Peptide Profile
Macimorelin
FDA-approved diagnostic test for adult growth hormone deficiency
Dose Range
25-60mg
Frequency
As needed
Route
Oral (reconstituted solution)
Cycle Length
4-6 weeks
Onset
Rapid (hours to days)
Evidence
Strong
Compound Profile
Scientific & Efficacy Data
C26H30N6O3
Molecular Formula
474.6 g/mol
Molecular Weight
4.1 hours
Half-Life
Oral; food decreases Cmax and AUC by approximately 50-55% — must be administered fasting
Bioavailability
381231-18-1
CAS #
9804938
PubChem ID ↗
Developed By · 2003
Aeterna Zentaris Research Team
Aeterna Zentaris Inc. (now distributed by Novo Nordisk Inc.)
Primary Benefits
Macimorelin achieves 87% sensitivity and 96% specificity for AGHD diagnosis with 97% reproducibility on retesting, providing highly reliable results for clinical decision-making.
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.
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.
Amino Acid Sequence
N/A — Non-peptide peptidomimetic small molecule (synthetic growth hormone secretagogue)Dosing
How much
do I take?
Timing
Best time to take
Early morning, after fasting overnight for at least 4 hours
With food?
Must be taken on an empty stomach. Food decreases absorption by 50-55%. Do not eat for at least 1 hour before and 30 minutes after administration.
If stacking
This is a single-use diagnostic test, not a compound to be stacked. Administered once with baseline and stimulated GH measurements at 30, 45, and 60 minutes post-dose.
Adjusting Your Dose
Increase if
- +Initial test is inconclusive and retesting is clinically indicated
- +Patient weight changes significantly between test administrations
- +Clinical suspicion remains high despite initial negative result
Decrease if
- -Not applicable — single fixed dose based on weight
- -Dose is calculated once at 0.5 mg/kg body weight
Signs of right dose
- ✓Clear GH response meeting diagnostic criteria at 2-minute interval
- ✓GH level above 2.8 ng/mL threshold (positive test)
- ✓Minimal side effects or patient tolerability issues
Dosing Calculator
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Suitability
Is this
right for me?
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
Who Should Avoid
Do not use if
- ×Patient has QT interval prolongation or history of torsades de pointes
- ×Patient is critically ill from acute medical or surgical illness
- ×Patient has known hypersensitivity to macimorelin
- ×Patient cannot maintain fasting state before test
- ×Patient is taking multiple QT-prolonging medications
- ×Patient has baseline QTc interval greater than 450 msec (males) or 460 msec (females)
Use with caution if
- !Patient has cardiac arrhythmias or structural heart disease
- !Patient is taking medications that inhibit CYP3A4
- !Patient has electrolyte abnormalities (especially hypokalemia or hypomagnesemia)
- !Patient has family history of sudden cardiac death or long QT syndrome
- !Patient is elderly or has compromised renal function
- !Patient is pregnant or breastfeeding (limited safety data)
Administration
How do I
use it?
Reconstitution
What you need
- •Macrilen powder vial (25 mg or 50 mg)
- •Sterile water for injection (pharmaceutical grade)
- •Small glass of water (8 oz) for dilution
- •Graduated cylinder or syringe for measuring
- •Small spoon or stirring device
Injection
Route
Not applicable — oral solution only
Best sites
- •Not applicable (oral administration)
Technique
- 1.Reconstitute powder with sterile water according to package instructions
- 2.Stir gently until completely dissolved
- 3.Solution should be clear and colorless
- 4.Drink entire solution immediately or within specified timeframe
- 5.Do not inject or inhale
Storage
Signs of degradation
Sample Daily Schedule
Safety
Is it
safe?
Safety Profile
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.
Interactions
With other peptides
- !Should not be combined during diagnostic testing as it will confound results
- !Discontinue GH therapy 4+ weeks before testing if possible to avoid false negatives
With medications
- !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.
- !CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir) - May increase macimorelin exposure and side effects. Monitor for increased QT prolongation.
- !Medications affecting electrolytes (diuretics, amphotericin B) - Electrolyte abnormalities increase QT prolongation risk. Ensure normal K+ and Mg+ before testing.
- !Corticosteroids or opioids - May blunt GH response to macimorelin. Discontinue if possible before testing.
With supplements
- ✓Supplements affecting heart rhythm (stimulants with high caffeine) - Extreme caffeine intake before testing may increase arrhythmia risk. Recommend abstinence 24 hours before test.
Effectiveness
Does it
work?
Evidence Level
Strong human trials
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
"Macimorelin as a Diagnostic Test for Adult GH Deficiency"
Garcia JM, et al., 2018
Finding: Macimorelin proved just as accurate as the dangerous insulin tolerance test for diagnosing growth hormone deficiency—catching real deficiencies 87% of the time and ruling out false positives 96% of the time. Plus, it's much safer and easier: just drink it instead of risking dangerous blood sugar drops from insulin shots.
View Study"Molecular recognition of two approved drugs Macimorelin and Anamorelin by the growth hormone secretagogue receptor"
Wang RL, et al., 2025
Finding: Scientists used incredibly powerful electron microscopes to photograph macimorelin actually grabbing onto the growth hormone receptor at atomic-scale resolution, revealing exactly how the drug tricks the brain into releasing natural growth hormone instead of just injecting synthetic GH.
View Study"Diagnosis and testing for growth hormone deficiency across the ages"
Yuen KCJ, et al., 2023
Finding: A major medical review concluded that macimorelin is safer, easier, and more practical than the traditional dangerous insulin test for diagnosing low growth hormone in adults of all ages, making it the new preferred way to test for growth hormone problems.
View StudyFrequently Asked Questions