Growth hormone & secretagogues

GH and IGF-1 together support protein synthesis in muscle and connective tissue, shift body composition toward lean mass and away from fat, and are involved in bone turnover and skin collagen. In people with a genuine deficiency, whether from pituitary disease or in childhood, replacing GH is transformative and uncontroversial.

The interesting question is what happens in people who are not deficient, and here the literature is unusually clear and unusually deflating. A systematic review pooled the controlled trials of GH in healthy elderly adults and found a consistent picture: lean body mass rose by roughly 2 kg and fat mass fell by roughly the same, with no accompanying improvement in strength ^[12]. Adverse events were substantially more common in the treated groups: soft-tissue oedema, joint pain, carpal tunnel syndrome and gynaecomastia, and a higher rate of new-onset diabetes or impaired fasting glucose.

A companion review asked the athletic question directly and found that GH increased lean mass without increasing strength, power or endurance, and increased lactate during exercise ^[13]. Read together, those two results are the honest ceiling for the whole category. The body-composition change is real and the functional payoff has not been demonstrated. A large part of the early lean-mass gain is also water, because GH causes sodium and fluid retention, and a scan cannot tell water from muscle.

IGF-1 also carries a complication that a marketing page will never mention. It is a growth signal, and higher circulating IGF-1 is associated in epidemiology with a higher incidence of several cancers, while lower IGF-1 signalling is one of the more reproducible ways to extend lifespan in laboratory animals. Nothing here proves that raising IGF-1 modestly in a healthy adult causes harm, and nothing rules it out. It is a genuinely open question, and reviews of GH use outside medicine name it as one of the main unresolved safety concerns ^[14].

The pituitary takes three inputs. GHRH from the hypothalamus tells it to make and release GH. Somatostatin, also from the hypothalamus, is the brake. Ghrelin, mostly from the stomach, is a third input that both pushes GH release and lifts the somatostatin brake. The drugs in this category copy the first or the third, and which one they copy tells you almost everything about how they behave.

The history of the second lever is worth telling, because it happened backwards. Synthetic growth-hormone-releasing peptides were made in the 1970s and 1980s and clearly released GH, but they did not act at the GHRH receptor and nobody knew what they did act on ^[2]. In 1996 a Merck group cloned the receptor they bound and named it the growth hormone secretagogue receptor, GHS-R1a, and it was an orphan: a receptor with no known natural ligand ^[3]. The ligand was found three years later, in stomach tissue rather than in the brain, and named ghrelin ^[4]. So an entire drug class existed, and worked, for two decades before anyone knew which natural signal it was imitating. Structural work has since shown how ghrelin and the synthetic agonist ibutamoren occupy that receptor's pocket ^[15].

The two levers have genuinely different characters. GHRH analogues are constrained by the somatostatin brake and by IGF-1 feedback, so they tend to amplify the natural pulse rather than create a new one, and their effect is self-limiting. Ghrelin-mimetics push harder, are less constrained, and bring the rest of ghrelin's biology with them: appetite, gastric motility, and in several of them adrenal and prolactin effects.

That last point separates the members and is the single most useful fact in this section. In healthy volunteers, GHRP-2 and hexarelin release GH strongly but also raise ACTH, cortisol and prolactin ^[6]. Ipamorelin was developed specifically to fix that: in the original characterisation it released GH with a potency comparable to GHRP-6 while, unlike GHRP-6 and GHRP-2, producing no increase in ACTH or cortisol even at high doses ^[5]. That is what "selective" means in this family. It is not a claim about muscle versus fat; it is a claim about which other pituitary hormones come along.

Pairing a GHRH analogue with a ghrelin-mimetic produces more GH than either alone, and more than the two added together. That synergy is real and it has a clean mechanistic explanation rather than being a marketing claim ^[2].

Three things happen at once. The GHRH receptor signals through Gs and cyclic AMP, which drives synthesis and release of GH. GHS-R1a signals through Gq and phospholipase C, a different second-messenger pathway acting on the same cell, so the two signals are additive at the level of the somatotroph rather than competing for one mechanism. And separately, ghrelin-mimetics reduce hypothalamic somatostatin tone, meaning the brake comes off at the same moment the accelerator is pressed. A pulse released against a lifted brake is much larger than one released against a held brake.

This is the clearest real-world illustration of the logic described in nootropic stacking: combining mechanisms that converge on one output through separate routes, rather than stacking two things that do the same job. It is also why the classic pairings put one of each on the list.

There is a design argument buried in this that is worth making explicit. The reason short-acting compounds are usually preferred is that GH's effects depend on the pulse, and a drug that holds GH and IGF-1 elevated around the clock is closer to injecting GH than to enhancing the body's own rhythm. CJC-1295 with its albumin linker produced sustained GH and IGF-1 elevation for six days or more after a single dose in healthy adults, with mean IGF-1 rising by up to about 1.5 to 3 fold and staying up ^[7]. That is a different pharmacological object from a nightly pulse, and it inherits more of GH's problems, particularly the glucose ones.

This is the section the marketing skips. Secretagogues reliably do the pharmacological thing: they raise GH and IGF-1, often substantially, and that has been demonstrated many times. What has repeatedly failed to appear is the clinical consequence.

The most informative single trial is a two-year, randomised, placebo-controlled study of oral MK-677 in healthy older adults. It worked exactly as designed: GH and IGF-1 were restored to the levels of healthy young adults and stayed there for two years. Fat-free mass rose by about 1.6 kg against placebo. Strength did not improve, function did not improve, and fat mass did not fall. Fasting blood glucose rose by about 0.3 mmol/L and insulin sensitivity fell ^[9].

The two large disease trials landed the same way. In patients recovering from hip fracture, MK-677 did not improve the primary functional endpoint ^[10]. In mild to moderate Alzheimer's disease, a one-year randomised trial found no effect on cognitive or functional decline despite the expected rise in IGF-1 ^[11]. Those are proper trials with proper endpoints, and they are negative.

The exception proves the rule, and it is worth being precise about it. Tesamorelin has an approval, but it earned it in a specific population with a specific problem: HIV-associated lipodystrophy, where a phase 3 trial showed a roughly 15 percent reduction in visceral adipose tissue against placebo, with IGF-1 rising as expected ^[8]. Visceral fat in that setting was the disease. Generalising that result to a healthy adult who wants to recompose is not supported by it.

None of this means the compounds do nothing. It means the demonstrated effect is a change in the way the body stores tissue, without, so far, a demonstrated change in what the body can do.

Raising GH is not free, and the side-effect list is remarkably consistent across GH itself, the secretagogues and the disease acromegaly, which is what chronic GH excess looks like ^[1]. That consistency is the point: these are GH's effects, not quirks of a particular molecule.

Fluid retention comes first and fastest. GH promotes sodium reabsorption, so the early weight gain is partly water. Puffiness in the hands and face, joint aches and stiffness, and carpal tunnel symptoms all follow from the same mechanism, with the nerve symptoms caused by swelling in a fixed compartment ^[12].

Glucose is the one to take seriously. GH is a counter-regulatory hormone: it is what the body raises to keep blood sugar available during fasting. It drives lipolysis, and the resulting rise in circulating free fatty acids directly impairs insulin action in muscle and liver. So a sustained GH elevation reliably nudges fasting glucose up and insulin sensitivity down, and this has been measured directly with MK-677 over two years ^[9] and seen across the pooled GH trials as more new diabetes and impaired fasting glucose ^[12]. Anything that holds GH up continuously carries more of this risk than something that produces a nightly pulse; MK-677 is the clearest example on both counts.

Appetite surges with the ghrelin-mimetics, because appetite is ghrelin's day job. Some people want that and some are blindsided by it. Cortisol and prolactin rise with the non-selective GHRPs, which matters because chronically raised cortisol undoes a good deal of what people are taking these compounds for; see the HPA axis and cortisol. Ipamorelin is the member specifically designed to avoid that ^[5].

Desensitisation is a real and underdiscussed limit. GHS-R1a agonists lose effect with continuous exposure, hexarelin fastest of the group, which is the pharmacological reason intermittent dosing is the norm rather than a folk custom.

Finally, these are unapproved research peptides with the sourcing, purity and sterility caveats of the whole peptide category, and they are prohibited at all times in tested sport, where the growth-hormone secretagogues are banned as a class alongside GH itself and detection methods for both have been developed and deployed ^[14]. Not medical advice.

It is worth being precise about status, because the market blurs it deliberately. Exactly one compound on this page is an approved medicine for a body-composition indication: tesamorelin, for HIV-associated excess visceral fat ^[8]. Sermorelin was an approved medicine and was withdrawn for commercial rather than safety reasons. MK-677 was developed by a major pharmaceutical company through phase 3 and abandoned after its trials failed ^[10][11]. Everything else in the table is a research chemical that has never had a phase 3 programme at all.

Some naming traps are worth flagging, because they cause real confusion. CJC-1295 with DAC and CJC-1295 without DAC are not the same drug; the second is Modified GRF 1-29, a short-acting peptide, while the first carries the albumin-binding linker and lasts for days. MK-677 is not a SARM, despite being sold next to them; it is a ghrelin-receptor agonist and has nothing to do with the androgen receptor. See SARMs for that distinction.

What is genuinely not known is easy to state and rarely stated. Nobody has run a long-term controlled trial of a GH secretagogue in healthy adults with a functional endpoint, so whether a sustained modest IGF-1 elevation over years is net beneficial, neutral or harmful is unanswered. The cancer question that follows IGF-1 around is unresolved in exactly the same way: an association in epidemiology, a mechanism that is plausible, and no trial that could settle it. The two-year MK-677 study is the closest thing available, and it was not powered for anything like that ^[9].

The reasonable reading is that this class does something measurable and well characterised at the level of hormones, has repeatedly failed to convert that into a measurable functional benefit, and carries a metabolic cost that scales with how continuously it is applied.