Capromorelin vs MK-677 (Ibutamoren)
An oral ghrelin agonist originally developed by Pfizer that stimulates growth hormone and appetite. FDA-approved for veterinary use (Entyce and Elura) and investigated in human trials for age-related muscle wasting and appetite support.
The oral growth hormone secretagogue that tricks your body into producing more GH — no needles required.
Written by Michael Carroll — Owner, Director of Research · Reviewed by the Peptide Initiative Research Team · Editorial standards
Quick comparison
Dose range
Capromorelin
10–10 mg
MK-677 (Ibutamoren)
25–25 mg
Frequency
Capromorelin
Once daily
MK-677 (Ibutamoren)
Once daily
Administration
Capromorelin
Oral (capsule or tablet)
MK-677 (Ibutamoren)
Oral (capsule or liquid)
Cycle length
Capromorelin
4-6 weeks
MK-677 (Ibutamoren)
8-12 weeks
Onset speed
Capromorelin
Moderate (1-2 weeks)
MK-677 (Ibutamoren)
Moderate (1-2 weeks)
Evidence level
Capromorelin
Moderate human trials (Phase 1-2)
MK-677 (Ibutamoren)
Moderate human trials (Phase 1-2)
Benefit ratings
GH & IGF-1 Stimulation
Physical Function
Oral Convenience
Muscle and body composition
Recovery and athletic performance
Sleep quality and metabolic health
Compound specifications
Capromorelin
Molecular formula
C28H35N5O4
Molecular weight
505.6 g/mol
Half-life
1-2.4 hours in humans
Bioavailability
Good oral bioavailability
CAS number
193273-66-4
MK-677 (Ibutamoren)
Molecular formula
C27H36N4O5S
Molecular weight
528.7 g/mol
Half-life
4-6 hours (IGF-1 elevation persists ~24 hours)
Bioavailability
High oral bioavailability
CAS number
159634-47-6
Dosing compared
Capromorelin
Oral
3 mg
Twice daily · Initial weeks
One of the lower doses tested in the year-long study in older adults; raised growth hormone and IGF-1 [1].
10 mg
Once daily at bedtime · Ongoing
A simple once-daily bedtime dose used in the human trial that improved lean body mass [1].
10 mg
Twice daily · Up to 12 months in the trial
The highest daily amount studied in older adults, giving the largest gains in muscle mass and physical function [1].
MK-677 (Ibutamoren)
Oral
Weeks 2-4
10 mg
Once daily · 2-4 weeks initial
Lower entry dose used in practice to limit water retention and appetite surge before titrating up.
Studied up to 1-2 years
25 mg
Once daily · 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].
Best suited for
Capromorelin
Age-Related Muscle Wasting
Capromorelin showed significant improvements in physical function — including tandem walk and stair climb — in elderly adults, making it a strong candidate for combating sarcopenia.
Appetite Stimulation
As an FDA-approved veterinary appetite stimulant, capromorelin has extensive real-world evidence for boosting food intake and body weight in mammals.
Growth Hormone Research
One of the best-studied oral ghrelin agonists with human Phase 2 data, offering researchers a well-characterized tool for studying GH/IGF-1 pathways.
MK-677 (Ibutamoren)
Older Adults (60+) Seeking Body Composition Improvements
MK-677 was designed for aging populations. It restores GH and IGF-1 to youthful levels, [7] 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 [1] 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. [2] It stimulates appetite (a feature in these populations) and preserves muscle tissue—making it useful for those losing mass despite good nutrition.
Side effects
Capromorelin
Common
- Appetite stimulation
- Fluid retention and weight gain
Uncommon
- Joint and muscle aches
- Fatigue or lethargy
- Peripheral edema (swelling)
Serious
- Hyperglycemia
MK-677 (Ibutamoren)
Common
- 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
Serious
- Glucose dysregulation and diabetes risk
Safety & evidence
Capromorelin
Evidence level
Moderate human trials (Phase 1-2)
FDA status
FDA approved for this use
Safety overview
Capromorelin was generally well-tolerated in Phase 2 human trials involving nearly 400 elderly participants. Most side effects were mild to moderate. Its FDA-approved veterinary use provides additional long-term safety data in mammalian species.
Contraindications
- Active cancer or history of cancer (GH may stimulate growth)
- Uncontrolled diabetes or severe metabolic disorders
- Severe liver or kidney disease
- Pregnancy and lactation
MK-677 (Ibutamoren)
Evidence level
Moderate human trials (Phase 1-2)
FDA status
Research compound
Safety overview
MK-677 has been studied in hundreds of humans over 2+ years and is generally well tolerated. [1][3] 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.
Contraindications
- Uncontrolled diabetes or severe glucose dysregulation
- Active cancer or high cancer risk requiring caution with IGF-1 elevation
- Pregnancy or breastfeeding
Which is right for you?
Choose Capromorelin if...
- Stimulating growth hormone production in elderly adults
- Supporting appetite and weight gain in wasting conditions
- Improving muscle mass and physical performance
- Anti-aging and longevity optimization
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