MK-677

Origin and structure

MK-677 comes from Merck's research laboratories, where it carried the codes L-163,191 and MK-0677; its generic name is ibutamoren, and it is usually made as the mesylate salt. Merck described it in 1995 as a potent growth hormone secretagogue that is active by mouth: in dogs, oral doses as low as 0.125 mg per kg raised growth hormone, with little effect on other hormones apart from a modest rise in cortisol, and it was selected for clinical studies.

Chemically it is a spiroindoline compound with no amino-acid chain, so although it is sold alongside peptides it is not one. Nor is it a SARM: those act on the androgen receptor, while ibutamoren acts on the ghrelin receptor and works through the body's own growth hormone.

It was tested through the 1990s and 2000s in older adults, obese men, people with Alzheimer's disease, hip-fracture patients and women with osteoporosis, but none of these programmes led to an approval. In 2017 the European Commission gave ibutamoren mesilate an orphan designation for growth hormone deficiency, and today the US company Lumos Pharma develops it for children with that condition under the code LUM-201.

How it works

Ibutamoren activates the growth hormone secretagogue receptor, better known as the ghrelin receptor, in the pituitary gland and the hypothalamus. Ghrelin is a hunger hormone made in the stomach, and the receptor it shares with ibutamoren does two things: it amplifies the pulses in which the pituitary releases growth hormone, and it stimulates appetite. The extra growth hormone makes the liver produce more IGF-1, which carries much of growth hormone's effect on muscle, bone and other tissue.

In older adults 25 mg a day brought growth hormone and IGF-1 up to the levels of healthy young adults, and one dose a day was enough to keep IGF-1 raised. The effect needs a pituitary that can still respond: in the phase 2 trial in children, 22 of 104 did not respond well enough to a test dose to go on, and the phase 3 trial excludes children whose pituitary barely responds to standard stimulation tests.

The same pathway explains most side effects. Growth hormone counteracts insulin, so blood glucose tends to rise; activating the ghrelin receptor increases hunger; and cortisol and prolactin rose in some trials, cortisol by 47 nmol/L over a year in older adults.

Is MK-677 approved?

No. No medicines regulator has authorised ibutamoren for any use. In the EU the only formal step is the orphan designation of June 2017 for growth hormone deficiency, a status that supports the development of medicines for rare conditions but is not a marketing authorisation. In the Netherlands it is not a registered medicine, so it may not be sold, advertised or supplied for use in people.

In the United States it is an investigational drug. Lumos Pharma's phase 3 trial started on 20 May 2026: about 150 previously untreated children with growth hormone deficiency who have not yet reached puberty are being randomised to ibutamoren or placebo capsules for 12 months, at sites in the US, the UK, Australia, New Zealand, India, Thailand, Chile and Argentina. Its estimated completion date is January 2028.

Meanwhile the FDA lists ibutamoren mesylate in Category 2 of its compounding lists, for bulk substances that raise significant safety risks: since 29 December 2022 for outsourcing facilities (503B) and since 29 September 2023 for pharmacies (503A). The reason it gives is the hip-fracture trial that was stopped early over a possible congestive heart failure signal. When the FDA last updated that page, in April 2026, ibutamoren was still on it.

In sport it is prohibited at all times. The WADA Prohibited List names ibutamoren (MK-677) in class S2.2.4, growth hormone releasing factors, as an example of a growth hormone secretagogue, alongside ipamorelin and ghrelin itself, and the Dutch anti-doping rules apply the same list.

What the research shows

Ibutamoren has a larger human record than most compounds sold next to peptides: more than 1,000 people took part in the trials below, most of them in the 1990s and 2000s. The pattern is consistent. It raises growth hormone and IGF-1, adds fat-free mass, and raises appetite, weight and blood glucose, but it has not turned those changes into proven gains in strength, function or disease outcomes.

• Older adults (Nass et al., 2008): 65 healthy people aged 60 to 81 received 25 mg or placebo daily for a year, followed by an exploratory second year. Fat-free mass rose 1.1 kg against a 0.5 kg fall on placebo, and body weight rose 2.7 kg against 0.8 kg, with more fat in the limbs. Visceral fat did not change, and strength and function did not improve. Fasting glucose rose 0.3 mmol/L (5 mg/dL) and insulin sensitivity fell.
• Obese men (Svensson et al., 1998): 24 men aged 18 to 50 received 25 mg or placebo for 8 weeks. IGF-1 rose about 40% and fat-free mass increased, but total and visceral fat did not change, and glucose tolerance worsened.
• Alzheimer's disease (Sevigny et al., 2008): 563 patients with mild to moderate disease received 25 mg or placebo for 12 months, and 416 completed. IGF-1 rose 60% at 6 weeks and 73% at 12 months, yet on none of four clinical scales did the disease progress more slowly than on placebo.
• Hip fracture (Adunsky et al., 2011): a phase IIb trial randomised 123 older patients to 25 mg or placebo for 24 weeks. Gait speed improved slightly, but stair-climbing power and most other function measures did not, and the trial was stopped early because of a congestive heart failure signal in a small number of patients.
• Bone (Murphy et al., 2001): 292 women aged 64 to 85 with low bone density received ibutamoren 25 mg, alendronate 10 mg, both, or placebo for 12 months, with bone density followed to 18 months. Added to alendronate, ibutamoren raised femoral-neck bone density by 4.2% against 2.5% with alendronate alone, but made no difference at the spine, the total hip or the whole body.
• Sleep (Copinschi et al., 1997): in 8 young adults, a week of 25 mg at bedtime lengthened deep (stage IV) sleep by about 50% and REM sleep by more than 20% compared with placebo; in 6 adults aged 65 to 71, REM sleep rose by nearly 50%. No larger trial has repeated these measurements.
• Children with growth hormone deficiency (OraGrowtH210, results posted June 2026): 104 children received a test dose, and the 82 who responded were randomised to 0.8, 1.6 or 3.2 mg per kg a day or to daily growth hormone injections. After six months they were growing at 6.8, 8.2 and 7.7 cm a year on ibutamoren, against 10.6 cm on injections.
• Fatty liver (NCT05364684): an open-label pilot gave 25 mg daily to 12 adults with fatty liver disease; 7 completed six months. Liver fat did not fall: it rose by 3.9 percentage points on average, not significantly different from historical placebo controls.

Side effects and warnings

In the trials the most frequent complaints were increased appetite, which in the year-long trial in older adults faded within a few months, mild and passing swelling of the lower legs, and muscle pain. Weight gain is part of the effect: 2.7 kg in a year against 0.8 kg on placebo in that trial.

Glucose control is the main metabolic concern. Fasting glucose rose and insulin sensitivity fell over a year in older adults, and glucose tolerance was impaired within two weeks in obese men. Side effects typical of growth hormone were also reported in the osteoporosis trial, though few women stopped because of them.

The most serious signal came from frail patients: the hip-fracture trial was stopped early when congestive heart failure appeared in a small number of patients, and this is the reason the FDA gives for listing ibutamoren as a significant safety risk. The published report does not give the number of cases.

No trial lasted longer than two years and none was designed to detect effects on cancer risk, which matters because IGF-1 stimulates cell growth.

Frequently asked questions

Is MK-677 legal in the Netherlands?

It is not a registered medicine in the Netherlands or anywhere else, so it may not be sold, advertised or supplied for use in people, whatever the label says. Its only EU status is a 2017 orphan designation for growth hormone deficiency, which is not an approval. For athletes it is prohibited under the anti-doping rules regardless of how it was obtained.

Is MK-677 banned in sport?

Yes, at all times, in and out of competition. The WADA Prohibited List names ibutamoren (MK-677) in class S2.2.4, growth hormone releasing factors, as a growth hormone secretagogue in the same group as ipamorelin and ghrelin itself, and the Dutch anti-doping rules apply the same list.

Is MK-677 a SARM or a peptide?

Neither. It is a non-peptide small molecule that acts on the ghrelin receptor and raises the body's own growth hormone. SARMs act on the androgen receptor, the target of testosterone. MK-677 is often sold next to both, which is where the confusion comes from, and unlike most peptides it is active by mouth.

Does MK-677 build muscle?

It raises fat-free mass, which is not the same as muscle strength. In the year-long trial in older adults fat-free mass rose 1.1 kg against a 0.5 kg loss on placebo, but strength and physical function did not change, and in obese men fat-free mass rose over 8 weeks while fat mass stayed the same. Fat-free mass includes water, and fluid retention is a known effect. No published trial has tested it in young adults who train.

Does MK-677 raise blood sugar?

In the trials, yes. Over a year in older adults fasting glucose rose by an average of 0.3 mmol/L (5 mg/dL) and insulin sensitivity fell, and in obese men an oral glucose tolerance test was already worse after two weeks. The likely cause is the extra growth hormone, which counteracts insulin.

Does MK-677 improve sleep?

One small study suggests it changes sleep structure. In 8 young adults a week of 25 mg at bedtime lengthened deep sleep by about 50% and REM sleep by more than 20% against placebo, and in 6 older adults REM sleep rose by nearly 50%. That is 14 people in a 1997 study, and no larger trial has repeated it.

Storage and handling

Store according to product-label instructions and batch handling notes. Lyophilized research peptides are commonly kept dry, protected from light, and handled with clean laboratory technique.

References

Checked against these sources on 28 October 2026.