At a Glance
Quickcomparison
Dose Range
Macimorelin
25–60 mg
MK-677 (Ibutamoren)
10–25 mg
Frequency
As needed
Once daily
Administration
Oral (reconstituted solution)
Oral (capsule or liquid)
Cycle Length
4-6 weeks
8-12 weeks
Onset Speed
Rapid (hours to days)
Moderate (1-2 weeks)
Evidence Level
Strong human trials (Phase 3 or FDA approved)
Moderate human trials (Phase 1-2)
Efficacy
Benefitratings
Diagnostic Accuracy
Safety & Tolerability
Convenience & Patient Experience
Muscle and body composition
Recovery and athletic performance
Sleep quality and metabolic health
Technical Data
Compoundspecifications
Molecular Formula
C26H30N6O3
Molecular Weight
474.6 g/mol
Half-Life
4.1 hours
Bioavailability
Oral; food decreases Cmax and AUC by approximately 50-55% — must be administered fasting
CAS Number
381231-18-1
C27H36N4O5S
528.7 g/mol
4-6 hours (IGF-1 elevation persists ~24 hours)
High oral bioavailability
159634-47-6
Protocols
Dosingtiers
standard
0.5 mg/kg
Single dose
One-time diagnostic test
FDA-label (Macrilen) single oral dose for diagnosing adult growth hormone deficiency. Granules (60 mg) are reconstituted in 120 mL water (0.5 mg/mL) and taken after an 8-hour fast; serum GH below 2.8 ng/mL across the 30/45/60/90-minute draws confirms deficiency [4].
starting
10 mg
2-4 weeks initial
Lower entry dose used in practice to limit water retention and appetite surge before titrating up.
25 mg
Studied up to 1-2 years
25 mg once-daily oral is the dose used in the randomized trial in healthy older adults; increased GH/IGF-1 and fat-free mass [1]. Most-studied dose [2].
Applications
Bestsuited for
Diagnosis of Adult Growth Hormone Deficiency
Macimorelin is FDA-approved specifically for diagnosing AGHD in adults. It replaces the insulin tolerance test (ITT) as a safer, more convenient, and equally accurate diagnostic option. Phase 3 trials showed 87% sensitivity and 96% specificity.
Patients Intolerant or Contraindicated for ITT
For patients who cannot safely undergo insulin tolerance testing due to cardiac risk, seizure history, or other contraindications, macimorelin offers an evidence-based non-invasive alternative.
Clinical Confirmation Testing
With 97% reproducibility on retesting, macimorelin can reliably confirm GH deficiency diagnoses and monitor therapeutic responses over time with excellent consistency.
Older Adults (60+) Seeking Body Composition Improvements
MK-677 was designed for aging populations. It restores GH and IGF-1 to youthful levels, increases lean mass, and improves physical function in people over 60. A 2-year clinical trial showed a 1.1 kg gain in fat-free mass with only modest fat gain—ideal for maintaining strength in older age.
Athletes and Fitness Enthusiasts Wanting Recovery Support
Oral dosing makes MK-677 convenient for athletes. It enhances muscle protein synthesis, speeds recovery after intense training, and supports lean mass building without the hassle of injections.
People with Chronic Wasting or Protein Deficiency
Clinical trials show MK-677 helps treat protein-energy wasting in kidney disease and other conditions. It stimulates appetite (a feature in these populations) and preserves muscle tissue—making it useful for those losing mass despite good nutrition.
Safety Profile
Sideeffects
Common
Dysgeusia (abnormal taste)
Dizziness
Headache
Fatigue
Nausea
Increased Appetite
Serious
Cardiovascular events
Increased appetite
Mild water retention or edema
Elevated fasting glucose
Uncommon
Joint or muscle pain
Carpal tunnel syndrome or nerve compression symptoms
Mild nausea or digestive upset
Glucose dysregulation and diabetes risk
Research Status
Safety& evidence
FDA Status
FDA approved for this use
Safety Overview
Macimorelin is well-tolerated with an excellent safety profile in clinical trials. No serious adverse events were reported in Phase 3 studies (n=154). Most side effects are mild and transient. QT prolongation of approximately 11 msec occurs but is generally not clinically significant. However, avoid combining with other QT-prolonging drugs.
Contraindications
xPatients with QT prolongation or QT/RR prolongation disorder
xConcurrent use with drugs known to prolong QT interval
xPatients with critical illness (acute medical or surgical illness)
xKnown hypersensitivity to macimorelin or any components
xPatients unable to fast or follow fasting requirements
Research compound
MK-677 has been studied in hundreds of humans over 2+ years and is generally well tolerated. Most side effects are mild and manageable, especially with lower starting doses. The lack of injection requirement makes it safer than injectable GH in terms of infection risk and sterility concerns. However, it is not FDA approved and should be viewed as a research compound.
xUncontrolled diabetes or severe glucose dysregulation
xActive cancer or high cancer risk requiring caution with IGF-1 elevation
xPregnancy or breastfeeding
Decision Guide
Which isright for you?
Choose Macimorelin if...
Diagnosing adult growth hormone deficiency (AGHD)
Patients who cannot tolerate or are contraindicated for insulin tolerance test
Clinical settings requiring convenient, repeatable hormone testing
Healthcare providers seeking FDA-approved diagnostic alternatives
Choose MK-677 (Ibutamoren) if...
Building lean muscle mass and strength
Improving overall body composition in aging adults
Enhancing athletic recovery and performance
Supporting metabolic health and fat loss