Macimorelin, explained simply

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

A worked example

For a 70 kg patient: 70 kg × 0.5 mg/kg = 35 mg total dose. If using a 25 mg vial, reconstitute with appropriate volume of water to achieve desired concentration.

Working out your dose

Dose (mg) = Patient body weight (kg) × 0.5 mg/kg. Round dose to nearest mg if needed. Single oral dose dissolved in water, taken on empty stomach.

Do not use it 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)

Be extra careful if…

• Patient has cardiac arrhythmias or structural heart disease
• Patient is taking a strong CYP3A4 inducer (e.g. carbamazepine, phenytoin, rifampin) — stop it and allow a washout, as it can cause a false positive result ^{5Source 5MACRILEN (macimorelin) for oral solution — US Prescribing Information, revised 07/2026Aeterna Zentaris / U.S. Food and Drug Administration · 2026}
• 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)

When to stop

• 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.

Medications

• Avoid
  QT-prolonging drugs (quinidine, disopyramide, amiodarone, sotalol, dofetilide, antipsychotics, antiemetics)

  Raises the risk of QT prolongation (a heart rhythm change) and torsades de pointes (a dangerous irregular heartbeat). Avoid the combination or use extreme caution with ECG (heart tracing) monitoring.

• Avoid
  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 (a drug-free waiting period) before testing ^{5Source 5MACRILEN (macimorelin) for oral solution — US Prescribing Information, revised 07/2026Aeterna Zentaris / U.S. Food and Drug Administration · 2026}

• Avoid
  Medications affecting electrolytes (diuretics, amphotericin B)

  Electrolyte abnormalities (imbalances of body salts) raise the risk of QT prolongation. Make sure potassium (K+) and magnesium (Mg+) levels are normal before testing.

• Avoid
  Corticosteroids or opioids

  May blunt the GH response to macimorelin. Stop them before testing if possible.

Other peptides

• Avoid
  Other growth hormone secretagogues (ibutamoren, anamorelin, etc.)

  Should not be combined during diagnostic testing, because it will throw off the results

• Avoid
  GH replacement therapy

  Stop GH therapy at least one week before testing; GH products can blunt the response to macimorelin ^{5Source 5MACRILEN (macimorelin) for oral solution — US Prescribing Information, revised 07/2026Aeterna Zentaris / U.S. Food and Drug Administration · 2026}

Supplements

• Avoid
  Supplements affecting heart rhythm (stimulants with high caffeine)

  Extreme caffeine intake before testing may increase arrhythmia risk. Recommend abstinence 24 hours before test.

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 Who Cannot Safely Have the Insulin Tolerance Test (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

Retesting agreed with the first result in 97% of people in the published trial. ^{1Source 1Macimorelin as a Diagnostic Test for Adult GH DeficiencyGarcia JM, et al. · 2018} It is a diagnostic test only: GH therapy must be stopped at least one week before testing, so it cannot be used to monitor treatment. ^{5Source 5MACRILEN (macimorelin) for oral solution — US Prescribing Information, revised 07/2026Aeterna Zentaris / U.S. Food and Drug Administration · 2026}