IGF-1 LR3: What It Is, Where to Get It Prescribed, and What It Costs

Insulin-like growth factor 1 is a 70-amino-acid peptide hormone. Most of it is made by the liver in response to growth hormone, and it is the messenger through which growth hormone does most of its work on muscle, bone and connective tissue. It gets its name from its structural resemblance to insulin, a resemblance that turns out to matter clinically. In the blood, almost all of it travels bound to a family of six IGF binding proteins, which hold it in reserve, extend its life in circulation and control where and how fast it is released to tissue. Free IGF-1 is cleared within minutes; the bound reservoir lasts hours.

IGF-1 LR3 — "Long R3 IGF-1" — is a laboratory-designed analogue of that hormone, first described by an Australian group in the Journal of Molecular Endocrinology in 1992. It has 83 amino acids: the 70 of human IGF-1 plus a 13-amino-acid extension at the front, a leftover of the fusion-protein method used to make it in bacteria, and one substitution, an arginine in place of the glutamate at position 3 (the "R3"). That single swap is the point of the molecule. It sharply reduces binding to the IGF binding proteins, so far more of the peptide stays free and active. The 1992 paper reported it to be more potent than native IGF-1 in a rat muscle-cell assay even though it binds the receptor somewhat less tightly, precisely because the binding proteins were no longer capturing it.

IGF-1 LR3 was not designed as a medicine. Its main legitimate use for three decades has been as an ingredient in serum-free cell-culture media in biomanufacturing: because it is not neutralised by the binding proteins present in culture, a very small amount keeps cells growing and producing. Most of what is published about it comes from that world and from 1990s animal physiology, not from clinical medicine. Its use as a prescribed injection in adults is a later development that rests on the mechanism, on animal work, and on what is known about native IGF-1 in people.

It is not growth hormone, and it is not a growth hormone secretagogue. It does not prompt the pituitary to release anything; it delivers the downstream signal directly, and in animal work it lowers the body's own growth hormone and IGF-1 output while it is being given. It is not insulin, although it can lower blood glucose, which is covered below.

It is also not mecasermin. Mecasermin, sold as Increlex, is native human IGF-1 made by recombinant technology, and it is an FDA-approved biologic for growth failure in children aged two and over with severe primary IGF-1 deficiency. IGF-1 LR3 shares 70 of its 83 amino acids with that drug, but it is a different molecule with different pharmacokinetics. It has never been through an FDA approval process, it has no approved indication in anyone, and none of the trial data behind Increlex belongs to it. This page keeps the two strictly apart, and any source that borrows the approved drug's evidence to describe the analogue is misleading you.

IGF-1 LR3 acts on the same receptor as the native hormone, the IGF-1 receptor, a tyrosine kinase found on muscle and most other tissues. Binding activates the PI3K–Akt cascade, which drives protein synthesis through mTOR and, through a second arm, switches off the FoxO transcription factors that run muscle protein breakdown. A 2007 review in In Vivo lays out this signalling in skeletal muscle in detail: activation and proliferation of satellite cells, their fusion into existing fibres, and increased protein synthesis in the fibre itself. This is the logic behind describing IGF-1 as anabolic — it is described as pushing synthesis up and breakdown down at the same time.

In the body, more than 99 percent of IGF-1 is bound to IGF binding proteins, and a 2018 review in Frontiers in Endocrinology describes why there are so many of them: they extend the hormone's life, keep it out of tissue until it is wanted, and buffer the insulin-like effects of the free fraction. The R3 substitution defeats most of that. The result described in cell work is more free peptide per microgram and a longer effective action. It is the whole reason the molecule was made, and it is also the reason the safety cautions later on this page are sharper than for native IGF-1: the binding proteins are a brake as well as a reservoir, and this analogue is built to bypass them.

IGF-1 and proinsulin are structurally related, and at high concentrations IGF-1 activates the insulin receptor as well as its own. Either route lowers blood glucose by pushing it into muscle and fat. This is not theoretical: a 1987 paper in the New England Journal of Medicine reported that recombinant native IGF-1 given to healthy adults produced hypoglycaemia, and the approved native product carries a warning for severe hypoglycaemia including seizures. For a version that is not held back by binding proteins, this is the mechanism a physician will be thinking about first.

In animals, IGF-1 delivered locally into muscle is described as producing hypertrophy and supporting regeneration. A 1998 paper in the Journal of Applied Physiology reported that infusing IGF-1 directly into a rat muscle increased its mass. A 1998 paper in the Proceedings of the National Academy of Sciences and a 2001 paper in Nature Genetics used viral or transgenic expression of IGF-1 confined to muscle in mice and reported preservation of muscle mass and function into old age and sustained regeneration. Note what those methods have in common: the IGF-1 was native, and it was placed in or made by the muscle rather than injected under the skin. They support the biology; they are not studies of the product on this page.

IGF-1 is the signal the pituitary reads to decide how much growth hormone to release, so supplying it from outside turns the body's own output down. A 1995 paper in the Journal of Endocrinology infused LR3 IGF-1 into guinea pigs and reported that circulating native IGF-1, IGF-2 and the binding proteins all fell while organs grew. That is the fundamental difference between this peptide and a secretagogue such as sermorelin or CJC-1295 / Ipamorelin, which act upstream and leave the feedback loop intact.

The classic LR3 studies used rats made catabolic with the steroid dexamethasone. A 1992 paper in the Biochemical Journal reported that IGF-1 and, more so, its modified variants including LR3 improved nitrogen balance and body-weight gain in those animals. A companion 1992 paper in the Journal of Endocrinology reported that the same analogues produced marked growth of the gut in the same model. Two things are worth holding onto: the animals were being actively wasted by a drug, which is not the situation of a healthy adult who trains, and the growth reported was not selective for skeletal muscle.

The muscle work is mostly native IGF-1 delivered locally or genetically, as described above: hypertrophy in the rat with local infusion, preserved force and regeneration in ageing mice with muscle-restricted expression. The endpoints were muscle mass, fibre size and measured force in the animal. There is no equivalent body of work on subcutaneous IGF-1 LR3 in healthy animals, let alone people.

The 1995 guinea pig study is the one that describes what a systemic, binding-protein-free IGF-1 does when it is simply infused: organs grew, and the animal's own IGF-1, IGF-2 and binding proteins fell. It is an honest picture of a signal that grows whatever tissue it reaches, and it is why the cancer-history question is not a formality on the intake.

Native IGF-1 has real human data, all of it with mecasermin. A 2007 paper in the Journal of Clinical Endocrinology and Metabolism reported long-term treatment of children with severe IGF-1 deficiency caused by growth hormone insensitivity, and the approved labelling pools five studies in 71 children: height velocity rose from about 2.8 to 8.0 cm a year in the first year of treatment, with hypoglycaemia reported at least once in 42 percent of them. The 1987 New England Journal of Medicine study described the metabolic effects of a single dose in healthy adults, including hypoglycaemia.

None of this can be read across to IGF-1 LR3. The population was children who could not make IGF-1; the drug was the native hormone, which is buffered by binding proteins; the doses were weight-based and titrated against glucose. The analogue was designed not to be buffered, which changes both what it might do and what it might do wrong.

This is the honest part. There are no randomised controlled trials of IGF-1 LR3 in humans, and there are essentially no published human clinical studies of it at all. The peer-reviewed human literature that mentions LR3 comes largely from anti-doping science; a 2010 paper in Growth Hormone & IGF Research, for example, describes analysing a black-market product sold as LR3 and finding a research-grade version with a purification tag still attached. What is prescribed rests on the mechanism, on animal work from the 1990s, on what native IGF-1 does in people, and on prescribers' clinical experience. That is a thinner base than for most of the peptides on this site, and this page says so plainly. Any source telling you it is "clinically proven" for muscle is wrong.

• It does not establish any effect on muscle size, strength or body composition in humans. That study has not been run.
• It does not establish a dose–response or a timeline in humans. Timelines below are clinical experience.
• It does not establish long-term safety, and the theoretical concern with any growth-promoting signal is exactly the kind that would only show up over years.
• It does not establish that findings for native IGF-1, favourable or unfavourable, transfer to the analogue at any particular dose.
• It does not establish efficacy for any condition as an approved treatment. It is not approved for anything.

Nothing visible happens. What matters in this window is what you notice, not what you see: shakiness, sweating, sudden hunger or light-headedness after a dose are the signs of low blood sugar, and they are a reason to eat something and call your physician, not to push on. Most people report nothing, which is normal.

This is where prescribers describe the things people started for, if they happen — mainly recovery between training sessions and a sense of muscle fullness. They are reports, not measurements, and they are inseparable from the training, sleep and protein that any change depends on. The first vial covers this window.

This is where a physician commonly looks at bloodwork — fasting glucose, and IGF-1 as a marker of what the axis is doing — and decides whether the next refill is the right call. Refills run on a 28-day cycle so that decision is built in rather than left to you.

Animal work describes the body's own IGF-1 and binding proteins being suppressed while the analogue is given, and recovering afterwards; there is no human data on how long any effect persists. Whatever was driven by training stays if the training stays. There is no withdrawal described, and no rebound has been reported, because the studies that would report it have not been done.

The directions are written in millilitres and in insulin-syringe units because that is what you can read off the barrel. 0.25 mL is 25 units on a U-100 syringe. You are not calculating anything — the number is printed on your medication, and the strength on the label tells your physician how many micrograms that volume delivers.

Subcutaneous means into the fat layer, not the muscle. Abdomen and thigh are the usual sites, rotated so the same spot is not used repeatedly. The reviewed directions specify Monday through Friday at bedtime, a five-days-on, two-off rhythm that prescribers use for this peptide. Because IGF-1 can lower blood glucose, the approved native product is labelled to be given close to a meal or snack; your own directions on timing and food are the ones to follow, and if they are unclear, ask before the first dose rather than after.

20 doses at five per week is four weeks, which is why refills run on a 28-day cycle. One vial per fill, always — not a stockpile.

Refrigerated, and it ships that way in insulated packaging with ice packs. Beyond-use dating runs from first puncture and is on the label. Keep it out of the freezer and out of light.

A secretagogue such as CJC-1295 / Ipamorelin or sermorelin acts on the pituitary; your own gland releases growth hormone in pulses, your liver makes the IGF-1, and rising IGF-1 feeds back to slow the pulses. IGF-1 LR3 delivers the end signal directly and, in animal work, turns that axis down while it is given. The secretagogues have human trials showing the hormone rise, and the feedback makes them self-limiting in a way this is not. For most people asking about "growth hormone peptides," a secretagogue is the first conversation and this is not.

Same receptor, different molecule, different world. Mecasermin is FDA-approved, for children with severe primary IGF-1 deficiency, with weight-based dosing titrated against blood glucose under specialist supervision; it is buffered in the blood by the binding proteins the way the body's own IGF-1 is. IGF-1 LR3 is built to avoid those proteins, has no approved use in anyone, and has no clinical trials. Nothing from the approved drug's labelling can be transferred to the analogue except the cautions, which is the direction this page transfers them.

Identical peptide, different context. The ASCEND blend pairs IGF-1 LR3 with CJC-1295, ipamorelin and tesamorelin in one vial, so the direct IGF-1 signal sits alongside upstream secretagogues. Whether that combination is appropriate, or whether a single peptide is, is your physician's decision, and the same cautions about blood sugar and cancer history apply to the blend.