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
Enter your weight to see your dose.
Compound profile
Macimorelin 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.
Dosing
How much Macimorelin do I take?
0.5 mg/kg · single dose
0.5 mg/kg
Single dose
Covers timing · dose-adjustment guidance.
Suitability
Is Macimorelin 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.
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 Macimorelin?
Oral (reconstituted solution)
Route
Not applicable — oral solution only
Best sites
Covers reconstitution · step-by-step technique · storage · a sample daily schedule.
Safety
Is Macimorelin safe?
6 common side effects · 2 serious
Macimorelin is given once, as a single oral dose for a diagnostic test.
In the registration trial (154 people dosed), the most common reactions were taste disturbance (4.5%), dizziness, headache and fatigue (3.9% each), and nausea and hunger (3.2% each). [5] No serious adverse events were reported for macimorelin.
[1] It raises the corrected QT interval by about 11 msec; QT prolongation can lead to torsade de pointes, so the label says to avoid drugs that prolong the QT interval. [5] The label lists no contraindications. [5]
The label rests on a randomized, open-label crossover trial against the insulin tolerance test (ITT) in 157 adults; 140 completed both tests. [5] Using a cut-off of 2.8 ng/mL, negative agreement with the ITT was 94% and positive agreement 74%.
[5] The published trial reports 87% sensitivity, 96% specificity and 97% reproducibility on retesting. [1] One of 154 macimorelin tests failed for a technical reason, against 27 of 157 ITTs that failed to reach the required hypoglycemia. [5]
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 replacement therapy — Stop GH therapy at least one week before testing; GH products can blunt the response to macimorelin [5]
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:Strong CYP3A4 inducers (carbamazepine, phenytoin, rifampin, St. John's wort, modafinil) — Lower macimorelin levels and can cause a false positive result. Stop them and allow a washout before testing [5]
- 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 Macimorelin is 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 stimulates growth hormone release by activating growth hormone secretagogue receptors in the pituitary and hypothalamus. [5] Plasma levels peak 0.5–1.5 hours after an oral dose, and GH peaks 30–90 minutes after dosing, so blood is drawn at 30, 45, 60 and 90 minutes.
[5] A maximally stimulated GH below 2.8 ng/mL confirms adult GH deficiency. [5] It is metabolised mainly by CYP3A4 and has a mean terminal half-life of 4.1 hours. [5] A liquid meal cut its Cmax by 55% and AUC by 49%, so it is given after at least an 8-hour fast.
[5] Because it acts downstream of the hypothalamus, it can miss GH deficiency early in hypothalamic disease. [5]
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
- Fast for at least 8 hours before the test [5]
- Avoid QT-prolonging medications if possible
Test Day (During Diagnostic Protocol)
What you might notice
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 30–90 minutes after dosing [5]
What's next
- Test concludes after the 90-minute blood draw [5]
- 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
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 — the patient may not have fasted for 8 hours, may still be on GH therapy (stop at least one week before), or may be taking a drug that affects GH release [5]
- Interfering medications — check for strong CYP3A4 inducers, which can cause a false positive [5]
- Technical issue — ensure blood is drawn at 30, 45, 60 and 90 minutes and samples are handled properly [5]
- Retest needed — with 97% reproducibility, repeat testing can confirm equivocal initial results
Key research
Clinical trials
Macimorelin 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 about Macimorelin
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?
What happens during the macimorelin test?
A healthcare professional dissolves the granules in water and you drink your weight-based dose (0.5 mg/kg) within 30 seconds, after fasting for at least 8 hours. [5] Blood is drawn for growth hormone at 30, 45, 60 and 90 minutes. [5] If the highest GH level stays below 2.8 ng/mL, the test is positive, which confirms adult GH deficiency; a higher peak is a negative result. [5]
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?
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
Macimorelin 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.