Macimorelin
FDA-approved diagnostic test for adult growth hormone deficiency
Written by Michael Carroll — Owner, Director of Research · Reviewed by the Peptide Initiative Research Team · Editorial standards
Suggested dose
0.5 mg/kg
Compound profile
Scientific & efficacy data
Growth Hormone
Peptide profile
Strong human trials
Macimorelin
0.5 mg/kg · As needed
Molecular formula
C26H30N6O3
- Mol. weight
- 474.6 g/mol
- CAS number
- 381231-18-1
- PubChem
- 9804938
- Developed · 2003
- Aeterna Zentaris Research Team
Aeterna Zentaris Inc. (now distributed by Novo Nordisk Inc.)
Amino acid sequence
N/A — Non-peptide peptidomimetic small molecule (synthetic growth hormone secretagogue)Diagnostic Accuracy
Macimorelin achieves 87% sensitivity and 96% specificity for AGHD diagnosis with 97% reproducibility on retesting, providing highly reliable results for clinical decision-making.
Safety & Tolerability
No serious adverse events reported in Phase 3 trials, QT prolongation is mild (~11 msec), and common side effects are transient and mild, making it suitable for broad patient populations.
Convenience & Patient Experience
Single oral dose eliminates painful injections, lengthy procedures, and hypoglycemia risk associated with insulin tolerance testing, while delivering diagnostic results in one 60-minute appointment.
Dosing
How much do I take?
0.5 mg/kg · single dose
0.5 mg/kg
Single dose
Covers timing · dose-adjustment guidance.
Suitability
Is this right for me?
Best for diagnosing adult growth hormone deficiency (aghd) & patients who cannot tolerate or are contraindicated for insulin tolerance test
Best 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.
Consider alternatives if
Do not use if
Use with caution if
Not sure?
Compare Macimorelin with similar peptides to find the best fit for your goals.
Administration
How do I use it?
Oral (reconstituted solution)
Route
Not applicable — oral solution only
Best sites
Covers reconstitution · step-by-step technique · storage · a sample daily schedule.
Safety
Is it safe?
6 common side effects · 2 serious
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.
Safety data from Phase 3 clinical trial (NCT02558829, 157 subjects) and FDA approval process demonstrating efficacy and safety equivalent to insulin tolerance test with superior convenience and tolerability.
Common side effects · experienced by some users
Dysgeusia (abnormal taste)
Metallic or unpleasant taste in mouth affecting 4.5% of patients. Often described as a bitter or medicinal aftertaste. Occurs within minutes of taking the dose and typically resolves within 1-2 hours.
Management: Eat something sweet or drink a flavored beverage after the required fasting window. Swish mouth with water. Taste usually resolves spontaneously. Does not interfere with test results.
Dizziness
Mild dizziness or lightheadedness in 3.9% of patients. Usually mild and transient, resolving within 1-2 hours. May be related to fasting state and GH stimulation.
Management: Sit or lie down if dizziness occurs. Stay well-hydrated. Avoid sudden position changes. Dizziness resolves as GH levels normalize. No intervention usually needed.
Headache
Mild to moderate headache in 3.9% of patients. Typically mild and non-throbbing. Occurs within first few hours after dosing. Similar incidence to placebo in some analyses.
Management: Rest in a quiet, dark environment. Over-the-counter pain relief (acetaminophen or ibuprofen) is safe if needed. Drink plenty of water. Usually resolves within 2-4 hours without intervention.
Fatigue
Mild fatigue or tiredness in 3.9% of patients. May be related to morning testing and fasting state. Typically mild and short-lived.
Management: Schedule test when able to rest afterward. Avoid strenuous activity immediately after testing. Fatigue usually resolves within 2-4 hours. Eat a light meal after the testing window concludes.
Nausea
Mild nausea in 3.2% of patients. Rarely progresses to vomiting (less than 1% of cases). Related to fasting state and GH stimulation. Transient and mild when it occurs.
Management: Remain still and breathe deeply. Avoid sudden movements. Eat bland foods after testing window. Ginger tea or peppermint may help. Does not typically interfere with diagnostic test.
Increased Appetite
Hunger or increased appetite stimulation in 3.2% of patients. Expected physiologic response since macimorelin stimulates ghrelin/hunger pathways. Occurs during test window.
Management: This is expected during the diagnostic test. Eat a meal immediately after the 60-minute testing window concludes. Hunger resolves once food is consumed. Does not affect test validity.
Stop and seek help if
- Test has been completed and diagnostic results obtained (single-use diagnostic tool)
- Patient develops signs of QT prolongation or cardiac arrhythmias
- Patient experiences severe hypersensitivity reaction
- Contraindications emerge that preclude continued testing
- Physician determines additional testing is not clinically warranted
Macimorelin is a single-dose diagnostic test, not an ongoing therapy. Once administered and blood samples collected, the diagnostic protocol is complete. Additional testing requires a separate administration if clinically indicated by the healthcare provider.
With other peptides
- Caution:Other growth hormone secretagogues (ibutamoren, anamorelin, etc.) — Should not be combined during diagnostic testing as it will confound results
- Caution:GH or IGF-1 replacement therapy — Discontinue GH therapy 4+ weeks before testing if possible to avoid false negatives
With medications
- Caution:QT-prolonging drugs (quinidine, disopyramide, amiodarone, sotalol, dofetilide, antipsychotics, antiemetics) — Increases risk of QT prolongation and torsades de pointes. Avoid combination or use extreme caution with ECG monitoring.
- Caution:CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir) — May increase macimorelin exposure and side effects. Monitor for increased QT prolongation.
- Caution:Medications affecting electrolytes (diuretics, amphotericin B) — Electrolyte abnormalities increase QT prolongation risk. Ensure normal K+ and Mg+ before testing.
- Caution:Corticosteroids or opioids — May blunt GH response to macimorelin. Discontinue if possible before testing.
With supplements
- Safe:Supplements affecting heart rhythm (stimulants with high caffeine) — Extreme caffeine intake before testing may increase arrhythmia risk. Recommend abstinence 24 hours before test.
Effectiveness
How do I know it's working?
Strong human trials · first signs before testing (preparation)
Evidence level
Strong human trials
(Phase 3 or FDA approved)
Regulatory status
FDA approved for this use
Onset of effects
Rapid
(hours to days)
How it works
Macimorelin is a small molecule that stimulates growth hormone release by binding to ghrelin receptors in the brain and pituitary.
Unlike natural growth hormone, macimorelin triggers your body's own growth hormone production, maintaining normal hormone pulses while increasing overall GH levels.
The deeper mechanism
Macimorelin is a non-peptide peptidomimetic selective agonist of the growth hormone secretagogue receptor (GHSR), also known as the ghrelin receptor.
At the molecular level, macimorelin binds to GHSR transmembrane receptor in the anterior pituitary and hypothalamus, activating G-protein coupled signaling cascades.
This triggers release of growth hormone-releasing hormone (GHRH) from hypothalamic neurons and direct stimulation of somatotroph cells in the pituitary. GH is then released into the bloodstream within 15-30 minutes. Peak GH responses occur at 30-60 minutes post-dose.
A GH level above 2.8 ng/mL indicates functional GH axis. The 4.1-hour half-life allows complete GH response measurement within the 60-minute testing window.
Food significantly reduces oral bioavailability (50-55% decrease in Cmax/AUC) through delayed gastric emptying, necessitating fasting administration.
What to expect
Before Testing (Preparation)
What you might notice
- Healthcare provider orders GH deficiency diagnostic testing
- Informed consent obtained, QT risk assessment performed
- Baseline lab work (electrolytes, ECG if clinically indicated)
What's normal
- No symptoms — baseline visit and screening
- ECG performed to rule out QT prolongation
- Instructions provided for fasting and test day preparation
What's next
- Schedule macimorelin diagnostic test for early morning
- Begin 4-hour fast the night before
- Avoid QT-prolonging medications if possible
Test Day (During Diagnostic Protocol)
What you might notice
- Arrive after 4+ hour fast in early morning (8-9 AM typical)
- Baseline blood sample drawn (time 0 minutes)
- Reconstituted macimorelin solution taken orally
- Mild side effects may appear: taste changes, mild nausea, dizziness
- Additional blood samples drawn at 30, 45, and 60 minutes
What's normal
- Dysgeusia (bad taste) in 4.5% of patients
- Mild hunger increase during fasting window
- Mild headache, dizziness, or fatigue in 3-4% of patients
- GH response peaks at 30-60 minute window
What's next
- Test concludes at 60-minute mark
- Eat a meal to end fasting period
- Side effects resolve within 2-4 hours
- Lab results analyzed within 1-2 days
Post-Testing (1-2 Days)
What you might notice
- Any mild side effects have completely resolved
- Lab results returned to healthcare provider
- GH level compared to diagnostic cutoff (2.8 ng/mL)
- Diagnosis confirmed: GH sufficient (negative) or deficient (positive)
What's normal
- Negative result (GH >2.8 ng/mL) — GH deficiency ruled out
- Positive result (GH ≤2.8 ng/mL) — consistent with AGHD
- 97% reproducibility if retesting needed
What's next
- If positive: discuss GH replacement therapy options with endocrinologist
- If negative: investigate other causes of symptoms
- Retest only if clinically indicated and initial results equivocal
Signs it's working
Diagnostic Accuracy Indicators
- Clear GH response visible in 30-60 minute window
- GH stimulation above 2.8 ng/mL threshold (positive diagnostic test)
- Consistent results on retesting (97% reproducibility)
- Results align with clinical presentation and other GH markers (IGF-1, cortisol)
Test Quality Metrics
- Patient tolerates dose well with minimal side effects
- Adequate blood samples obtained at all timepoints
- Clear diagnostic result (definitively positive or negative, not equivocal)
- No technical issues interfering with sample quality
Not seeing results? Common reasons
- Equivocal result (GH borderline near 2.8 ng/mL threshold) — may require retesting for confirmation
- Blunted GH response — patient may not have fasted adequately, or may be on suppressant medication (corticosteroids, opioids)
- Medications interfering — check for CYP3A4 inhibitors or GH-suppressing drugs
- Technical issue — ensure proper blood draw timing (baseline, 30, 45, 60 min) and sample handling
- Retest needed — with 97% reproducibility, repeat testing can confirm equivocal initial results
Key research
Clinical trials
Tested in people
5 registered studies, none currently enrolling.
By phase
A trial spanning two phases is counted in both, so these can sum above the total. Observational studies carry no phase.
What kind of research
About 402 people took part in the studies that actually administered Macimorelin. Observational studies analyse the records of people already taking it — nobody was given anything for the study, so they are counted separately and cannot show cause and effect.
Most recent
A Research Study of How Well Macimorelin Works to Find Out if Children Have a Lack of Growth Hormone and How Safe it is
AEterna Zentaris
Copeptin After an Oral Stimulation With Macimorelin in Healthy Volunteers
University Hospital, Basel, Switzerland
Validation of Macimorelin as a Test for Adult Growth Hormone Deficiency
AEterna Zentaris
Sources: ClinicalTrials.gov, the EU Clinical Trials Information System and ISRCTN, deduplicated so a study registered twice is counted once. A study is counted when Macimorelin is named as an intervention; studies that only mention it in passing are not.
Questions
Frequently asked
What exactly is macimorelin used for?
Macimorelin (brand name Macrilen) is FDA-approved specifically for diagnosing adult growth hormone deficiency (AGHD) in patients suspected of having low GH. It's a single-dose diagnostic test, not a therapy. You take it once, and doctors measure your blood GH response to determine if your GH system is working normally.
How is macimorelin different from the insulin tolerance test (ITT)?
Both are FDA-approved tests for diagnosing GH deficiency, but macimorelin is safer and more convenient. ITT requires insulin injection, blood sugar monitoring, and watching for hypoglycemia symptoms. Macimorelin is simply drinking a reconstituted powder. Macimorelin has 87% sensitivity and 96% specificity — nearly identical accuracy to ITT — but with better tolerability and no serious adverse events in trials.
Do I have to fast before taking macimorelin?
Yes, absolutely. You must fast for at least 4 hours (ideally overnight) before the test. Food decreases the amount of macimorelin absorbed by 50-55%, which would make the test invalid. Your healthcare provider will give you specific fasting instructions.
What happens during the macimorelin test?
You arrive in the morning after fasting. A nurse draws a baseline blood sample (time 0). You drink the reconstituted macimorelin solution on an empty stomach. Then blood samples are drawn at 30, 45, and 60 minutes to measure your GH response. If your GH level goes above 2.8 ng/mL, the test is negative (you have enough GH). If it stays below 2.8 ng/mL, the test is positive (consistent with GH deficiency).
What are the common side effects?
Most side effects are mild and short-lived. The most common is dysgeusia (abnormal taste like metallic or bitter) affecting 4.5% of patients. Others include dizziness, headache, fatigue, nausea, and increased hunger — each occurring in 1-4% of patients. These usually resolve within 1-4 hours. No serious adverse events were reported in clinical trials.
Is macimorelin safe if I have heart problems?
Macimorelin causes a mild QT interval prolongation (approximately 11 msec increase). For most people this is not a concern, but if you have existing QT prolongation, take QT-prolonging medications, or have electrolyte abnormalities, you need careful evaluation. Your doctor will review this before the test and may recommend an ECG.
How accurate is macimorelin for diagnosing GH deficiency?
Clinical trials showed 87% sensitivity (correctly identifying people with deficiency) and 96% specificity (correctly identifying people without deficiency). Results are 97% reproducible on retesting. Macimorelin has 84% overall agreement with the insulin tolerance test, making it highly reliable for diagnostic purposes.
Can I use macimorelin if I'm already taking GH replacement therapy?
No, you should discontinue GH therapy for at least 4 weeks before macimorelin testing. Taking GH will suppress your body's natural GH response, giving a false negative result. Your doctor will advise you on when to stop and restart therapy.
What if my first test result is unclear or borderline?
If your GH level is borderline near the 2.8 ng/mL cutoff, your doctor may recommend retesting. With 97% reproducibility, a repeat test will give a consistent result. The test can be repeated as needed for diagnostic confirmation.
Further reading
History & related research
History · since 2017
The Simple Drink That Replaced a Dangerous Test
Macimorelin is an oral diagnostic tool that mimics a natural hormone. It helps doctors safely diagnose growth hormone deficiency without dangerous side effects.
Read the full history of MacimorelinReady for the protocol?
Every dosing tier, administration route, timing note, and dose-adjustment rule for Macimorelin, on one page.