Macimorelin dosing & administration
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
Dosing
How much do I take?
Oral: Taken by mouth as a capsule, tablet, or liquid.
Bioavailability Varies by compound — many peptides are largely broken down in the gut, so little reaches the blood intact; some small molecules absorb well.
0.5 mg/kg
Frequency
Single dose
Duration
One-time diagnostic test
FDA-label (Macrilen) single oral dose for diagnosing adult growth hormone deficiency. Granules (60 mg) are reconstituted in 120 mL water (0.5 mg/mL) and taken after an 8-hour fast; serum GH below 2.8 ng/mL across the 30/45/60/90-minute draws confirms deficiency [4].
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
Administration
How do I use it?
Reconstitution
What you need
Injection
Route
Not applicable — oral solution only
Best sites
Storage
Before reconstitution
Store in refrigerator at 2-8°C (36-46°F). Protect from light. Keep in original container until time of use.
After reconstitution
Once reconstituted in water, solution must be used within 30 minutes. Do not refrigerate after reconstitution. Discard any unused portion.
Signs of degradation — discard the vial
Sample daily schedule
Early morning (8-9 AM typical)
0.5 mg/kg body weight (25-60 mg based on weight) injection
Site: Oral administration
Patient must fast overnight (minimum 4 hours). After dosing, measure blood GH at baseline, 30, 45, and 60 minutes. Positive test = GH > 2.8 ng/mL.
Safety
Is it safe?
Side effects
Commonly reported: Dysgeusia (abnormal taste), Dizziness, Headache, Fatigue, Nausea, Increased Appetite
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.
Flagged pairings
- Other growth hormone secretagogues (ibutamoren, anamorelin, etc.) — Should not be combined during diagnostic testing as it will confound results
- GH or IGF-1 replacement therapy — Discontinue GH therapy 4+ weeks before testing if possible to avoid false negatives
- 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.
- Supplements affecting heart rhythm (stimulants with high caffeine) — Extreme caffeine intake before testing may increase arrhythmia risk. Recommend abstinence 24 hours before test.
Published research
What the studies show
Garcia JM, et al. · 2018
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.
Wang RL, et al. · 2025
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.
Yuen KCJ, et al. · 2023
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.
Aeterna Zentaris · 2017
FDA label: single oral dose of 0.5 mg/kg after 8-hour fast (60 mg granules reconstituted in 120 mL water, 0.5 mg/mL); stimulated serum GH below 2.8 ng/mL confirms adult GH deficiency.
Head to head
Macimorelin compared
Want the full picture?
The complete Macimorelin research profile: mechanism of action, clinical studies, effectiveness timeline, and FAQ.