IGF-1 LR3

Summary

IGF-1 LR3 is not a drug that failed trials. It is a laboratory reagent — a deliberately engineered variant ("mutein") of human IGF-1 built to have low affinity for the IGF-binding proteins that normally sequester IGF-1 in the bloodstream, and high affinity for the IGF-1 receptor. Strip away the binding proteins and you get a molecule that stays active far longer, which is exactly what you want when you are feeding cells in a flask and exactly what makes it attractive to people who assume more active means more muscle.

We could not find a single human clinical study of IGF-1 LR3. A PubMed search for the molecule restricted to human-tagged records returns fourteen papers; every one is a cell-culture experiment using human cell lines, plus one analytical case report of a seized black-market vial. There is no human trial, no human dose, and no human safety data.

The animal record does not straightforwardly support the muscle claim either. LR3 increased organ growth in guinea pigs and body weight in normal fetal sheep — but it reduced average daily gain and food intake in pigs, and produced no growth benefit in growth-restricted fetal sheep in a 2025 study. In several species, including pigs and guinea pigs, it suppressed the animal's own circulating IGF-1.

A different molecule — recombinant human IGF-1, mecasermin — is FDA-approved, for a rare pediatric growth disorder, with a contraindication for malignancy and warnings about hypoglycemic seizures. That approval does not extend to LR3, and the two are not the same thing.

What people use it for

Muscle growth is the whole proposition. IGF-1 sits downstream of growth hormone and is the mediator through which much of GH's anabolic signalling actually happens, so the logic runs: skip the GH secretagogues, take the end product directly, and take the version that the body's binding proteins cannot switch off.

Localized growth is the second claim and a more specific one — the belief, common in bodybuilding forums, that injecting into a lagging muscle produces growth in that muscle. LR3 is sold and discussed with site-injection protocols attached.

Recovery and healing rounds it out, on the general reasoning that a growth factor should accelerate tissue repair.

None of these has been tested in a human being that we could find.

Human evidence

There is none for IGF-1 LR3. That is the finding, and it is worth being precise about how we established it rather than asserting it. Searching PubMed for the exact molecule returns 47 records. Restricting to records tagged with human subject matter returns fourteen, and reading them, all fourteen are in vitro: IGF-binding-protein signalling in cultured fibroblasts, keratinocyte proteolysis, and proliferation studies in human breast and prostate cancer cell lines. The one remaining paper is a 2010 forensic analysis of a black-market injection vial. A search for human clinical trials of the compound returns nothing at all.

So the honest statement is not "the evidence is weak." It is that no one has published a study of what this molecule does in a person.

The approved relative. Recombinant human IGF-1 — mecasermin, sold as Increlex — is FDA-approved for growth failure in children aged 2 and older with severe primary IGF-1 deficiency (height and IGF-1 both at or below −3.0 standard deviations, with normal or elevated growth hormone) or with GH gene deletion who have developed neutralizing antibodies to GH. That is a real approval for a real and rare condition, and it grades S+ on its own terms.

It is also the source of most of the confusion on this page. Mecasermin is native-sequence human IGF-1. LR3 is an engineered variant designed to behave differently in the body — that is the entire point of engineering it. The label's own limitation of use is instructive: Increlex is not indicated even for secondary forms of IGF-1 deficiency such as GH deficiency or malnutrition, let alone for healthy adults wanting more muscle.

Animal / preclinical evidence

This is where LR3's actual literature lives, and it is more interesting — and less flattering — than the marketing suggests.

The effect depends on the species. In guinea pigs, a seven-day infusion stimulated organ growth. In finisher pigs, a four-day infusion at 180 µg/kg/day decreased average daily gain and food intake. The authors of that study noted the pattern directly: IGF-I analogues that bind poorly to binding proteins stimulate growth in the rat but inhibit growth in the pig. An anabolic agent whose direction of effect flips between two mammals is not a molecule anyone should be confident about in a third.

It suppresses the body's own IGF-1. In the pig study, LR3 reduced plasma IGF-I, IGFBP-3 and insulin, and cut mean growth hormone by 23% with a 60% reduction in the area under the GH peaks. The guinea-pig study likewise found LR3 infusion reduced plasma IGF-I, IGF-II and binding-protein concentrations. This is negative feedback working as designed: a long-acting IGF-1 signal tells the pituitary to stop making GH, and endogenous IGF-1 falls with it. Nothing about "the binding proteins cannot switch it off" prevents the axis upstream from switching itself off.

The most recent study is negative. A 2025 study infused LR3 into growth-restricted fetal sheep for a week. Body weights did not differ from vehicle. Circulating amino acids — including the branched-chain amino acids that support protein accretion — decreased in the treated group. The same lab's earlier work in normal fetal sheep found that LR3 at 6.6 µg/kg/h did raise body weight, but lowered insulin concentrations and glucose-stimulated insulin secretion.

And it is used to make cells proliferate — including cancer cells. Several of the human-cell papers above use LR3 precisely as a proliferation stimulus in breast and prostate cancer cell lines. That is a description of how researchers use the reagent, not a claim that injecting it causes cancer in people. It is relevant context all the same, and it connects to the safety section below.

Taken together: real preclinical work, genuinely mixed results, at least two clear negatives on growth, and a mechanism that partly undercuts itself. Combined with zero human data, the human muscle claim rests on community practice rather than on evidence, which is what B means.

Anecdotal / community reports

Low-confidence. These are community reports, not evidence. Not medical guidance.

The community reports are detailed and confident: dosing schedules in micrograms, injection timing around training, site-injection protocols for specific muscles, cycle lengths. The specificity is worth noticing precisely because none of it descends from a study — there is no human trial to have produced a schedule, so every number in circulation originated with somebody's guess and then got repeated.

Two reported effects are consistent with the pharmacology and worth taking seriously as warnings rather than benefits: hypoglycemia — IGF-1 has insulin-like activity, and the approved IGF-1 product's label carries a warning about hypoglycemic seizures — and a distinctive fullness or swelling in injected muscle, which reports frequently interpret as growth and which no measurement has ever distinguished from local fluid.

Doses used in published studies

Context only — not a recommendation. PeptideIQ Base does not provide dosing advice.

There is no human dose of IGF-1 LR3, because there is no human study of IGF-1 LR3. Everything in the table above is one of two things, and neither transfers.

The first is the approved label dose of a different molecule: mecasermin at 0.04–0.08 mg/kg twice daily, up to 0.12 mg/kg, given subcutaneously shortly before or after food, in children with a rare deficiency, with blood-glucose monitoring until a tolerated dose is established. Different molecule, different population, different purpose.

The second is animal infusion rates: 6.6 µg/kg/h in normal fetal sheep, 180 µg/kg/day in pigs. Both were continuous infusions in species whose responses to LR3 differ from one another, and one of them is the study where growth went down.

Any dosing figure presented as a human protocol for LR3 was not derived from a trial. There is no trial.

Safety & side effects

The safety profile of IGF-1 LR3 in humans is not merely thin — it is absent. No study has measured it. What follows is inference from the approved IGF-1 product and from the epidemiology, and inference is what it should be labelled as.

Hypoglycemia is the immediate concern. The Increlex label warns that severe hypoglycemia leading to hypoglycemic seizures has been observed with treatment, because IGF-1 has insulin-like hypoglycemic effects, and directs that it be given within about 20 minutes of a meal with glucose monitoring during dose-finding. LR3 was engineered to escape the binding proteins that restrain IGF-1's activity, so there is no reason to expect this risk to be smaller and some reason to expect it to be larger.

The malignancy signal is on the label, not hypothetical. Increlex is contraindicated in patients with malignant neoplasia or a history of malignancy, and the warnings state that several cases of malignant neoplasia have been observed in pediatric patients treated with it, with instructions to discontinue if evidence of malignancy develops. Separately, a pooled analysis of ten prospective cohort studies plus the EPIC cohort found a modest association between higher serum IGF-1 and colorectal cancer risk — a relative risk of 1.07 (95% CI 1.01–1.14) per standard deviation increase. That is an observational association with naturally varying levels, not evidence that injecting an IGF-1 analog causes cancer, and it should not be inflated into one. It is also not nothing, and it sits alongside a label contraindication and a research literature that uses this exact molecule to grow cancer cells in dishes.

The other label warnings for recombinant IGF-1 include anaphylaxis requiring hospitalization, intracranial hypertension (with funduscopic examination recommended), tonsillar and adenoidal hypertrophy, slipped capital femoral epiphysis, and progression of scoliosis. Several of those are specific to growing children. The first two are not.

And then there is what is actually in the vial. In 2010, analytical chemists examined a black-market injection vial and identified its contents as Long-R³-IGF-I carrying a His₆ purification tag still attached at the C-terminus. His-tags are added during protein synthesis to make purification convenient and are normally cleaved off; this one was not. The authors' conclusion is the sentence to take away from this page: the effects of His-tagged Long-R³-IGF-I in humans have not been elucidated or described, and the product was likely a by-product of biochemical research rather than something synthesised for injection.

Regulatory / legal status

IGF-1 LR3 is not an approved medicine anywhere. It has no FDA approval and no approval from any other regulator we could identify. It is manufactured and sold as a research and cell-culture reagent — the peer-reviewed description is that it is "usually produced for biochemical studies" — and material sold for injection has, in at least one analysed case, been exactly that: laboratory protein with the purification tag left on.

Mecasermin (Increlex) is approved, and it is a different molecule. Its indication is narrow: growth failure in children 2 and over with severe primary IGF-1 deficiency, or GH gene deletion with neutralizing antibodies to GH. It is contraindicated in malignant neoplasia or a history of malignancy, in closed epiphyses, and in known hypersensitivity. The label states explicitly that it is not a substitute for GH in approved GH indications and is not indicated for secondary IGF-1 deficiency. Presenting LR3 as a research-grade version of this drug is the C-graded claim above.

In sport, insulin-like growth factor 1 and its analogues are named on the WADA Prohibited List under S2.3, growth factors and growth factor modulators. Everything in class S2 is prohibited at all times, in and out of competition, and is a non-Specified Substance — the category with the least room for a reduced sanction.

Status can change and varies by country; this is not legal or medical advice.

Podcast / media mentions

IGF-1 LR3 rarely gets named in mainstream coverage. Where it appears is in bodybuilding and enhancement content, usually as the advanced option — the thing you graduate to after growth hormone, discussed with the fluency that comes from a long oral tradition rather than a literature.

The premise underneath it is not silly. IGF-1 really is the mediator through which much of growth hormone's anabolic signalling runs, and an analog that evades the binding proteins really does stay active longer. What the confident version omits is everything downstream of that premise: that the analog was designed for cell culture, that it suppresses the body's own IGF-1 and growth hormone in the animals where that was measured, that it reduced growth in pigs and did nothing in growth-restricted sheep, that no human has been studied on it, and that when someone finally analysed a vial from the market, what was inside was a research by-product with its purification handle still attached.