Macimorelin vs MK-677 (Ibutamoren)
The distinction that mattersBoth are orally active ghrelin receptor agonists. Macimorelin is FDA-approved for a single diagnostic dose; MK-677 is an unapproved compound taken daily, and its long-term trial found a heart failure signal.
These are not equivalent in evidence Macimorelin is rated 5/5 and MK-677 (Ibutamoren) is rated 3/5 — a gap of 2 levels on our scale. Similar marketing does not mean similar proof.
| Macimorelin FDA Approved | MK-677 (Ibutamoren) Not a Peptide | |
|---|---|---|
| Evidence rating |
5/5 Very strong
Evidence rating 5 out of 5: Very strong
|
3/5 Moderate
Evidence rating 3 out of 5: Moderate
|
| Regulatory status | FDA-approved (2017) and EMA-approved for the diagnosis of adult growth hormone deficiency. | Not a peptide. Investigational drug; development discontinued. Not approved. Prohibited in sport. |
| Category | Growth Hormone Axis | Growth Hormone Axis |
| Drug class | Orally active ghrelin receptor agonist — peptidomimetic | Non-peptide, orally active ghrelin receptor agonist — listed here because it is universally marketed alongside peptides |
| Route | Oral, single diagnostic dose | Oral, once daily |
| Half-life | ~4 hours | ~4–6 hours (effects persist ~24h) |
| Studied in | Pivotal randomized crossover trial against the insulin tolerance test, the previous reference standard. | Multiple phase 2 trials including elderly hip-fracture recovery, sarcopenia, GH deficiency, and a 2-year trial in healthy older adults. |
| Who should avoid it |
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|
| Legal status | Prescription diagnostic agent, administered under medical supervision. | Unapproved investigational drug. Not lawful as a dietary supplement in the US. WADA-prohibited. |
Benefits — Macimorelin
- Replaces a genuinely dangerous test
The insulin tolerance test requires deliberately inducing hypoglycemia under supervision and is contraindicated in cardiac disease and epilepsy. Macimorelin achieves comparable diagnostic accuracy with an oral solution.
- High diagnostic agreement
The pivotal trial showed sensitivity and specificity closely matching the insulin tolerance test.
- Oral and outpatient
No infusion, no induced hypoglycemia, no intensive monitoring.
- Proves oral GH secretagogues are feasible
A useful reference point when assessing unapproved oral secretagogue claims.
Benefits — MK-677 (Ibutamoren)
- Reliable IGF-1 elevation, orally
Trials consistently show IGF-1 increases into the young-adult range in older subjects — no injection required.
- Increases fat-free mass
A 2-year randomized trial in healthy older adults showed a roughly 1.6 kg increase in fat-free mass versus placebo.
- Improved sleep architecture
Studies show increased slow-wave and REM sleep duration.
- Appetite stimulation
Substantial, and potentially useful in cachexia or during recovery from illness.
- Increased bone turnover markers
Suggests bone remodelling activity, though fracture-reduction benefit was not demonstrated.
Risks & cons — Macimorelin
- Diagnostic use only
It is not a treatment. There is no evidence base for repeated dosing, and no approval for therapeutic use.
- QT interval prolongation
The main safety concern; concurrent QT-prolonging drugs must be stopped before testing.
- Dysgeusia
A distorted or metallic taste is the most common adverse effect.
- False results with interfering drugs
Strong CYP3A4 inducers and recent GH therapy can invalidate the test.
- Fatigue and headache
Reported after the single dose.
Risks & cons — MK-677 (Ibutamoren)
- Failed its key clinical trial
The hip-fracture recovery program did not meet endpoints, and development was stopped. Fat-free mass gains did not translate into functional benefit.
- Congestive heart failure signal
A trial in frail elderly patients was halted after an increase in congestive heart failure cases in the treatment arm. This is the most serious concern with the compound.
- Insulin resistance and hyperglycemia
Consistent and dose-dependent; fasting glucose rises and insulin sensitivity falls.
- Fluid retention and oedema
Common, sometimes marked, especially early in use.
- Increased appetite and weight gain
Often unwanted; a substantial fraction of gained weight is not lean tissue.
A comparison is not a recommendation. Neither column is being suggested for you. Which — if either — is appropriate depends on your diagnosis, history and medications, and that is a conversation for a qualified clinician.