# IGF-1 LR3: what the lab-made growth factor is, and what is known about it > Source: https://peptidewellnessusa.com/peptides/igf-1-lr3/ > Field guide · Peptide deep-dive > By PeptideWellness Editorial Team · Medically reviewed by Ksenia Petrushkina, PA-C, MPAS · Updated November 11, 2026 · 6 min read IGF-1 LR3 is a lab-made version of insulin-like growth factor 1, 83 amino acids long, built so that the proteins that normally hold IGF-1 in check barely bind it. It was designed for research and cell culture. It has never been tested in a human trial, it is banned in sport, and the approved IGF-1 drug it is often compared with carries warnings for low blood sugar and more. IGF-1 LR3 (also written LR3-IGF-1 or Long R3 IGF-I) is a modified version of insulin-like growth factor 1, the hormone through which growth hormone does much of its work. It was built in the early 1990s by adding 13 amino acids to the front of IGF-1 and changing one more, so that the binding proteins that normally hold IGF-1 in the blood barely recognize it [1]. That makes more of it free to act, at least in a dish and in rats. It was designed as a research tool and is still made as one. No study has ever given it to people: there is no published human trial and no registered one [9]. It is banned in sport [6]. And because it is sold online as a "research peptide" for muscle growth, the most useful comparison is with the one form of IGF-1 that is an approved drug, mecasermin, whose label lists low blood sugar, raised pressure in the skull and other serious effects [5]. ## What IGF-1 LR3 is Natural IGF-1 is a single chain of 70 amino acids held in shape by three internal bridges [5]. LR3 differs in two places: - **"L" for long.** A 13-amino-acid extension at the front: the first 11 amino acids of methionyl porcine growth hormone, then two more, valine and asparagine [1]. The full molecule is 83 amino acids. - **"R3" for arginine at position 3.** IGF-1 normally has glutamic acid there; LR3 has arginine [1]. At 83 amino acids, LR3 is a small protein rather than a peptide by most definitions. Our page on [what a peptide is](https://peptidewellnessusa.com/learn/what-is-a-peptide/) explains where the line usually falls. Both changes do the same job: they stop IGF-1 from being caught by IGF-binding proteins, the six proteins that carry most IGF-1 in the blood and control how much reaches tissue. ## How it behaves The early laboratory work set out the trade-off [1][2]: - In cell cultures whose cells release binding proteins, LR3 was more potent than IGF-1, because the binding proteins could not mop it up. - In cells that release no binding proteins, LR3 was less potent than IGF-1. On its own it binds the IGF-1 receptor about three times less tightly [2]. So LR3 is not a stronger IGF-1. It is an IGF-1 that escapes its usual brakes, and whether that matters depends on how much binding protein is around. The same feature cuts the other way in the body. Binding proteins keep IGF-1 in the blood for hours; without them, LR3 is cleared faster [3]. In rats it disappeared from the blood within about four hours of an injection into muscle [6]. ## What the animal studies found The studies are in rats, mostly from one Australian group in the 1990s. - **Rats made catabolic with a steroid.** Over seven days by continuous pump, LR3 was about two and a half times as potent as IGF-1 at reversing weight loss and nitrogen loss. Part of the gain was in gut weight, which rose by up to 45% [2]. - **Pump against injection.** When given by once- or twice-daily injection instead of a continuous pump, LR3's advantage shrank sharply. For reversing the loss of muscle in steroid-treated rats, it was barely as potent as IGF-1 [3]. - **Food-restricted rats.** LR3 kept body weight up by 3% to 8%, but muscle protein was not spared. In adult rats it increased muscle protein breakdown [4]. The pattern is not what the muscle-building claims suggest. The extra weight went partly to organs such as the gut, and the muscle effect, which is the one sellers advertise, was the weakest. LR3 is also used to study growth in fetal sheep and in cell culture, where it is added to growth media. These uses are why it is made commercially. ## Human evidence There is none. We searched PubMed for every common spelling of LR3 among papers indexed as human clinical trials, and ClinicalTrials.gov for any study using it as an intervention. Both returned zero on 10 November 2026 [9]. A 2021 anti-doping study states plainly that it has never been approved for use in humans [6]. There is also no evidence that natural IGF-1 improves physical performance in healthy people; this has not been studied in a proper trial [10]. ## Risks, from the approved relative With no human data on LR3, the nearest guide is mecasermin (Increlex), recombinant human IGF-1, approved for children whose growth fails because their bodies cannot make IGF-1 [5]. Its label is a list of what too much IGF-1 action can do: - **Low blood sugar.** IGF-1 acts partly like insulin. In mecasermin's trials, 42% of children had hypoglycemia at least once; five had severe episodes and four had seizures or lost consciousness. The label says to give it with a meal [5]. - **Raised pressure inside the skull,** with swelling of the optic nerve, vision changes and headache [5]. - **Enlarged tonsils and adenoids,** with snoring and sleep apnea [5]. - **Hip and spine problems** in growing children [5]. - **Cancer.** Mecasermin must not be used by anyone with cancer or a history of it. Cancers have been reported in children treated with it, more often at higher than recommended doses; whether the drug caused them is not known [5]. Whether LR3 behaves the same way in people is unknown. Escaping the binding proteins means more free IGF-1 activity per molecule while it lasts, which is the mechanism behind these effects. There is a product risk on top. Anti-doping laboratories testing black-market vials have found oxidized, lower-quality forms of LR3 [6], and one vial contained a version still carrying the chemical tag used to purify it in the lab, a form whose effects in people have never been described [8]. ## Legal status and sport IGF-1 LR3 is not an approved drug and has no FDA status for compounding. It is sold as a research chemical, which cannot lawfully be sold for people to inject. IGF-1 and its analogues are on the World Anti-Doping Agency's prohibited list, in and out of competition, and anti-doping laboratories have methods to detect LR3 in blood [6][7]. For the hormones upstream of IGF-1, see [sermorelin](https://peptidewellnessusa.com/peptides/sermorelin/), [ipamorelin](https://peptidewellnessusa.com/peptides/ipamorelin/) and [tesamorelin](https://peptidewellnessusa.com/peptides/tesamorelin/), and the [performance category](https://peptidewellnessusa.com/peptides/performance/) for how they compare. ## Frequently asked ### What is IGF-1 LR3? A lab-made analogue of insulin-like growth factor 1, 83 amino acids long, with a 13-amino-acid extension and arginine at position 3. Both changes stop IGF-binding proteins from holding it [1]. ### Is IGF-1 LR3 a peptide? By length it is a small protein: 83 amino acids, more than natural IGF-1's 70. It is sold alongside peptides [1][5]. ### Has IGF-1 LR3 been tested in humans? No. There is no published or registered human trial [9]. ### Does IGF-1 LR3 build muscle? Not shown. In rats it raised body weight, partly through organ growth, and was barely more effective than IGF-1 at protecting muscle when injected [2][3][4]. ### What are the risks of IGF-1 LR3? Unknown in people. The approved IGF-1 drug, mecasermin, carries warnings for low blood sugar, raised pressure in the skull and other effects, and is not used in anyone with cancer [5]. ### Is IGF-1 LR3 legal? It is not an approved drug and is banned in sport [6][7]. ## Human studies on record Counted from PubMed and ClinicalTrials.gov on October 9, 2026. A study counts only if participants were given IGF-1 LR3. [How we count, and the same figures for every peptide we cover](https://peptidewellnessusa.com/learn/human-trials-by-peptide/). **Neither registry lists a trial in which people were given IGF-1 LR3.** ## References 1. [Francis GL, 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. *J Mol Endocrinol.* 1992. PMID 1378742.](https://pubmed.ncbi.nlm.nih.gov/1378742/) 2. [Tomas FM, et al. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. *Biochem J.* 1992. PMID 1371669.](https://pubmed.ncbi.nlm.nih.gov/1371669/) 3. [Tomas FM, et al. Superior potency of infused IGF-I analogues which bind poorly to IGF-binding proteins is maintained when administered by injection. *J Endocrinol.* 1996. PMID 8708565.](https://pubmed.ncbi.nlm.nih.gov/8708565/) 4. [Tomas FM, et al. Insulin-like growth factor-I (IGF-I) analogue, LR3IGF-I, ameliorates the loss of body weight but not of skeletal muscle during food restriction. *Growth Horm IGF Res.* 2001. PMID 11472075.](https://pubmed.ncbi.nlm.nih.gov/11472075/) 5. [FDA. Increlex (mecasermin) injection, prescribing information, effective 18 May 2026: sections 1, 4, 5, 6 and 11. DailyMed, read 10 November 2026.](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d5b2b0e1-c523-468d-9a0b-4ae4e4a8dbce) 6. [Mongongu C, 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 Test Anal.* 2021. PMID 33587816.](https://pubmed.ncbi.nlm.nih.gov/33587816/) 7. [Thomas A, et al. Determination of LongR3-IGF-I, R3-IGF-I, Des1-3 IGF-I and their metabolites in human plasma samples by means of LC-MS. *Growth Horm IGF Res.* 2017. PMID 28668757.](https://pubmed.ncbi.nlm.nih.gov/28668757/) 8. [Kohler M, et al. Detection of His-tagged Long-R3-IGF-I in a black market product. *Growth Horm IGF Res.* 2010. PMID 20675162.](https://pubmed.ncbi.nlm.nih.gov/20675162/) 9. [PubMed (LR3 spellings, humans and clinical-trial filters) and ClinicalTrials.gov (IGF-1 LR3 as intervention), searched 10 November 2026.](https://clinicaltrials.gov/search?intr=IGF-1%20LR3) 10. [Guha N, et al. IGF-I abuse in sport. *Curr Drug Abuse Rev.* 2009. PMID 20443773.](https://pubmed.ncbi.nlm.nih.gov/20443773/) > Editorial note: Informational only — not medical advice. IGF-1 LR3 is not approved for any use in people.