The GH secretagogue family tree — Sermorelin, CJC-1295, Ipamorelin, GHRPs, MK-677, Hexarelin

The tree has two main branches. Every GH secretagogue in common use falls into one of two mechanistic categories, and the distinction is the single most important thing to understand about this class.

The first branch is the GHRH analogs. These compounds mimic growth hormone-releasing hormone — the peptide your hypothalamus naturally produces to stimulate GH release from the anterior pituitary. Sermorelin, CJC-1295 (with and without DAC), and tesamorelin all belong here. They work by binding to the GHRH receptor on pituitary somatotroph cells — the GHRHR — and triggering GH secretion through the same pathway that your own hypothalamus uses. The pituitary's normal regulatory machinery remains operational. Somatostatin — the inhibitory signal that acts as a brake on GH release — continues to function. The feedback loop stays intact. What these compounds do is essentially amplify the signal the pituitary is already listening for, rather than sending an entirely new kind of message.

The second branch is the GHRPs and ghrelin mimetics. GHRP-2, GHRP-6, Hexarelin, Ipamorelin, and MK-677 all work through a completely different receptor: the GHS-R1a, sometimes called the ghrelin receptor or the growth hormone secretagogue receptor. Ghrelin is a peptide hormone produced primarily in the stomach, involved in appetite and energy balance — and it also, separately, stimulates GH release through this receptor. The GHRPs and ghrelin mimetics capitalize on that second function. They send a GH-stimulating signal through a pathway entirely distinct from GHRH signaling, at a different point in the regulatory architecture.

This two-receptor reality is not just academic. It has a practical consequence that shapes how these compounds are often combined: GHRH-analog stimulation and GHS-R1a stimulation are additive, and in some research contexts appear to be synergistic. When you activate both pathways simultaneously, GH release exceeds what either compound would produce alone by more than simple addition predicts. This is the physiological rationale behind the pairing strategy — taking a GHRH analog like CJC-1295 or Sermorelin with a GHRP-class compound like Ipamorelin or GHRP-2. The pituitary receives two distinct, complementary signals at once, and the response is amplified accordingly. It's roughly analogous to pressing the accelerator while releasing the brake, rather than doing either alone.

Within the GHRH analog branch, the differences are primarily pharmacokinetic. Sermorelin is the shortest-acting: it has a plasma half-life of less than thirty minutes, which means it produces a discrete, short GH pulse, largely mimicking the endogenous pulsatile pattern. It requires more frequent dosing — typically daily, often at bedtime — and produces relatively modest, physiologically bounded GH and IGF-1 elevation. It was at one point FDA-approved for evaluating GH deficiency in children, and adult use through compounding pharmacies is largely off-label.

CJC-1295 without DAC (also called mod-GRF 1-29) is a modified GHRH fragment with a half-life of roughly thirty minutes to an hour — similar to Sermorelin in its short-acting character but chemically stabilized against enzymatic degradation, which makes it somewhat more potent dose-for-dose. CJC-1295 with DAC is an entirely different pharmacokinetic story. The DAC — Drug Affinity Complex — is a modification that allows the peptide to bind to albumin in the blood, dramatically extending its half-life to somewhere between six and eight days. A single injection produces sustained GH-stimulating activity for days. This fundamentally changes the character of its effect: rather than discrete pulses, it produces a blunted but prolonged elevation in GH and IGF-1 that departs from the physiological pulsatile pattern. Whether that departure is a trade-off worth accepting depends entirely on what you're trying to accomplish.

Tesamorelin is the GHRH analog with the most direct clinical evidence behind it — it's FDA-approved, under the brand name Egrifta, specifically for HIV-associated lipodystrophy, where visceral fat accumulation is a recognized complication of antiretroviral therapy. It's a chemically stabilized full-length GHRH analog with a meaningful half-life advantage over native GHRH, producing sustained pituitary stimulation and measurable visceral adipose reduction in that population.

In the GHS-R1a branch, the differences are primarily about selectivity and pleiotropic effects. Ipamorelin is the most selective compound in this class: it stimulates GH release with minimal effect on cortisol and prolactin. This makes it the cleanest GHRP-class option for people who want GH stimulation without engaging the stress-hormone axis. The trade-off is that it's also somewhat less potent per dose than the less selective compounds.

GHRP-2 and GHRP-6 are older, more broadly studied compounds that produce meaningful GH release but come with off-target effects. GHRP-2 elevates cortisol and prolactin in a dose-dependent fashion. GHRP-6 does the same, and adds significant appetite stimulation — a consequence of its ghrelin-receptor activity that goes beyond GH release and touches the appetite-regulation side of ghrelin's role. For some applications, appetite stimulation is a desired effect; for others it's an inconvenience or a real problem.

Hexarelin occupies a distinctive position in the family because its off-target effects are both more pronounced and more pharmacologically interesting. It produces potent GH release but also has documented activity at cardiovascular tissue — specifically at cardiac receptors independent of GH release — that has been studied in the context of cardiac function and cardioprotection. This pleiotropic profile makes Hexarelin the most pharmacologically complex compound in the GHRP class and the one that requires the most careful clinical consideration. It also shows more pronounced pituitary desensitization with repeated dosing than most of its relatives.

MK-677, also called ibutamoren, is the odd member of the ghrelin-mimetic family: it's an orally bioavailable, non-peptide GHS-R1a agonist. Every other compound in both branches requires subcutaneous injection. MK-677 is taken by mouth, which is a significant practical difference. Its half-life is long — around twenty-four hours — which means once-daily oral dosing produces relatively sustained GHS-R1a activation. This sustained activation pattern is quite different from the discrete pulses produced by short-acting injectable GHRPs. MK-677 raises GH and IGF-1 reliably, but the sustained rather than pulsatile character of that elevation comes with metabolic trade-offs, particularly around insulin sensitivity. Appetite increase — through the same ghrelin-receptor mechanism as GHRP-6 — is a common and well-documented effect.

The selectivity dimension runs across both branches and is worth treating as a distinct decision axis. On one end: Ipamorelin is essentially clean GH stimulation with minimal hormonal crosstalk. On the other end: GHRP-2, GHRP-6, Hexarelin, and MK-677 all activate the ghrelin receptor in ways that extend beyond GH — touching cortisol, prolactin, appetite, and in Hexarelin's case, cardiac tissue. The GHRH analogs as a class are relatively selective for GH-axis stimulation, with tesamorelin having the most documented evidence base for a specific tissue outcome.

How does this map translate to clinical decision-making? The combination principle argues for pairing one compound from each branch. The duration principle argues for matching pharmacokinetics to goal: short-acting compounds for people who want discrete pulsatile GH patterns that approximate physiology; longer-acting options for people prioritizing sustained IGF-1 elevation or convenience. The selectivity principle argues for choosing the cleanest compound in each class when the goal is GH support without hormonal side effects — which typically means Sermorelin or CJC-1295 no-DAC on the GHRH side, and Ipamorelin on the GHRP side.

None of the GHRPs are FDA-approved. CJC-1295 with or without DAC is not FDA-approved. MK-677 is not FDA-approved. Sermorelin's pediatric approval was commercially withdrawn, and adult use is off-label. Tesamorelin's FDA approval is specific to HIV-associated lipodystrophy. Every other use of every compound in both branches operates in off-label or entirely unapproved territory and requires clinical evaluation and an informed prescribing provider.

The family tree doesn't make the decisions simpler. What it does is make them more honest — replacing a menu of interchangeable-seeming names with a map of distinct mechanisms, distinct durations, distinct trade-offs. That's the starting point for a conversation worth having.