IGF-1 LR3 (Long R3 IGF-I) is a laboratory-modified version of insulin-like growth factor 1 — the hormone through which growth hormone does most of its tissue-building work. It was created in Adelaide in the early 1990s as a cell-culture reagent, and that remains its only documented use. Two changes to its structure leave it binding poorly to the carrier proteins in blood, so in cell culture and in animals it acts longer and more strongly than natural IGF-1. The promises attached to it — faster muscle growth, recovery, "local growth" — rest on that biology alone. There are no human studies whatsoever: none registered, none published. The only window onto its risks comes from the licensed medicine made of native IGF-1 (mecasermin) and from the epidemiology of IGF-1 itself.
What IGF-1 LR3 is
IGF-1 is a 70-amino-acid protein made mainly in the liver under the influence of growth hormone; it owes its name to a structure resembling insulin. In blood, almost all IGF-1 circulates bound to carrier proteins (IGFBPs), which extend its life but also ration its access to tissue. IGF-1 LR3 is the same hormone with two edits introduced by the group of Francis and Ballard in Adelaide: at position 3 glutamic acid was swapped for arginine ("R3"), and a thirteen-amino-acid extension was added to the start of the chain ("Long"). Both changes weaken binding to the carrier proteins. In the original 1992 paper the authors state plainly that such analogues have properties making them "very useful reagents" for investigating IGF-I action — they were built as a laboratory tool, not as a drug candidate.
How it works — the receptor and the carrier proteins
IGF-1 docks onto the IGF-1 receptor (a "switch" on the cell), which activates the pathways governing protein synthesis, cell division and cell survival. LR3 acts on that same switch, but — as Francis's work showed — not more forcefully. In cultured rat muscle cells every long analogue stimulated protein and DNA synthesis and suppressed protein breakdown more strongly than native IGF-1, and the order of potency in cultures that secrete carrier proteins ran: Long [Arg3]-IGF-I and des(1-3)IGF-I first, then Long [Gly3]-IGF-I, then Long IGF-I, and ordinary IGF-1 last. The decisive result, however, came from chicken embryo fibroblasts, which secrete no detectable carrier proteins: there Long [Arg3]-IGF-I was weaker than IGF-1. In other words, LR3 "acts more strongly" only where binding proteins are tying up the natural hormone; against the receptor itself it does worse. Tomas's paper from the same institute puts a number on it: three times worse.
What was studied — cells and animals
The animal data are surprisingly inconsistent. In rats made catabolic with dexamethasone, LR3 proved roughly 2.5 times more potent than native IGF-1 at halting the loss of body weight and nitrogen; the authors stressed that this was all the more striking because LR3 binds the receptor less well — that is, its potency comes from evading the carrier proteins. In the same experiment IGF-1 and its analogues increased gut weight by up to 45 %, which shows that this kind of "growth" is not selective for muscle.
In pigs the outcome was the opposite. A four-day infusion of LR3 lowered average daily weight gain, food intake, and the concentrations of insulin, IGFBP-3 and the animals' own IGF-1; mean growth hormone fell by 23 % and the area under the GH peaks by 60 %. The authors open the paper by noting that analogues binding poorly to carrier proteins "stimulate growth in the rat but, in contrast, inhibit growth in the pig". That matters twice over: it shows the effect depends on the species, and it confirms that IGF-1 given from outside switches off the body's own growth-hormone axis through feedback. In pigs, adding growth hormone did not reverse that suppression.
No human studies — how we checked
We searched PubMed for "Long R3 IGF-I", "LR3 IGF-1", "LongR3" and "long arginine3 IGF-I", including with the human-studies filter and the randomised-trial filter. The result: not a single paper in which this analogue was given to a person. In the ClinicalTrials.gov registry none of those terms returns a single study. A 2026 review confirms this independently: it grades the evidence for GH-IGF-1-axis peptides on a four-tier scale and places IGF-1 LR3 in the lowest tier, D — "no peer-reviewed human studies", with the evidence resting solely on preclinical extrapolation and user narratives. The same review points out that the literature does not even record a route of administration or a half-life in humans.
This is not an accusation against the molecule; it is a description of the state of knowledge. An absence of human studies means neither "safe" nor "ineffective"; it means nobody has checked. For a compound whose mechanism is anabolic and mitogenic (cell-division promoting) at once, that gap is the single most important fact in this article.
The nearest window onto risk — mecasermin (rhIGF-1)
Native recombinant IGF-1 is a licensed medicine (mecasermin) for children with severe primary IGF-1 deficiency, in whom growth hormone does not work. It is a different molecule from LR3 — no extension, no R3 swap, and therefore normal binding to the carrier proteins — but it acts on the same receptor, which makes its record the best available approximation of the risks. In Chernausek's study, 76 children were treated for up to 12 years: height velocity jumped from 2.8 to 8.0 cm per year in the first year, and the commonest adverse event was hypoglycaemia, reported by 49 % of those treated, followed by tissue overgrowth at injection sites (32 %) and tonsillar or adenoidal hypertrophy (22 %). The European Eu-IGFD registry of 306 patients records 0.11 hypoglycaemia events per patient per year, of which 0.01 were serious; the risk was higher in people with a previous history of hypoglycaemia and in those with Laron syndrome. The hypoglycaemia follows from IGF-1 being a relative of insulin and lowering blood glucose at high concentrations. We deliberately give no methods of use and no doses.
IGF-1 and cancer risk
Here the human data are extensive — they simply concern concentrations of the body's own IGF-1 rather than administration of an analogue. A meta-analysis of individual participant data from 19 studies, covering up to 10,554 men with prostate cancer and 13,618 controls, found that men in the highest fifth of the IGF-1 distribution had an odds ratio for prostate cancer of 1.29 (confidence interval 1.16-1.43) against the lowest fifth. After mutual adjustment for the other proteins measured, only IGF-1 remained associated with risk; the authors write of "strong evidence that IGF-I is highly likely to be involved in prostate cancer development".
This needs reading carefully in both directions. Observational studies do not prove that giving IGF-1 causes cancer — they show that a higher concentration coincides with a higher risk. The association is moderate and spans the natural range of values, not the levels reached after injecting an analogue. On the other hand the mitogenic mechanism is the same one, so with no long-term safety data for LR3 at all, caution in anyone with a history of cancer is biologically justified even though no study has measured it.
Safety and the quality of the material
With a compound that has no human studies, the most tangible risk becomes what is actually in the vial. German anti-doping analysts described a black-market product in which they found Long-R3-IGF-I still carrying a histidine tag attached to the end of the chain. Such a tag is added to proteins to make them easy to purify in the laboratory, and it is normally removed afterwards. The authors state plainly that the effects of that molecule in humans "have not been elucidated or described", and that the product is more likely a by-product of biochemical research than a preparation made for injection.
The French anti-doping agency, while developing a detection method for these analogues, found "abundant signs of lower quality, oxidized peptide forms" in black-market products — common enough that the method had to be adapted to detect both forms. The same paper recalls that IGF-I analogues "were never approved for use in humans", and shows how fragile LR3 is: after a single injection in rats the unchanged peptide had disappeared within 4 hours, while its degradation products remained detectable for up to 16 hours.
The wider context — the bottom of the same cascade
IGF-1 LR3 sits at the end of a pathway whose upper reaches belong to growth hormone and to the peptides that coax the pituitary into releasing it — GHRH analogues such as tesamorelin, or ghrelin mimetics such as ipamorelin. The difference is fundamental: secretagogues leave the whole regulatory loop with the body, native growth hormone overrides it, and IGF-1 LR3 overrides one storey more — the liver and its rationing by carrier proteins. The pig data show that such a molecule does not "add to" the GH axis but shuts it down: the animals' own growth hormone and own IGF-1 both fell.
Summary
IGF-1 LR3 is a laboratory analogue of IGF-1 with two edits that weaken its binding to carrier proteins. Its advantage over the natural hormone comes precisely from that evasion rather than from a stronger pull on the receptor — it binds the receptor three times less well, and in cells without carrier proteins it performs worse than ordinary IGF-1. In catabolic rats it was 2.5 times more effective than IGF-1; in pigs the reverse, slowing growth and switching off the animals' own growth-hormone axis. In humans it has never been studied once — neither in PubMed nor in ClinicalTrials.gov, which a 2026 review confirms by placing it in the lowest evidence tier. The known risks come from the medicine made of native IGF-1 (hypoglycaemia in half the children treated, tonsillar hypertrophy in one in five) and from epidemiology, in which a higher IGF-1 concentration is associated with a higher risk of prostate cancer (odds ratio 1.29). State of the evidence: no human data; everything beyond the chemistry and the cell cultures is reasoning by analogy.
Sources
- Travis RC, Appleby PN, Martin RM, et al. A Meta-analysis of Individual Participant Data Reveals an Association between Circulating Levels of IGF-I and Prostate Cancer Risk. Cancer Research. 2016;76(8):2288-2300. PMID: 26921328. DOI: 10.1158/0008-5472.CAN-15-1551. pubmed.ncbi.nlm.nih.gov/26921328
- Chernausek SD, Backeljauw PF, Frane J, et al. Long-term treatment with recombinant insulin-like growth factor (IGF)-I in children with severe IGF-I deficiency due to growth hormone insensitivity. The Journal of Clinical Endocrinology and Metabolism. 2007;92(3):902-910. PMID: 17192294. DOI: 10.1210/jc.2006-1610. pubmed.ncbi.nlm.nih.gov/17192294
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- Tomas FM, Knowles SE, Owens PC, et al. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. The Biochemical Journal. 1992;282(Pt 1):91-97. PMID: 1371669. DOI: 10.1042/bj2820091. pubmed.ncbi.nlm.nih.gov/1371669
- Mongongu C, Coudoré F, Domergue V, et al. Detection of LongR3-IGF-I, Des(1-3)-IGF-I, and R3-IGF-I using immunopurification and high resolution mass spectrometry for antidoping purposes. Drug Testing and Analysis. 2021;13(7):1256-1269. PMID: 33587816. DOI: 10.1002/dta.3016. pubmed.ncbi.nlm.nih.gov/33587816
- Francis GL, Ross M, Ballard FJ, et al. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. Journal of Molecular Endocrinology. 1992;8(3):213-223. PMID: 1378742. DOI: 10.1677/jme.0.0080213. pubmed.ncbi.nlm.nih.gov/1378742
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For in-vitro laboratory research only. It is not a human medicine and is not for treatment.