This page is for general education. It compares two compounds that get shopped for the same reasons — more muscle, less fat, better sleep, slower aging — but that sit on opposite ends of the regulatory spectrum. On one side, HGH 191AA (Somatropin) is an approved biologic with a narrow list of labeled indications. On the other, MK-677 (Ibutamoren) is an abandoned oral drug candidate that survives only as a research chemical. Nothing here is medical advice, a protocol, or a recommendation.
The interesting part is what happens when you set the regulatory gap aside and read the trials. Both compounds do raise growth hormone and IGF-1. Both reliably add lean body mass. And in randomized trials of healthy adults, neither reliably adds strength or function. That convergence — a real body-composition effect that does not become a real performance effect — is the single most useful thing to know before comparing anything else.
First, a category error worth fixing
MK-677 is not a peptide. It is a non-peptide small molecule — a spiro-piperidine scaffold rather than a chain of amino acids — engineered at Merck precisely so it could be swallowed instead of injected. The paper introducing it describes an orally active, mechanistically nonpeptide secretagogue that raised GH in dogs at oral doses as low as 0.125 mg/kg (PNAS 1995). It is also not a SARM: it has no androgen receptor activity and no steroid structure. It gets filed next to SARMs because the same vendors sell both — the FDA warning letter that names “MK-677 Ibutamoren” lists it in the same product lineup as RAD-140, ostarine and LGD-4033 (FDA, 2023). Commerce, not chemistry, put it in that aisle. If the peptide-versus- small-molecule distinction is new, see what peptides actually are.
Somatropin, by contrast, genuinely is the hormone: recombinant human growth hormone with the same 191-amino-acid sequence as the pituitary product, which is where the “191AA” label comes from.
Head-to-head
| Dimension | MK-677 (ibutamoren) | HGH 191AA (somatropin) |
|---|---|---|
| Molecule type | Non-peptide small molecule (spiro-piperidine). Not a peptide, not a SARM, not a steroid. | Recombinant protein, 191 amino acids — sequence-identical to endogenous human GH. |
| Route and schedule | Oral, once daily. Oral bioavailability is the compound's entire reason for existing; trials used a single daily dose. | Subcutaneous injection. Approved dosing divides the weekly amount across daily injections, adult doses titrated against IGF-I. |
| Mechanism | Agonist at the growth hormone secretagogue receptor (GHS-R1a) — the ghrelin receptor. Amplifies the body's own pulsatile GH release. | Binds growth hormone receptors directly and drives hepatic IGF-1 production. Supplies the hormone rather than requesting it. |
| GH / IGF-1 effect | Dose-dependent: 25 mg daily raised mean 24-hour GH about 97% and lifted IGF-I from 141 to 265 µg/L in four weeks in elderly subjects (n=32). | Direct and dose-proportional; IGF-I is the routine monitoring marker on label, and adult doses are adjusted against it. |
| Body-composition outcome | +1.1 kg fat-free mass vs −0.5 kg on placebo at 12 months in healthy older adults (n=65). | +2.1 kg lean body mass and −2.1 kg fat mass in healthy elderly across 18 study populations; +2.1 kg lean mass in young athletic samples. |
| Strength / function outcome | No change. The trial states plainly that increased fat-free mass did not produce changes in strength or function. | No change. Strength and exercise capacity did not improve; exercise lactate was higher in 2 of 3 studies measuring it. |
| Regulatory status | No approval anywhere. Development stopped. Products sold for human use are unapproved new drugs in the FDA's framing, and it is not a lawful supplement ingredient. | FDA-approved biologic with defined indications — pediatric growth failure from GHD, Prader-Willi, small for gestational age, Turner syndrome, idiopathic short stature, and adult GHD. |
| Cost and access reality | Cheap and frictionless: capsules, tablets or flavored liquids ordered online with no prescription, no pharmacy, and no guarantee of identity, purity or dose. | Expensive and gated: a prescription biologic dispensed through pharmacies in pens, cartridges or vials, with product-specific labeling and pharmacovigilance behind it. |
| Anti-doping | Prohibited at all times. Named on the WADA list under S2.2.4 as a growth hormone secretagogue. | Prohibited at all times. Growth hormone and its analogues sit under S2.2.3. |
| Key risks | Rising fasting glucose and falling insulin sensitivity; marked appetite stimulation; fluid retention; a congestive-heart-failure safety signal that ended a hip-fracture trial early. | Soft-tissue edema, arthralgia, carpal tunnel syndrome and gynecomastia; more incident diabetes and impaired fasting glucose; label contraindications including active malignancy and acute critical illness. |
Two ways to raise one axis
The mechanistic difference is easy to state and easy to overrate. Somatropin is the hormone, so it works regardless of what the pituitary is willing to do; it also suppresses the endogenous feedback loop the way any exogenous hormone does. MK-677 works upstream, at the ghrelin receptor, asking the pituitary for more of its own GH — the same target as injectable secretagogues such as ipamorelin, and adjacent to the GHRH analogues such as sermorelin. Because secretagogues amplify a pulse rather than replace it, they are often described as more physiological.
That framing is fair as biology and weak as a promise. MK-677 is long-acting enough that one daily oral dose keeps GH and IGF-1 elevated around the clock, which is not what a pulse looks like; and in the two-year trial, IGF-1 stayed at young-adult levels throughout continuous dosing (Ann Intern Med 2008). More to the point, the metabolic downside showed up at the same doses that produced the hormonal upside — in both compounds.
What the trials found: mass without strength
This is where the two literatures agree, and it is the uncomfortable finding neither marketing tradition mentions.
- MK-677. A two-year randomized trial in 65 healthy adults aged 60–81 restored GH and IGF-1 toward young-adult levels. Fat-free mass rose 1.1 kg in the treated group while falling 0.5 kg on placebo (P < 0.001). The authors then note that the increased fat-free mass did not result in changes in strength or function (Nass et al., Ann Intern Med 2008).
- HGH in healthy older adults. A systematic review of 31 articles covering 18 unique study populations (220 GH-treated participants) found lean body mass up 2.1 kg and fat mass down 2.1 kg — with no change in body weight or bone density, and significantly more soft-tissue edema, arthralgia, carpal tunnel syndrome and gynecomastia. The conclusion is a single sentence: GH cannot be recommended as an antiaging therapy (Liu et al., Ann Intern Med 2007).
- HGH in young athletes. A companion review of 27 study samples (303 GH-treated participants, mean age 27) found the same 2.1 kg lean-mass increase — and strength and exercise capacity that did not improve. Lactate during exercise was significantly higher in 2 of 3 studies that measured it, and edema and fatigue were more frequent. The authors conclude that claims growth hormone enhances physical performance are not supported by the literature (Liu et al., Ann Intern Med 2008).
Two different molecules, two different mechanisms, three independent evidence bases, one result: the DEXA scan moves, the person does not get measurably stronger. Some of that lean-mass gain is very likely water — edema is among the most consistently reported effects of both.
The exception is worth naming, because it is the one context where GH genuinely earns its keep. In adults with diagnosed growth hormone deficiency, a meta-analysis of 22 placebo-controlled randomized trials (591 GH-treated, 562 placebo) found lean body mass up 2.61 kg versus 0.04 kg and fat mass down 2.19 kg versus a 0.31 kg gain, with improvements in total and LDL cholesterol (Pituitary 2015). Replacing a hormone someone is missing is a different proposition from adding hormone to someone who is not.
The shared metabolic flag
Both compounds push glucose in the wrong direction, which follows directly from GH being a counter-regulatory hormone to insulin.
- In the two-year MK-677 trial, fasting blood glucose rose an average of 0.3 mmol/L (about 5 mg/dL, P = 0.015) and insulin sensitivity declined (Ann Intern Med 2008). Rising fasting glucose also appears in the earliest dose-ranging work (JCEM 1996).
- In the healthy-elderly GH review, treated participants had higher rates of new-onset diabetes and impaired fasting glucose (Ann Intern Med 2007).
No published dose of either compound separates the hormonal effect from the glycemic one. That matters most to anyone with prediabetes, existing diabetes, or a family history — a group covered in who tends to be advised away from these compounds.
Where they stop being comparable
Everything above treats the two as peers. The regulatory picture does not.
Somatropin carries a real label. Genotropin, a representative product, is indicated for pediatric growth failure due to GH deficiency, Prader-Willi syndrome, small for gestational age, Turner syndrome and idiopathic short stature, plus adult-onset or childhood-onset GHD in adults — and it is contraindicated in acute critical illness, active malignancy, active proliferative or severe non-proliferative diabetic retinopathy, closed epiphyses, and severely obese Prader-Willi patients with airway compromise. Two placebo-controlled trials in non-GH-deficient ICU patients (n=522) found significantly increased mortality with somatropin, 42% versus 19% (Genotropin label, DailyMed). Anti-aging and athletic use appear nowhere on that list. That is the shape of a regulated medicine: narrow indications, named contraindications, quantified harms.
MK-677 has none of that, and not for lack of testing. Merck and academic collaborators ran an unusually broad program. A 563-patient, 12-month Alzheimer's trial raised serum IGF-1 by 60.1% at six weeks and 72.9% at twelve months and found no significant difference between treatment groups on cognitive or functional measures — target engaged, disease unchanged (Neurology 2008). A phase IIb hip-fracture trial in 123 elderly patients was terminated early over a congestive-heart-failure safety signal, with the authors concluding the compound has an unfavorable safety profile in that population (Arch Gerontol Geriatr 2011). The compound reached late-stage trials and stopped there; what is sold today is the residue of that abandoned program, not a product of it.
The practical consequence is asymmetric uncertainty. With somatropin, the questions are about whether an off-label use is appropriate. With MK-677, the questions start earlier — whether the capsule contains what the label says. A 2025 forensic survey of seized performance products found ibutamoren the single most frequently detected molecule among SARMs, metabolic modulators and secretagogues, with 65% of seized products represented as medicines and some samples overdosed relative to their stated content (Drug Test Anal 2025). This is part of why many clinicians decline to engage with gray-market compounds at all, and it belongs in any honest reading of the risks attached to this category.
Anti-doping: both are prohibited
There is no version of this comparison where one option is safer for a tested athlete. The WADA Prohibited List places both in section S2, prohibited at all times, in and out of competition:
- S2.2.3 covers growth hormone, its analogues and fragments — somatropin included.
- S2.2.4 covers growth hormone secretagogues and their mimetics, naming ibutamoren (MK-677) explicitly alongside ipamorelin, anamorelin and ghrelin, and covering GHRH analogues such as CJC-1295 and sermorelin in the same subsection (WADA Prohibited List).
Because MK-677 is sold openly in supplement-adjacent channels, it is a recurring source of violations from products the buyer did not think of as a drug.
How to dive deeper
- MK-677 (Ibutamoren) overview · benefits · side effects · dosing in trials · research
- HGH 191AA (Somatropin) overview · benefits · side effects · dosing concepts · research
- Injectable secretagogues on the same receptor: ipamorelin · sermorelin acetate
- Background questions: what are peptides · risks of peptides · who should avoid them · why some doctors decline to prescribe
- More side-by-sides: compare hub