IGF-1 LR3: what has been measured about safety

IGF-1 LR3 has its own published literature, which several compounds on this site do not, and the effect that recurs across it is not the effect it is sold for. A one-week infusion into late-gestation fetal sheep reduced glucose-stimulated insulin secretion, which the 2021 paper traces to a defect intrinsic to the pancreatic islets [2]. A 2023 follow-up found that the attenuation seen during an acute infusion did not persist once the islets were isolated [3]. The 1999 infusion in beef heifers markedly reduced plasma glucose and every amino acid measured [4].

Those are measurements in fetal sheep and in cattle, at infusion rates chosen for those experiments, and none of them establishes what happens in an adult person. What they do establish is that the compound's own record contains a repeated pancreatic effect, which is worth a reader's attention precisely because it sits on a system the compound is not marketed to touch.

The second thing on this page is about the design rather than a study. This molecule is engineered with low affinity for the binding proteins that normally hold circulating IGF-1 in reserve [1], so it removes a control the body uses. Nothing published describes what removing that control does in an adult, because no study in a person has been run.

What this page reports, and what it does not

We report what has been documented. We do not prescribe what should be done.

Every sentence below reports what a named source measured or searched, in what system, and what it found. Where the answer is that nobody has looked, the page says which study would have looked and names the document recording that it was not done.

This page publishes no dose, no schedule, no route instruction and no list of people who should avoid anything. A list of who should not use a compound is a directions-for-use document, and it only makes sense if the reader is going to use the thing. What replaces it is a description of the compound and its record, written about the molecule rather than about the reader.

One distinction runs through everything here. Mecasermin is the unmodified 70 amino acid protein and it is an approved medicine with a label, a manufacturer and two post-marketing registries. This analog carries a substitution and a thirteen amino acid extension specifically so that it behaves differently, so the approved protein's file is not this molecule's file in either direction.

What has been measured

The finding the literature keeps returning to

Three published experiments name this analog and measure something about insulin or glucose, and all three land in the same place. A one-week infusion into late-gestation fetal sheep reduced glucose-stimulated insulin secretion, and the 2021 paper attributes that to a defect intrinsic to the islets rather than to anything upstream of them [2]. A 2023 study of an acute infusion in the same preparation found the attenuation did not persist once islets were isolated, which narrows what the effect is rather than removing it [3]. And the 1999 intravenous infusion in beef heifers markedly reduced plasma glucose along with the plasma concentrations of every amino acid measured [4].

The receptor overlap gives a plain reason to expect this rather than to be surprised by it. The IGF-1 receptor and the insulin receptor are closely related, and a molecule engineered to reach the first at higher free concentrations is operating next to the second.

Be exact about what those experiments are. Two are infusions into a fetus in utero and one is an eight hour infusion into cattle under restricted feeding, at rates chosen to answer a physiological question about those preparations. None of it is a measurement in an adult person, and none of the three was designed to characterise safety. [Animal] [2] [3] [4]

What the modification removes

Circulating IGF-1 is almost entirely bound to carrier proteins, and those carriers decide how much reaches tissue at any moment. That is a control system rather than an obstacle. The 2025 sheep study describes this analog in exactly those terms: low affinity for the IGF binding proteins and high affinity for the IGF-1 receptor [1].

So the design is not a stronger version of the hormone. It is a version that evades a regulator the body uses, and the point of the modification is that it behaves differently from the native protein. Nothing in the published record addresses what that produces in an adult over time, in any species, because the study has not been run.

The same 2025 study is also the most recent measurement of what the analog does, and its title reports the result: it did not promote growth in the late-gestation growth-restricted fetal sheep it was given to [1]. That is an efficacy finding rather than a safety one, and it is on this page because it is the only recent experiment that administered the analog and reported what happened. [Animal] [1]

What the one review of actual use reports

A 2026 review in Frontiers in Endocrinology covers performance-enhancing peptides marketed as research compounds and names IGF-1 Long R3 among them, alongside the growth hormone releasing hormone analogues, the growth hormone releasing peptides, ipamorelin, hexarelin and AOD-9604. It describes reported adverse effects across that group spanning endocrine and metabolic disturbance [5].

That is the only current published work addressing this compound as it is actually used rather than as an infusion in a research animal, and its limits are the limits of its design. A review of self-administration practice and clinical encounters is not a trial: it does not measure how often anything happens, it does not separate one compound from another within the group it covers, and it cannot attribute an effect to this molecule specifically. [Anecdote] [5]

Why the approved protein's file does not answer here

Mecasermin is the same protein without the modification, and it is an approved medicine: a biologics licence application, a named manufacturer, a label carrying warnings and monitoring requirements, and two long-running post-marketing registries [8]. That file is the most complete safety documentation for anything in this protein family.

It is documentation about a different molecule. This analog exists because someone wanted the hormone to behave differently in circulation, and it does: the whole modification is a change in how much free hormone reaches tissue and for how long. Evidence about the native protein under its own binding-protein control is not evidence about a version built to evade that control.

The direction matters too. Nothing in the mecasermin record covers an adult with normal IGF-1, because the licensed population is children with a severe deficiency of it. What that record does hold is set out, with its sources, on the mecasermin safety record, linked under Safety records this page refers to at the foot of this page.

What has not been characterised

Set out plainly, so the shape of the gap is visible rather than implied. Nothing is published on what a single large exposure to this analog does in a person, what repeated exposure does over weeks or months, whether it affects DNA, what it does in pregnancy or to offspring, or whether long-term exposure produces tumours. A registry sweep on 23 August 2026 returned 29 records naming mecasermin or recombinant human IGF-1 as an intervention and not one of them names LR3 or Long R3 [6].

Nothing is published on the injected route in a person, which is the route the analog is sold for. There is no human half-life, no exposure figure and no route comparison. The animal work delivered continuous infusions into research preparations, one of them a fetus in utero, and an infusion rate chosen for a sheep is not a route a person uses.

Whether the pancreatic effects measured in fetal sheep have any counterpart in an adult is unaddressed anywhere in the published record. That is the single most specific open question on this page, and it is open in both directions.

Our takeThe most useful thing on this page is the direction the compound's own literature points. Three published experiments administered this analog and measured something, and all three landed on insulin and glucose rather than on muscle. That is not a warning: fetal sheep and cattle under restricted feeding are not adults buying a vial, and none of those studies was built to characterise harm. It is a description of where the only measurements that exist actually are, which is somewhere other than where the marketing is.

Talk to a licensed clinician about anything concerning your health.

Frequently asked questions

References

1. White A, Stremming J, Wesolowski SR, Al-Juboori SI, Dobrinskikh E, Limesand SW, Brown LD, Rozance PJ. IGF-1 LR3 does not promote growth in late-gestation growth-restricted fetal sheep. Am J Physiol Endocrinol Metab. 2025. PMID 39679943 DOI 10.1152/ajpendo.00259.2024 Describes the analog as having low affinity for IGF-binding proteins and high affinity for the IGF-1 receptor. Read as an abstract; the full text was not read.
2. White A, Stremming J, Boehmer BH, Chang EI, Jonker SS, Wesolowski SR, Brown LD, Rozance PJ. Reduced glucose-stimulated insulin secretion following a 1-wk IGF-1 infusion in late gestation fetal sheep is due to an intrinsic islet defect. Am J Physiol Endocrinol Metab. 2021. PMID 33938236 DOI 10.1152/ajpendo.00623.2020
3. White A, Stremming J, Brown LD, Rozance PJ. Attenuated glucose-stimulated insulin secretion during an acute IGF-1 LR3 infusion into fetal sheep does not persist in isolated islets. J Dev Orig Health Dis. 2023. PMID 37114757 DOI 10.1017/S2040174423000090
4. Hill RA, Hunter RA, Lindsay DB, Owens PC. Action of long(R3)-insulin-like growth factor-1 on protein metabolism in beef heifers. Domest Anim Endocrinol. 1999. PMID 10370861 DOI 10.1016/s0739-7240(99)00015-6 Reports a tendency for whole-body and skeletal muscle protein to be conserved, alongside markedly reduced plasma concentrations of all amino acids measured and of glucose.
5. Dominikowski A, Rękoś Z, Olejarz M, Szczepanek-Parulska E, Domin R, RuchaŁa M. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis: bridging the gap between clinical evidence and patient self-administration. Front Endocrinol (Lausanne). 2026. PMID 42395176 DOI 10.3389/fendo.2026.1822475 Names IGF-1 Long R3 among agents marketed as research compounds. Read in full on 23 August 2026.
6. ClinicalTrials.gov. Registry sweep for mecasermin, Increlex, mecasermin rinfabate, iPlex, rhIGF-1, IGF-1 LR3 and Long R3 IGF-1, run through the v2 API on 23 August 2026 and filtered to records whose interventions name one of those drugs. Twenty-nine records survive the filter and none names LR3 or Long R3. An unfiltered query returns 135, because IGF-1 is a widely measured biomarker. ClinicalTrials.gov. 2026. ClinicalTrials.gov
7. World Anti-Doping Agency. The 2026 Prohibited List. Section S2.3, Growth Factors and Growth Factor Modulators, carries the entry: Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues. The S2 class heading states that the class is prohibited at all times, in and out of competition, and that all substances in it are non-Specified. Read in full on 23 August 2026. World Anti-Doping Agency. 2026. WADA 2026 Prohibited List
8. US Food and Drug Administration. INCRELEX (mecasermin) injection prescribing information, BLA021839, Ipsen Biopharmaceuticals, Inc. Read via the openFDA drug label API on 23 August 2026. Cited here for the approved population of the unmodified protein, which is a different molecule from the analog this page covers. openFDA drug label. 2026. Increlex label
9. Stremming J, Heard S, White A, Chang EI, Shaw SC, Wesolowski SR, Jonker SS, Rozance PJ, Brown LD. IGF-1 infusion to fetal sheep increases organ growth but not by stimulating nutrient transfer to the fetus. Am J Physiol Endocrinol Metab. 2021. PMID 33427051 Part of the same fetal sheep programme, cited for the programme's design rather than for a finding about the analog specifically.
10. Jonker SS, Giraud GD, Chang EI, Elman MR, Louey S. Coronary vascular growth matches IGF-1-stimulated cardiac growth in fetal sheep. FASEB J. 2020. PMID 32573852 Part of the same fetal sheep programme.
11. Stremming J, White A, Donthi A, Batt DG, Hetrick B, Chang EI, Wesolowski SR, Seefeldt MB, McCurdy CE, Rozance PJ, Brown LD. Sheep recombinant IGF-1 promotes organ-specific growth in fetal sheep. Front Physiol. 2022. PMID 36091374 Cited to show what the unmodified protein does in the same preparation the analog was tested in.
12. Lu Z, Liu N, Huang H, Wang Y, Tu T, Qin X, Wang X, Zhang J, Su X, Tian J, Bai Y, Luo H, Yao B, Zhang H. Recombinant expression of IGF-1 and LR3 IGF-1 fused with xylanase in Pichia pastoris. Appl Microbiol Biotechnol. 2023. PMID 37261455 Cited for how the analog is produced, not for any biological finding.