MK-677 (Ibutamoren Mesylate) Evidence Grade: B+
MK-677 (Ibutamoren) is a non-peptide, orally active growth hormone secretagogue that acts as a potent agonist of the ghrelin receptor (GHSR1a). Unlike injectable GH secretagogue peptides such as GHRP-6, GHRP-2, or ipamorelin, MK-677 is a small-molecule compound that can be taken orally, providing sustained elevation of growth hormone (GH) and insulin-like growth factor 1 (IGF-1) levels for up to 24 hours from a single daily dose.
Originally developed by Merck as a potential treatment for growth hormone deficiency, sarcopenia, and osteoporosis in elderly populations, MK-677 has been studied in multiple clinical trials involving hundreds of participants over periods of up to 2 years. While it was never approved as a pharmaceutical product, its clinical data provides a relatively robust evidence base for a research compound, and it remains one of the most widely studied oral GH secretagogues.
MK-677 elevates GH through the same physiological pathway as endogenous ghrelin — pulsatile pituitary release — rather than providing exogenous GH. This means it preserves the natural GH secretion pattern with amplified pulse amplitude. Average IGF-1 elevation of 40-89% has been documented across clinical trials, with sustained effects during chronic administration.
Table of Contents
Overview
MK-677, also known as ibutamoren or ibutamoren mesylate (formerly designated L-163,191), is a spiropiperidine-derived non-peptide compound developed by Merck Research Laboratories in the 1990s. It mimics the action of the endogenous hunger hormone ghrelin by binding to and activating the growth hormone secretagogue receptor type 1a (GHSR1a), also known as the ghrelin receptor, in the hypothalamus and pituitary gland [1].
The compound's primary pharmacological effect is to stimulate pulsatile growth hormone release from the anterior pituitary gland. A single 25 mg oral dose typically increases peak GH levels by 2-5 fold, and chronic administration raises IGF-1 levels by 40-89% depending on the population studied and duration of treatment [2][3]. Critically, MK-677 does not suppress the hypothalamic-pituitary axis: it amplifies the natural GH pulsatility pattern rather than replacing it, and negative feedback mechanisms remain intact.
MK-677 was investigated by Merck in clinical trials for growth hormone deficiency in the elderly, sarcopenia (age-related muscle loss), osteoporosis (bone mineral density preservation), and obesity-related metabolic dysfunction. Multiple Phase II trials were completed, but the compound was never advanced to Phase III or submitted for regulatory approval, likely due to concerns about insulin resistance and insufficient efficacy endpoints for specific indications.
Despite never receiving pharmaceutical approval, MK-677 has one of the most robust clinical evidence bases of any research compound. It is frequently classified as a "SARM" in the research community, though this is technically incorrect — it has no interaction with androgen receptors. Its oral bioavailability, once-daily dosing, and well-characterized safety profile from clinical trials have made it one of the most widely studied GH secretagogues in research settings.
Mechanism of Action
MK-677 binds to and activates the growth hormone secretagogue receptor type 1a (GHSR1a) in the hypothalamus and pituitary gland. This receptor is the same target as endogenous ghrelin. Activation triggers Gq/11 protein-coupled signaling, leading to phospholipase C activation, IP3-mediated calcium release from intracellular stores, and subsequent GH release from pituitary somatotrophs. The signal mimics the natural ghrelin pulse, amplifying GH secretion without disrupting the pulsatile pattern [1][4].
Unlike exogenous GH (which provides a flat, non-physiological GH profile), MK-677 amplifies endogenous GH pulse amplitude by 2-5 fold while maintaining the normal pulsatile rhythm. This means GH peaks are higher but the interpulse troughs are preserved, and the overall GH profile more closely resembles youthful physiology. This pulsatile pattern is believed to be more physiologically relevant for tissue-specific effects than continuous GH exposure [2].
The sustained GH elevation from daily MK-677 dosing drives hepatic production of insulin-like growth factor 1 (IGF-1). Clinical trials document IGF-1 increases of 40-89% above baseline with chronic administration. IGF-1 mediates many of GH's anabolic effects including protein synthesis, bone mineralization, and cellular proliferation. IGF-1 elevation is sustained for the duration of MK-677 use without evidence of desensitization [3].
As a ghrelin receptor agonist, MK-677 activates appetite-stimulating pathways in the hypothalamus (NPY/AgRP neurons). This results in increased hunger and caloric intake, particularly during the first weeks of use. Additionally, ghrelin receptor activation influences sleep architecture (promoting deeper sleep stages), cortisol regulation, and gastric motility [5].
GH elevation has well-characterized antagonistic effects on insulin signaling. MK-677-induced GH elevation can cause dose-dependent insulin resistance, evidenced by increased fasting glucose and insulin levels in clinical trials. This is a direct consequence of GH's anti-insulin metabolic effects (lipolysis promotion, hepatic glucose output, reduced peripheral glucose uptake) and represents the primary safety concern with chronic use [3][6].
Research Timeline
Discovery and initial characterization. Merck Research Laboratories identifies MK-677 (L-163,191) as a potent, orally active GH secretagogue. First publications demonstrate oral bioavailability and GH-elevating properties in animal models.
First human studies. Chapman et al. (1996) publish the first clinical data showing MK-677 increases GH and IGF-1 in healthy young and older adults. Single-dose and short-term studies establish dosing and PK parameters [2].
Obesity study. Svensson et al. demonstrate that MK-677 increases GH secretion, IGF-1, and fat-free mass in obese males over 8 weeks, while also documenting increased appetite and insulin resistance [6].
Bone density and body composition studies. Murphy et al. and Nass et al. investigate MK-677 for hip fracture recovery and age-related sarcopenia. Positive effects on lean body mass and bone turnover markers documented [7].
2-year chronic study published. Nass et al. publish results of a 2-year randomized controlled trial in healthy elderly subjects. Sustained IGF-1 elevation (+89%) confirmed without tachyphylaxis. Body composition and functional strength outcomes mixed [3].
Sleep and metabolic research. Studies document MK-677's effects on sleep architecture (improved REM and Stage IV sleep), cortisol regulation, and metabolic parameters. Research interest shifts from pharmaceutical development to research applications.
Research community adoption. MK-677 becomes widely used in research settings as an oral GH secretagogue. Classified as a research compound without pharmaceutical development pathway. WADA includes GH secretagogues in prohibited list.
Clinical Evidence Grade: B+
N=65 healthy elderly subjects, randomized, double-blind, placebo-controlled. 2 years of daily MK-677 25 mg. Results: IGF-1 increased by 89% at 1 year, sustained at 2 years. GH levels restored to young adult values. Fat-free mass increased by 1.1 kg. No significant improvement in functional strength endpoints. Fasting glucose increased by ~0.3 mmol/L. No tachyphylaxis observed for GH/IGF-1 effects.
N=32. Short-term crossover study. MK-677 25 mg increased 24-hour integrated GH concentrations by 97% and IGF-1 by 55% in elderly subjects. GH pulsatility pattern was maintained. Demonstrated that oral MK-677 can restore GH/IGF-1 axis in elderly to levels approximating young adults.
N=24 obese males, 8-week treatment. MK-677 25 mg vs placebo. Results: 24-h GH increased by ~40%, IGF-1 by ~40%. Fat-free mass increased by 3.0 kg (lean mass increase). Body weight increased due to fluid retention and appetite stimulation. Fasting glucose increased, insulin sensitivity decreased.
N=161 elderly patients post hip fracture. MK-677 25 mg daily for 6 months. Improved IGF-1 levels and trends toward improved functional outcomes, though primary endpoints did not reach statistical significance. Demonstrated safety in a frail elderly population.
Demonstrated that MK-677 improves sleep quality by increasing duration of REM sleep and Stage IV (deep) sleep. GH secretion during sleep was amplified. Suggests a mechanism for the subjective sleep quality improvement reported by research subjects.
Evidence Summary
| Endpoint | Evidence Volume | Quality | Outcome |
|---|---|---|---|
| GH/IGF-1 Elevation | 6+ clinical trials | High | Consistent, 40-89% IGF-1 increase |
| Body Composition | 4+ trials | Moderate-High | Modest lean mass increase (+1-3 kg) |
| Bone Density | 2 trials | Moderate | Improved turnover markers, limited BMD data |
| Sleep Quality | 2 trials | Moderate | Improved REM/Stage IV sleep |
| Functional Strength | 2 trials | Moderate | Mixed / Not significant |
Dosing & Administration
Research Dosing
| Parameter | Conservative | Standard | High |
|---|---|---|---|
| Dose | 10 mg/day | 25 mg/day | 50 mg/day |
| Frequency | Once daily | Once daily | Once daily |
| Cycle Length | 8-12 weeks | 12-24 weeks | 8-16 weeks |
| Timing | Evening/bedtime | Evening/bedtime | Evening/bedtime |
| IGF-1 Increase | ~20-40% | ~40-60% | ~60-89% |
Note: 25 mg was the standard dose across Merck clinical trials. The 10 mg dose is based on dose-response studies showing meaningful GH elevation at lower doses with reduced side effects. The 50 mg dose was studied but showed diminishing returns with increased side effects.
Administration Notes
- Route: Oral — typically as capsule or liquid solution
- Timing: Once daily due to ~24-hour half-life. Bedtime dosing is common to leverage sleep quality benefits and minimize daytime appetite stimulation
- Food interaction: Can be taken with or without food. Some protocols suggest fasting administration to maximize GH pulse amplitude, though clinical significance is uncertain
- No reconstitution needed: MK-677 is a small molecule, not a peptide; it does not require reconstitution or injection
- No PCT required: MK-677 does not suppress the HPG axis or testosterone production. No post-cycle therapy is needed
Pharmacokinetics
| Parameter | Value |
|---|---|
| Half-Life | ~24 hours |
| Oral Bioavailability | ~60-70% |
| Time to Peak (Tmax) | 1-2 hours |
| Steady State | ~7 days |
| Protein Binding | Not well characterized |
| Metabolism | Hepatic (CYP3A4 mediated) |
| Elimination | Fecal (primary), renal (minor) |
| GH Peak After Dose | ~1-2 hours post-dose |
| IGF-1 Steady State | ~2-4 weeks of daily dosing |
Key PK Features
- Long half-life: The ~24-hour half-life enables true once-daily dosing with sustained receptor occupation. This distinguishes MK-677 from injectable GH secretagogues (GHRP-2/6, ipamorelin) which have half-lives of minutes and require multiple daily injections.
- Oral bioavailability: As a small molecule (~529 Da), MK-677 has excellent oral absorption, a major advantage over peptide-based GH secretagogues that must be injected.
- No tachyphylaxis: Unlike some GH secretagogues that show reduced efficacy over time, MK-677 maintains GH/IGF-1 elevation for at least 2 years of continuous use without desensitization of the GHSR1a receptor [3].
- CYP3A4 metabolism: Hepatic metabolism via CYP3A4 suggests potential drug interactions with CYP3A4 inhibitors (ketoconazole, grapefruit juice) and inducers (rifampin).
Side Effects & Safety
Common Effects
- Increased appetite (very common, ghrelin-mediated)
- Water retention / edema (common, especially weeks 1-4)
- Mild lethargy or drowsiness
- Tingling/numbness in extremities (GH-mediated)
- Joint stiffness (GH-mediated)
- Vivid dreams / improved sleep depth
- Transient increase in fasting blood glucose
Significant / Dose-Dependent
- Insulin resistance: Fasting glucose elevation of 0.3-0.5 mmol/L documented in clinical trials. Dose-dependent and may worsen with chronic use
- HbA1c elevation: Modest increases noted in 2-year study
- Increased cortisol (modest, within normal range)
- Increased prolactin (modest, typically not clinically significant)
- Facial puffiness (water retention)
- Carpal tunnel-like symptoms at high doses
Primary Safety Concern — Insulin Resistance: The most clinically significant adverse effect of MK-677 is dose-dependent insulin resistance. In the 2-year Nass et al. trial, fasting glucose increased by ~0.3 mmol/L, and some subjects developed impaired fasting glucose. Individuals with pre-diabetes, metabolic syndrome, or type 2 diabetes should use extreme caution. Regular fasting glucose and HbA1c monitoring is strongly recommended during any research protocol [3][6].
Contraindications
- Type 2 diabetes or pre-diabetes (insulin resistance concern)
- Active cancer (GH/IGF-1 may promote tumor growth)
- Pituitary disorders (may interfere with normal regulation)
- Pregnancy and breastfeeding (no safety data)
- Concurrent use of diabetes medications (altered glucose handling)
Required Monitoring (Research Protocols)
| Test | Baseline | During Use | Post-Cycle |
|---|---|---|---|
| Fasting Glucose | Required | Monthly | 4 weeks post |
| HbA1c | Required | Every 3 months | 3 months post |
| IGF-1 | Recommended | 4-8 weeks | 4 weeks post |
| Fasting Insulin | Recommended | Monthly | Optional |
| Cortisol | Optional | Optional | Optional |
Stacking & Synergies
| Combination | Rationale | Evidence |
|---|---|---|
| BPC-157 | GH/IGF-1 elevation amplifies BPC-157's tissue repair effects. MK-677 upregulates GHR expression that BPC-157 also targets. Oral MK-677 + SubQ BPC-157 = convenient combined protocol. | Moderate (theoretical/mechanistic) |
| TB-500 | TB-500's cell migration + MK-677's GH-mediated anabolism provide complementary tissue repair mechanisms. | Low-Moderate (theoretical) |
| Ipamorelin / CJC-1295 | Additional GH secretagogue stacking. GHRH (CJC-1295) + ghrelin mimetic (MK-677) can provide additive GH pulses, though this also increases side effects. | Moderate (mechanistic) |
| Berberine / Metformin | Insulin sensitizers may counteract MK-677's insulin resistance effects. Some research protocols combine low-dose berberine (500 mg) or metformin to mitigate glucose impact. | Low-Moderate (theoretical/anecdotal) |
Regulatory Status
| Jurisdiction | Status | Details |
|---|---|---|
| United States (FDA) | Not approved | Never submitted for NDA. Research chemical classification. Not a controlled substance. |
| WADA | Prohibited | Listed under S2.5 "Growth Hormone Secretagogues" — prohibited in and out of competition. Detectable in urine testing. |
| European Union | Not approved | Research chemical. Regulatory status varies by member state. |
| Australia (TGA) | Not approved | Schedule 4 (prescription only for related compounds). Not available through standard pharmaceutical channels. |
| China | Manufactured | Primary source of research-grade MK-677 synthesis. |
Note: MK-677 is frequently sold alongside SARMs (selective androgen receptor modulators) but is NOT a SARM. It has no interaction with androgen receptors and does not suppress testosterone. However, it is prohibited by WADA and may be prohibited in competitive sports settings.
Frequently Asked Questions
Is MK-677 a steroid or a SARM?
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References
- Patchett AA, et al. "Design and biological activities of L-163,191 (MK-0677): a potent, orally active growth hormone secretagogue." Proc Natl Acad Sci USA. 1995;92(15):7001-5. PMID: 7624358
- Chapman IM, et al. "Stimulation of the growth hormone (GH)-insulin-like growth factor I axis by daily oral administration of a GH secretagogue (MK-677) in healthy elderly subjects." J Clin Endocrinol Metab. 1996;81(12):4249-57. PMID: 8954023
- Nass R, 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):601-11. PMID: 18981485
- Howard AD, et al. "A receptor in pituitary and hypothalamus that functions in growth hormone release." Science. 1996;273(5277):974-7. PMID: 8688086
- Copinschi G, et al. "Effects of a 7-day treatment with a novel, orally active, growth hormone (GH) secretagogue, MK-677, on 24-hour GH profiles, insulin-like growth factor I, and adrenocortical function in normal young men." J Clin Endocrinol Metab. 1996;81(8):2776-82. PMID: 8768828
- Svensson J, et al. "Two-month treatment of obese subjects with the oral growth hormone (GH) secretagogue MK-677 increases GH secretion, fat-free mass, and energy expenditure." J Clin Endocrinol Metab. 1998;83(2):362-9. PMID: 9467534
- Murphy MG, et al. "Effect of alendronate and MK-677 (a growth hormone secretagogue), individually and in combination, on markers of bone turnover and bone mineral density in postmenopausal osteoporotic women." J Clin Endocrinol Metab. 2001;86(3):1116-25. PMID: 11238496
- Bach MA, et al. "Is there a role for growth hormone secretagogues in the treatment of frailty in the elderly?" Endocrine. 1997;7(2):173-7. PMID: 9549041
- Smith RG, et al. "Peptidomimetic regulation of growth hormone secretion." Endocr Rev. 1997;18(5):621-45. PMID: 9331545
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Medical Disclaimer: This article is provided for educational and research reference purposes only. MK-677 is not approved by any regulatory agency for human use. All information is derived from clinical research and should not be interpreted as clinical guidance. Consult a qualified healthcare professional before considering any research compound. See our full Medical Disclaimer.
