Ipamorelin vs MK-677: what each one is and what studies show

What each compound is

Ipamorelin is a chain of five amino-acid units. Researchers at Novo Nordisk described it in 1998 as Aib-His-D-2-Nal-D-Phe-Lys-NH2. The NH2 means the chain ends in an amide group.

Three of its five units are unusual. Aib and D-2-Nal are amino acids that the body does not use to build proteins. D-Phe is the mirror-image form of phenylalanine. FDA says ipamorelin contains unnatural amino acids, which make it harder to characterize.

Merck researchers described MK-677 in 1995 under the code L-163,191. Its other names include MK-0677 and ibutamoren. A 2005 receptor study classed MK-677 as a nonpeptide compound.

Both are called growth hormone secretagogues. A secretagogue is a substance that makes cells release another substance, here growth hormone. Both act at the ghrelin receptor, a target on cells that the hormone ghrelin also uses.

A shared target does not make the two the same kind of molecule. Ipamorelin is built from amino acids joined in a chain. The 2005 receptor study classed MK-677 as nonpeptide.

How each was studied in people

The 1999 ipamorelin study gave it as a 15-minute infusion. The 2014 ipamorelin trial gave it into a vein. In the 2008 MK-677 trial, people took it by mouth. The 2011 trial abstract does not state the route.

The 1995 Merck paper called MK-677 orally active, meaning it works when swallowed. That paper tested rat cells and dogs. It does not show how well MK-677 works in people.

The study route matters for what a result covers. A finding after an infusion into a vein does not describe a product taken another way. FDA says it lacks safety information for ipamorelin given by certain other injection routes.

What the human studies of ipamorelin show

A 1999 study gave ipamorelin as a 15-minute infusion to healthy male volunteers. It tested five dose levels, with eight men at each level. Each dose caused one short episode of growth hormone release.

A phase 2 trial, published in 2014, tested ipamorelin after bowel surgery. A phase 2 trial is an early test of whether a drug helps. Of 117 adults enrolled, 114 were analyzed.

Some patients got ipamorelin and some got a placebo, a look-alike with no drug. Neither patients nor researchers knew who got which.

The main measure was time until a patient could eat a standard solid meal. The median, or middle value, was 25.3 hours with ipamorelin and 32.6 hours with placebo. Statistically, that gap was too small to rule out chance.

The authors report no clear benefit on the main measure or on other measures of whether the drug worked. They describe ipamorelin as well tolerated in that setting.

Adverse events, meaning any health problems during the trial, were common in both groups: 87.5% with ipamorelin and 94.8% with placebo.

FDA cites a published study that reported serious adverse events, including death, when ipamorelin was given into a vein to help gut movement. FDA does not name that study on its page.

How strong is the evidence? Emerging. Some studies in people exist. They show a short hormone response in healthy men and no proven benefit after bowel surgery.

What the human studies of MK-677 show

A 2008 trial gave MK-677 or placebo by mouth to 65 healthy adults aged 60 to 81. It ran two years, with main results at one year. Growth hormone and a related blood marker, IGF-1, rose to levels seen in healthy young adults.

Fat-free mass, meaning everything in the body except fat, rose 1.1 kg with MK-677. It fell 0.5 kg with placebo. Fat deep in the belly and total body fat did not differ from placebo.

Strength and physical function did not change. The authors say the trial was too small and too short to judge function.

The same trial reported other changes. Body weight rose 2.7 kg with MK-677 and 0.8 kg with placebo. Fasting blood sugar rose by about 5 mg/dL, and the body responded less well to insulin. Cortisol, a stress hormone, also rose.

The most frequent side effects were increased appetite that eased within a few months, and short-lived, mild leg swelling and muscle pain. The authors report no serious adverse effects in that trial.

A 2011 trial tested MK-677 against placebo in 123 older adults recovering from a hip fracture, with results at 24 weeks. IGF-1 rose, and walking speed improved. Most other measures of physical function did not improve.

That trial stopped early because of a safety signal, an early warning of possible harm. It was congestive heart failure, meaning the heart could not pump well enough, in a small number of patients. The authors call MK-677's safety profile unfavorable in that patient group.

How strong is the evidence? Two trials in older adults show hormone and body changes. Neither showed better strength. One showed faster walking but no gain on most other function tests, and it stopped for a heart-failure signal.

Why the two cannot be ranked from these studies

A PubMed search on November 3, 2026 found no study that gave both compounds to people and compared them. Papers that name both were reviews, chemistry work or animal studies.

The ipamorelin studies tested healthy men and surgery patients for up to a week. The MK-677 trials tested older adults for up to two years. Different people, routes, lengths and measures do not make a fair comparison.

In pigs, the 1998 ipamorelin paper found that ipamorelin did not raise ACTH or cortisol more than GHRH did, even at very high doses. GHRH is the body's own growth hormone releasing hormone. ACTH signals cortisol release.

The 1995 Merck paper reported only modest cortisol increases with MK-677 in dogs. The 2008 MK-677 trial reported a cortisol rise in older adults. An animal result and a human result cannot settle which compound is more selective in people.

Claims that one compound works better for muscle, fat loss or sleep are not tested in the records reviewed here, for either compound.

FDA, sports and military records for each

A November 3, 2026 search of Drugs@FDA, FDA's database of approved drugs, found no product for either name. FDA's 2025 notice says ibutamoren is not approved by FDA and its safety and effectiveness have not been established.

Compounding means a pharmacy or outsourcing facility mixes a drug instead of a drug company making it. FDA's Category 2 list holds substances that FDA says may present significant safety risks in compounding.

Ibutamoren mesylate is in Category 2 under both FDA policies. The 503A policy covers pharmacies that fill one patient's prescription. The 503B policy covers outsourcing facilities. Ipamorelin acetate is in Category 2 under the 503B policy only. FDA lists a separate ipamorelin nomination as withdrawn.

For ibutamoren mesylate, FDA cites potential congestive heart failure in certain patients and the stopped hip-fracture trial. For ipamorelin acetate, FDA cites possible immune reactions, clumping, impurities and the serious events in vein-infusion research.

An October 28, 2026 policy from the U.S. Department of Health and Human Services (HHS) covers pharmacy compounding of six peptides. HHS does not plan to act against licensed pharmacies that compound them and meet its conditions. Ipamorelin and MK-677 are not among the six.

The World Anti-Doping Agency (WADA) sets anti-doping rules under the World Anti-Doping Code. Its 2026 Prohibited List names ibutamoren (MK-677) and ipamorelin as growth hormone secretagogues, in section S2.2.4. That class is banned at all times, in and out of competition.

The Department of Defense (DoD) keeps a Prohibited Dietary Supplement Ingredients List. Service members may not use supplements with a listed ingredient. A November 3, 2026 search returned a row for each compound.

Both rows give the reason "WADA S2: Peptide Hormones, Growth Factors, etc."

Where this comparison stops

A name on a label does not show what a product contains. FDA found undeclared ibutamoren in one named product in 2025. That finding covers the tested product only.

The study records here are abstracts, plus FDA, WADA, HHS and DoD pages. Full methods, all side-effect tables and funding details were not reviewed.

No doses, schedules or product choices are given here. For a question about your own health, ask a licensed clinician who knows your history.