Macimorelin (brand name Macrilen) is an FDA-approved oral drug used to diagnose adult growth hormone deficiency by stimulating the pituitary to release growth hormone.
What it does
Macimorelin works by activating growth hormone secretagogue receptors — proteins found in the pituitary gland and hypothalamus that, when triggered, signal the body to release growth hormone (GH). Think of these receptors as an 'on switch': macimorelin binds and flips the switch, producing a measurable GH surge within a predictable window. Because the response is dose-dependent and time-limited, clinicians can draw blood after administration and compare the GH peak against established thresholds to confirm or rule out deficiency.
Unlike older GH stimulation tests that use insulin-induced hypoglycemia (a stress response that carries real risks), macimorelin is taken as a single oral solution, making the test substantially safer and more convenient. Its half-life of roughly 4.1 hours means the pharmacological effect is short-lived and the test is completed in a single clinic visit.
What the evidence shows
Diagnosis of adult growth hormone deficiency (AGHD) Solid human trial data supporting FDA approval; validated against the insulin tolerance test as reference standard
A pivotal validation trial compared a single 0.5 mg/kg oral dose of macimorelin against the insulin tolerance test (ITT) — the historical gold standard for diagnosing AGHD — in adults with and without confirmed GH deficiency. The study demonstrated that macimorelin produced diagnostic sensitivity and specificity comparable to the ITT, with a GH cutoff of 2.8 ng/mL used to define a positive (deficient) result NCT02558829. This trial formed the basis of the FDA approval of Macrilen as the first oral GH stimulation test.
One practical limitation is that the test's safety and diagnostic accuracy have not been established in people with a BMI above 40 kg/m², a population where GH responses are blunted by obesity-related physiology. Results in that group should be interpreted cautiously NCT02558829.
Diagnosis of pediatric growth hormone deficiency Ongoing research; adult data does not automatically transfer to children
A dedicated pediatric trial is evaluating whether macimorelin can reliably identify GH deficiency in children, along with its safety profile in that age group NCT04786873. Adult pharmacokinetics and diagnostic cutoffs don't automatically apply to children, so this research is necessary before the test can be used routinely in pediatric practice. Results are pending.
Cancer cachexia (muscle and weight loss) Early-phase human trial; no efficacy conclusions available
A pilot trial tested repeated doses of macimorelin in patients with cancer cachexia — the severe muscle and weight loss that often accompanies advanced cancer — to assess safety and preliminary efficacy NCT01614990. This is a mechanistically plausible application, since GH secretagogues can increase appetite and lean mass, but no efficacy conclusions are available from this trial and macimorelin is not approved or established for this use.
How it's used
In the approved diagnostic protocol, the dose is 0.5 mg/kg taken orally as a single solution after an overnight fast of at least 8 hours. Blood is drawn at 45 and 90 minutes post-dose to measure serum GH. A peak GH below 2.8 ng/mL is considered consistent with adult GH deficiency NCT02558829. Macimorelin is not a daily supplement or a self-administered compound — it is a single-use diagnostic agent administered in a clinical setting. Certain medications that affect GH release (somatostatin analogues, glucocorticoids, proton pump inhibitors) need to be held before testing per prescribing guidelines Source 4.
Side effects and safety
In clinical trials, the most commonly reported side effects were mild and transient: headache, nausea, fatigue, dizziness, and diarrhea NCT02558829. Infection was noted at moderate severity in some participants. Because macimorelin is used as a single diagnostic dose, the side-effect profile relevant to most users is limited to that one-time exposure.
Relative contraindications include active malignancy, diabetes mellitus, and severe obesity (BMI > 40 kg/m²) — the last of which also compromises diagnostic accuracy. GH secretagogues as a class can affect insulin sensitivity, and with chronic high-dose use (not the approved single-dose context), edema and glucose dysregulation are theoretical concerns. Long-term safety data are not available and not relevant to the approved single-dose diagnostic use, but would matter if the compound were ever used repeatedly for other indications NCT01614990.
Bottom line
Macimorelin is the only FDA-approved oral test for adult growth hormone deficiency, backed by a proper validation trial against the insulin tolerance test. Its evidence base is narrow but solid for that specific diagnostic purpose. Outside of that context — self-administration, repeated dosing, or use as a GH-boosting compound — it has no established evidence and no regulatory backing.