Searches such as "ibutamoren vs ipamorelin" or "MK-677 vs HGH" usually ask which works better. No trial has compared them in people. What the published record does allow is a comparison of what each molecule is, how it reaches the growth hormone (GH) axis, what human studies found, adverse effects included, and its legal and sporting status. MK-677 and ipamorelin are sold here for laboratory research only, to adults. Somatropin, the medicine usually meant by "HGH", is prescription-only in the UK.
The three at a glance
| Feature | MK-677 (ibutamoren) | Ipamorelin | Somatropin (HGH) |
|---|---|---|---|
| What it is | Non-peptide small molecule | Synthetic five-amino-acid peptide | Recombinant human growth hormone, a 191-amino-acid protein |
| Molecular weight | 528.7 g/mol (free base) | 711.9 g/mol | About 22,000 g/mol (22 kDa) |
| Target | Ghrelin receptor (GHS-R1a) | Ghrelin receptor (GHS-R1a) | Growth hormone receptor |
| Where the GH comes from | The pituitary's own supply | The pituitary's own supply | Supplied directly, pituitary bypassed |
| Route in published human studies | By mouth | Mainly intravenous infusion | Injection (licensed products) |
| Human evidence | Several randomised trials, the longest two years | Mainly one pharmacokinetic study and one phase 2 trial | Licensed medicine with decades of clinical use |
| UK status | Not licensed, not controlled | Not licensed, not controlled | Prescription-only medicine, Class C controlled drug |
| WADA | S2, prohibited at all times | S2, prohibited at all times | S2, prohibited at all times |
What each one is
MK-677 (ibutamoren)
MK-677 was designed at Merck Research Laboratories in the 1990s under the codes L-163,191 and MK-0677, and its non-proprietary name is ibutamoren. It is a non-peptide molecule built to reproduce the action of the GH-releasing peptides while remaining active by mouth (formula C27H36N4O5S, CAS 159634-47-6), often supplied as the mesylate salt. It reached clinical trials but has never been licensed as a medicine anywhere.
Ipamorelin
Ipamorelin is a synthetic pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2 (C38H49N9O5, CAS 170851-70-4), developed at Novo Nordisk in Denmark and first described in 1998 as a selective GH secretagogue. Like MK-677 it acts at the ghrelin receptor. Why it is called selective, and the limits of that claim, are set out in our ipamorelin research guide.
Somatropin
Somatropin is the non-proprietary name for recombinant human growth hormone: a 191-amino-acid protein, identical in sequence to the main form of GH made by the pituitary, produced by recombinant DNA technology. It is the active substance of licensed GH medicines in the UK, given by injection and prescribed by specialists for diagnosed conditions such as GH deficiency in children and adults.
Is MK-677 a peptide?
No. It is a non-peptide molecule that mimics GH-releasing peptides at their receptor, which is why it is often listed with them. Ipamorelin is a peptide and somatropin a protein, and swallowed peptides and proteins are largely broken down by digestive enzymes, one reason somatropin is injected; our article on whether oral peptides work explains the barrier.
How each one acts on the growth hormone axis
Growth hormone is released from the pituitary in pulses, the largest of them during sleep. Two signals from the hypothalamus set the rhythm: growth hormone releasing hormone (GHRH) switches release on and somatostatin switches it off. Ghrelin, made mainly in the stomach, amplifies release through its own receptor, GHS-R1a. GH drives production of insulin-like growth factor 1 (IGF-1), largely in the liver, and both feed back to damp further release.
MK-677 and ipamorelin: signalling the pituitary
Both secretagogues bind GHS-R1a and add no hormone: they prompt the pituitary to release what it already makes, and the feedback loop stays in place. In a two-year trial in older adults, MK-677 enhanced the existing pulsatile pattern of GH secretion rather than replacing it with a flat level (Nass 2008). Ipamorelin acts briefly. Given by intravenous infusion to healthy men, it produced a single episode of GH release that peaked at about 40 minutes and fell back to negligible levels, and its terminal half-life was about two hours (Gobburu 1999).
Working through the pituitary means the response depends on how much pituitary function there is. In a study of nine young men with severe GH deficiency that began in childhood, MK-677 raised 24-hour mean GH and IGF-1 in every participant, but the GH response was largest in those who were least deficient at the start. Fasting and post-meal insulin and post-meal glucose also rose (Chapman 1997). A secretagogue cannot release hormone that the pituitary is unable to make.
Somatropin: replacing the hormone
Somatropin skips every step above and acts directly at the GH receptor in the liver and other tissues. The amount circulating is set by the injection rather than by the pituitary, and the resulting rise in GH and IGF-1 suppresses the body's own secretion. It can therefore replace what a failing pituitary cannot make, and in people with normal pituitary function it overrides the body's own feedback control of GH.
What the human research reports
None of the three literatures compares the compounds directly. At the time of writing, a PubMed search for ibutamoren or MK-677 together with ipamorelin returns reviews, chemistry and animal papers, and no randomised trial in people.
MK-677: body composition changed, glucose control worsened
The longest trial randomised 65 healthy adults aged 60 to 81 to MK-677 or placebo for up to two years (Nass 2008). GH and IGF-1 rose to young-adult levels. Over the first year, fat-free mass rose by 1.1 kg with MK-677 and fell by 0.5 kg with placebo, and body weight rose by 2.7 kg against 0.8 kg. Visceral and total fat mass did not differ between groups, and the gain in fat-free mass brought no change in strength or function. Fasting glucose rose by an average of 0.3 mmol/L, insulin sensitivity fell and cortisol rose. The most frequent side effects were increased appetite, which eased over a few months, transient mild lower-limb oedema and muscle pain. The authors noted that the study's size and duration gave it too little power to assess functional outcomes.
A phase IIb trial randomised 123 older patients recovering from hip fracture to MK-677 or placebo (Adunsky 2011). IGF-1 rose and one measure, gait speed, improved, but most other measures of physical function did not. The trial was stopped early after congestive heart failure occurred in a limited number of patients, and the authors judged the adverse-effect profile of MK-677 unfavourable in that population.
Ipamorelin: very little human data
Ipamorelin's human record consists mainly of the infusion study above, which measured only drug and GH levels, and one phase 2 trial. In that trial, 117 adults having bowel resection surgery were randomised to intravenous ipamorelin or placebo to test whether it shortened postoperative ileus, the temporary stalling of gut movement after surgery (Beck 2014). The time to the first tolerated solid meal did not differ significantly between groups, and adverse events were frequent in both arms. Neither study measured muscle, strength or body fat.
Somatropin in people without a deficiency
Two systematic reviews pooled randomised trials of GH in people with no diagnosed deficiency. In healthy older adults (220 people given GH across 18 study populations), fat mass fell by about 2.1 kg and lean body mass rose by about 2.1 kg, with no significant change in weight. Those treated were significantly more likely to have soft tissue oedema, joint pain, carpal tunnel syndrome and gynaecomastia, and somewhat more likely to develop diabetes or impaired fasting glucose. The reviewers concluded that GH could not be recommended as an anti-ageing treatment (Liu 2007).
In young, fit adults (303 people given GH across 27 study samples), lean body mass again rose by about 2.1 kg, but strength and exercise capacity did not appear to improve. Blood lactate during exercise was higher in two of the three studies that measured it, and soft tissue oedema and fatigue were more frequent. The reviewers concluded that claims of improved athletic performance were not supported (Liu 2008). Measured lean mass includes body water, and fluid retention appears in both reviews.
MK-677 vs HGH for muscle: the common thread
We make no outcome claims. In these trials, both changed measured body composition without matching changes in strength or function, with fluid retention and effects on glucose. Ipamorelin has no equivalent data, and none of this is a claim about any product sold here.
Regulatory status in the UK, EU and US
Is HGH legal in the UK?
Somatropin is a prescription-only medicine under the Human Medicines Regulations 2012, and it is also listed as a Class C drug under the Misuse of Drugs Act 1971. Licensed products, including pens, are prescribed by specialists, and supplying somatropin outside that route is an offence.
Are MK-677 and ipamorelin legal in the UK?
Neither is a controlled drug, and neither is a licensed medicine: the MHRA has never granted a marketing authorisation for either. Neither is an authorised food supplement or novel food, or a SARM, since neither acts at the androgen receptor. Both can be sold to adults for laboratory research if not presented for human consumption, which is how Free Muscle supplies them. Our page on MK-677 legal status in the UK sets out the detail.
Can MK-677 or ipamorelin be used instead of prescribed growth hormone?
No. Neither has been approved for GH deficiency or any other condition, neither is a substitute for somatropin or any prescribed medicine, and neither is sold here for any therapeutic purpose. Growth hormone treatment is a decision for a doctor.
European Union and United States
In the EU, somatropin medicines are authorised and available only on prescription, while MK-677 and ipamorelin have no authorisation as medicines or novel foods. In the US, somatropin is an FDA-approved prescription drug and its distribution as an anti-ageing agent is illegal (Liu 2007). Neither MK-677 nor ipamorelin has FDA approval for any use.
WADA and anti-doping tests
All three are prohibited at all times, in and out of competition, under class S2 of the WADA Prohibited List: growth hormone itself, and MK-677 and ipamorelin as growth hormone secretagogues. Accredited laboratories test for growth hormone in blood and for secretagogues and their metabolites in urine. No source cited here gives a detection window: detectable, window not established.
Common questions
Ibutamoren vs ipamorelin: which is stronger?
No head-to-head trial exists, and we do not rank research compounds by effect. Their studies differ in route, duration and population, so the results cannot be set side by side.
Is MK-677 available as tablets, capsules or liquid?
As a small molecule, MK-677 escapes the digestive breakdown that limits oral peptides, and the published trials gave it by mouth. A capsule holds a fixed fill of dry material, while a liquid depends on the solvent and on the compound staying evenly dissolved, which makes stability harder to document. The MK-677 collection lists the capsules stocked, by brand.
MK-677, ipamorelin and HGH dosage, cycles and before-and-after results
We do not publish dosing, frequency, cycle or combination guidance for any research compound, and we make no claims about results or before-and-after outcomes. The amounts used in the trials above were set by investigators under medical supervision and have no application outside those studies.
Somatropin pens, injections and prices in the UK
Licensed somatropin pens and injections are supplied only on prescription, through NHS or private specialist care, and this guide does not quote prices. A pen offered without a prescription is not a lawful supply.
How to verify a batch of MK-677 or ipamorelin
No regulator checks a research compound before sale, so a batch certificate of analysis (COA) from an independent laboratory is the main evidence of what is in the container. A useful one shows:
- A batch number that matches the one printed on the label.
- An identity result by mass spectrometry that matches the expected molecule: 528.7 g/mol for MK-677 as the free base, heavier as the mesylate salt, and 711.9 g/mol for ipamorelin.
- A purity percentage by HPLC, which measures how much of the sample is one compound, not which compound it is.
- The measured quantity per capsule, where the laboratory tested it.
- The laboratory's name and the test date.
Every Free Muscle-brand batch is assayed by Janoshik Analytical, an independent laboratory, and the certificate for a batch is sent by email on request; quote the batch number from the label. For other brands, a certificate is shown only where the manufacturer supplies one for the batch held. See the lab tests page and our guide to reading a certificate of analysis. UK delivery is free, tracked and plainly packaged, and unopened items can be returned within 30 days.
Related reading
- MK-677 (ibutamoren): every brand stocked
- Ipamorelin collection
- MK-777 (acetamoren) collection
- Research peptides collection
- MK-677 (ibutamoren): what the research says
- Ipamorelin: the selective GH secretagogue, its trials and UK status
- Do oral peptides actually work? The honest answer
- How to read a certificate of analysis
MK-677 and ipamorelin are sold by Free Muscle for laboratory research use only. They are not licensed medicines, have not been evaluated by the MHRA, EMA or FDA, are not food supplements and are not for human consumption. Customers must be 18 or over. Somatropin is a prescription-only medicine and Class C controlled drug in the UK, and no research compound replaces it or any prescribed treatment. Nothing here is medical, dosing or usage advice, and the studies described support no claim about any product.
References
- Chapman IM, Pescovitz OH, Murphy G, et al. Oral administration of growth hormone (GH) releasing peptide-mimetic MK-677 stimulates the GH/insulin-like growth factor-I axis in selected GH-deficient adults. J Clin Endocrinol Metab. 1997;82(10):3455-3463. PMID: 9329386.
- Nass R, Pezzoli SS, Oliveri MC, 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):601-611. PMID: 18981485.
- Adunsky A, Chandler J, Heyden N, et al. MK-0677 (ibutamoren mesylate) for the treatment of patients recovering from hip fracture: a multicenter, randomized, placebo-controlled phase IIb study. Arch Gerontol Geriatr. 2011;53(2):183-189. PMID: 21067829.
- Gobburu JV, Agersø H, Jusko WJ, Ynddal L. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharm Res. 1999;16(9):1412-1416. PMID: 10496658.
- Liu H, Bravata DM, Olkin I, et al. Systematic review: the safety and efficacy of growth hormone in the healthy elderly. Ann Intern Med. 2007;146(2):104-115. PMID: 17227934.
- Liu H, Bravata DM, Olkin I, et al. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med. 2008;148(10):747-758. PMID: 18347346.
- Beck DE, Sweeney WB, McCarter MD; Ipamorelin 201 Study Group. Prospective, randomized, controlled, proof-of-concept study of the Ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients. Int J Colorectal Dis. 2014;29(12):1527-1534. PMID: 25331030.