Merck then ran two disease trials. In mild to moderate Alzheimer's disease, a 12-month randomised trial in several hundred patients found that MK-677 raised IGF-1 as expected but had no effect on the progression of the disease on any clinical measure ^[9]. In patients recovering from hip fracture, a phase IIb randomised, placebo-controlled trial tested 25 mg a day for six months; the published report records that the programme was discontinued after a safety and efficacy review by the sponsor, and cases of congestive heart failure in this frail, elderly population have been reported as part of that decision ^[10]. No further Merck trials followed, and no other company has taken the compound through to approval.

Taken together, the human studies show a drug that does exactly what its mechanism predicts on hormone levels, produces small body-composition changes in healthy people, and has not shown benefit in any patient group large enough to license. That is a more complete picture than most research peptides can offer, and it is not a flattering one.

Across the trial programme, MK-677 was given once a day by mouth at doses from 10 to 50 mg. Healthy elderly subjects received up to 25 mg a day for two to four weeks ^[1]; healthy young men 5 or 25 mg for seven days ^[2]; dieting volunteers 25 mg for two weeks ^[4]; obese men 25 mg for two months ^[5]; elderly adults 25 mg for nine weeks in the bone-marker study ^[6]; postmenopausal women 25 mg for a year, with or without alendronate ^[7]; healthy older adults 25 mg for one year, with a second year in an extension ^[8]; Alzheimer's patients 25 mg for 12 months ^[9]; and hip-fracture patients 25 mg for six months ^[10]. The 25 mg dose was chosen early because it produced a near-maximal IGF-1 response, and the programme rarely went above it.

These are the doses used in published research under medical supervision, with blood tests for glucose, IGF-1 and other markers at intervals. They are not a recommendation, and this guide does not give one. The 25 mg capsules sold by peptide stores reflect the trial dose, which is unusual: with most research peptides the doses seen online come from forums, whereas here the number comes from Merck. That does not make it safe to copy. The trials that used 25 mg for a year also recorded rising fasting glucose and falling insulin sensitivity at that dose ^[8].

Anyone asking about MK-677 dosage should also note what the trials did not test: no study used more than 50 mg a day, no study used the multi-month cycling patterns described in bodybuilding forums, and no study combined MK-677 with anabolic steroids, IGF-1 LR3 or SARMs. Our peptide calculator covers the arithmetic for liquid drops sold by concentration, but the underlying numbers come from a trial programme that stopped.

The MK-677 side effects reported in trials are consistent across studies and follow from the mechanism. Increased appetite was the most common, reported in short studies and in the year-long trial in older adults ^[8][11]. Fluid retention, showing as oedema, peripheral swelling or a feeling of puffiness, was reported more often in treated groups than with placebo ^[8]. Muscle pain and transient lethargy or sleepiness were reported, the latter in keeping with the sleep findings ^[3][8]. Some studies reported raised prolactin or cortisol; the seven-day study in young men specifically examined adrenocortical function, and changes in cortisol have not been consistent across trials ^[2][11].

The metabolic effects deserve the most attention. Growth hormone opposes the action of insulin, so a drug that raises it for a year would be expected to push blood glucose up, and that is what the Annals trial found: higher fasting glucose and lower insulin sensitivity in the MK-677 group ^[8]. The obese-subject study also reported a rise in fasting glucose and insulin over two months ^[5]. In a healthy older person this is a shift within the normal range; in someone with prediabetes or a family history of type 2 diabetes it is a reason for caution, and no trial has studied that group.

The most serious signal came at the end of the programme. In the hip-fracture trial, a population of frail elderly patients, the sponsor's safety and efficacy review led to discontinuation, and heart failure has been reported among the concerns ^[10]. Fluid retention and its cardiovascular consequences are a known class effect of raising growth hormone, and the review of secretagogue safety published in 2018 flagged the absence of long-term safety data for all of them ^[11]. No trial has followed people taking MK-677 for the multi-year periods some users describe, and nothing is known about its effects on growth in adolescents, on existing tumours, or in pregnancy, because none of those groups has been studied.

For someone buying MK-677 from a peptide store, the usual product risks apply on top of the pharmacological ones: unverified purity, capsule content that does not match the label, and liquid products whose concentration cannot be checked without a lab. See our guides to third-party peptide testing and how to spot a fake peptide supplier.

MK-677 has never been approved as a medicine by any regulator, and Merck stopped developing it after the hip-fracture programme was discontinued ^[10]. That leaves it in the same grey zone as most products on PepFinder: not a licensed medicine, not a food supplement, and in most countries not a specifically controlled drug either. The answer to is MK-677 legal therefore depends on what you mean by legal and where you are. Our legal status overview covers the general rules for each market; the position for MK-677 in particular is as follows.

In the United Kingdom, MK-677 is not a licensed medicine and is not listed under the Misuse of Drugs Act. Selling it for human consumption would make it an unlicensed medicine, which is why UK stores sell it labelled for research use only; possession for personal use is not an offence. In the United States, it has no FDA approval and is not a scheduled controlled substance, but the FDA has warned that products marketed as supplements containing ibutamoren or SARMs are unapproved drugs, and a compound that has been tested as a drug cannot legally be sold as a dietary supplement. In the European Union it has no authorisation from the EMA or any national agency and falls under each member state's medicines law once it is sold for use in people.

Australia treats compounds in this category more strictly: substances sold for bodybuilding and hormone modulation, including SARMs and growth hormone secretagogues, are generally scheduled as prescription-only under the Poisons Standard, so importing or supplying MK-677 without a prescription can be an offence, and the current schedule should be checked before buying. Health Canada has not authorised any product containing ibutamoren and has issued public advisories about unauthorised bodybuilding products containing it; it cannot lawfully be sold for human use in Canada.

For athletes the position is unambiguous. The World Anti-Doping Agency lists growth hormone secretagogues, with ibutamoren (MK-677) named, under section S2 of the Prohibited List, which means it is banned at all times, in and out of competition, in every sport that follows the WADA Code ^[12]. Because it raises IGF-1 for as long as it is taken, and IGF-1 is routinely measured in athlete biological passports, it is also readily detected.

Because MK-677 is a small molecule rather than a peptide, storing it is simpler than storing BPC-157 or ipamorelin. Ibutamoren mesylate is a stable crystalline salt that keeps at room temperature in a sealed, dry container away from light; it does not need refrigeration as a powder or in capsules, and it does not need reconstitution. Much of the advice in our peptide storage guide about freeze-dried vials, cold chains and reconstituted shelf life does not apply here.

The liquid form sold by many stores is a different matter. MK-677 is not very soluble in water, so liquid products are usually suspensions or solutions in polyethylene glycol, glycerol, ethanol or a similar carrier, dispensed by dropper. These should be kept sealed, away from heat and light, and shaken before use if the product is a suspension, since the dose in a dropper depends on the compound being evenly distributed. Bacteriostatic water, which peptide buyers use to reconstitute lyophilised vials, is not the right solvent for MK-677 and is not needed for it.

Raw powder is sold by some suppliers for research use. Weighing a 25 mg dose from raw powder requires a milligram-accurate balance, which most buyers do not have, and the trial programme never used anything other than manufactured tablets or capsules of known content ^[1][8].

Against the injected ghrelin-receptor peptides, MK-677 offers the same mechanism by mouth with a much longer duration. Ipamorelin, GHRP-2, GHRP-6 and hexarelin each produce a growth hormone pulse lasting a few hours after a subcutaneous injection, so users inject them one to three times a day; MK-677 was dosed once daily throughout its trials and sustained IGF-1 around the clock ^[1][8]. The cost of that convenience is that the effect is always on, including the appetite and glucose effects, whereas short-acting peptides wear off between doses. GHRP-6 shares MK-677's strong hunger effect; ipamorelin is marketed on having less of it, though the human data for ipamorelin are far thinner than for MK-677.

Against the GHRH analogues, sermorelin, CJC-1295 and tesamorelin, the difference is the receptor. GHRH analogues stimulate the pituitary through the GHRH receptor; MK-677 acts through the ghrelin receptor. Tesamorelin is the only member of either group with a marketing authorisation, for HIV-associated abdominal fat in the US. Our ipamorelin versus sermorelin comparison covers that split in more detail.

Against recombinant hGH, MK-677 is indirect: it can only raise growth hormone as far as the pituitary will go, which in an older adult means back towards young-adult levels, not beyond ^[1]. hGH injections can exceed physiological levels and are a licensed medicine with its own extensive safety record. IGF-1 LR3 bypasses the pituitary and the liver altogether. For a reader comparing options for muscle growth, the honest summary is that MK-677 has the best human evidence of any product in the secretagogue category and that the evidence shows a kilogram or so of lean mass over a year with no gain in strength ^[8].

Finally, MK-677 is often sold and discussed as an MK-677 SARM. It is not a SARM. Selective androgen receptor modulators such as YK-11 act on the androgen receptor; MK-677 has no androgenic activity and does not suppress natural testosterone. It appears in SARM stacks because the same stores sell both and because its oral form makes it easy to add, not because of any shared pharmacology.

A typical MK-677 for sale listing on a peptide store comes in one of three forms. The commonest is capsules or tablets, usually 25 mg each, in bottles of 30, 60 or 90, mirroring the trial dose. The second is a liquid, most often 25 mg per mL in a 30 mL dropper bottle, sometimes 50 mg per mL. The third is raw powder in gram quantities, listed with a stated purity and aimed at buyers who intend to make their own capsules or solutions. All three are labelled for research use, and none is a licensed product.

PepFinder handles these forms differently. For raw powder and vials where the mass of MK-677 is stated, we compare listings per milligram, which is the same basis we use for peptides and lets a buyer see how much they are paying for the compound itself. For capsule and tablet listings we show the products and the price per bottle but do not rank them per milligram, because the stated content of a capsule cannot be checked against a certificate of analysis for the finished product in the way a powder can, and because the excipients, count and format make per-mg figures misleading. The MK-677 price comparison page shows both groups, and our methodology explains the scoring behind the store rankings.

When looking at a certificate of analysis for MK-677, look for HPLC purity on the batch and, ideally, an independent lab's identity test; our guide to reading a peptide COA explains the difference between a supplier's own certificate and a third-party one. Because MK-677 is a small molecule with a single known structure, it is easier to verify by mass spectrometry than a mixture like cerebrolysin, and an honest supplier should be able to show a test of the specific batch. Our guide to how peptide prices work covers why stated purity, batch size and form move the price, and the supplier directory lists the stores we track and the checks each has passed.

If you decide to buy MK-677, the checklist is the same as for any research compound: a batch-specific certificate, a store with a verifiable address and history, a price that is neither far below nor far above the market, and an understanding that you are buying an unapproved drug whose developer stopped working on it. The trials answer the question of what MK-677 does more completely than for almost any other product on these shelves, which is exactly why the marketing should be read against them.