01 What is IGF-1 LR3?
In plain English.
IGF-1 LR3 is a laboratory-made version of insulin-like growth factor-1 (IGF-1), the hormone the body uses downstream of growth hormone to drive cell growth and repair. It was designed in the early 1990s by an Australian research group (Francis and colleagues at CSIRO / GroPep) specifically as a reagent for cell culture, a tool to keep cells alive and dividing in a Petri dish. It was never intended, tested or approved as a medicine for humans. It is now widely diverted into the bodybuilding market, where it is injected for its anabolic effects.
This is the most important point on the page: IGF-1 LR3 is not the same molecule as mecasermin (Increlex). Mecasermin is recombinant human IGF-1, the natural sequence, and IS an FDA-approved drug, given under specialist supervision to children with severe primary IGF-1 deficiency. IGF-1 LR3 is a modified analogue: it has an arginine swapped in at position 3 and an extra 13-amino-acid tag bolted onto the N-terminus. Those modifications stop it binding to the body's IGF-binding proteins, which means it acts faster, harder and for longer than the natural hormone. It also means none of mecasermin's safety data transfers to it. Vendors blur the two; do not.
02 How it works
The simple version, then the science.
Native IGF-1 normally circulates bound to a family of carrier proteins called IGFBPs, which hold it in reserve and meter out how much actually reaches receptors. IGF-1 LR3 was engineered specifically to escape that brake: the modifications make it bind very poorly to IGFBPs, so a much larger fraction of the dose hits IGF-1 receptors on cells. That is why it works in a cell-culture flask at much lower concentrations than native IGF-1, and also why, when people inject it, the effects are stronger and the half-life is much longer (~20–30 hours vs ~12–15 hours).
Go deeper · the proposed mechanism
IGF-1 LR3 is an 83-amino-acid analogue: the parent 70-aa IGF-1 sequence, with Glu3→Arg3 and a 13-residue methionyl-porcine-GH-derived N-terminal extension. It retains agonist activity at the type 1 IGF receptor (IGF-1R) with similar affinity to wild-type IGF-1, but has dramatically reduced affinity for IGFBP-1 through IGFBP-6. The clinical and pharmacological consequences are predictable: higher free-fraction exposure, longer biological half-life, and, crucially for the safety profile, measurable cross-reactivity at the insulin receptor, because IGF-1 and insulin receptors are structurally related. That cross-reactivity is the mechanism behind the acute hypoglycaemia risk seen in users.
03 What it's used for
Each use graded by how strong the evidence actually is.
- PreclinicalCell culture growth supplementThe intended, validated use. LR3 IGF-I is used in serum-free mammalian cell culture (CHO, HEK293) to support growth and recombinant protein production, at concentrations roughly 200× lower than insulin (Voorhamme & Yandell 2006). This is laboratory science; it is not a human use.
- AnecdotalMuscle hypertrophy (bodybuilding)The reason it is sold. There are no controlled human trials of IGF-1 LR3 for muscle growth in healthy adults. The anabolic rationale is biologically plausible, IGF-1 signalling drives muscle protein synthesis, but plausible mechanism is not the same as proof of net benefit, and the safety profile (below) is the entire issue.
- AnecdotalRecovery and fat lossFrequently claimed online. No controlled human data in either direction. Often stacked with GH secretagogues; the combined safety profile of those stacks has never been studied.
04 What the evidence says
The pharmacology of IGF-1 LR3 itself is well-characterised, in test tubes and cell-culture flasks. Francis et al. (1992) and the subsequent GroPep papers established that the LR3 modifications produce an analogue with very low IGFBP affinity and ~3× the potency of native IGF-1 in cell-based assays. Tomas et al. (1996) showed that infused or injected LR3-IGF-I retains its superior potency over native IGF-1 in animal models. That is the entire credible evidence base for LR3 as a molecule. What does not exist is any controlled clinical trial of IGF-1 LR3 in humans for muscle, recovery, fat loss or any other purpose. The closest human data is for native IGF-1 (mecasermin) in the narrow indication of severe primary IGF-1 deficiency in children, and even that approved use carries a label flagging hypoglycaemia, intracranial hypertension, tonsillar hypertrophy and a need to monitor for benign and malignant neoplasms. None of those safety signals get smaller when you swap in a longer-acting, IGFBP-resistant analogue that no regulator has ever reviewed.
05 Dosing & administration
Reported in the literature, information not advice.
No safe or effective human dose has been established for IGF-1 LR3, because it has never been through human trials as a therapeutic. Online communities describe microgram-range subcutaneous doses, sometimes daily, sometimes pre- or post-workout, but these regimens are not medical protocols and they ignore the fact that the same dose can produce very different effective exposures depending on a user's endogenous IGFBP levels, body fat, recent food intake and concurrent insulin. The hypoglycaemia risk is dose- and timing-dependent and is what hospitalises people. A qualified clinician should be consulted before considering any peptide; for IGF-1 LR3 specifically, that conversation should also include why the approved IGF-1 drug (mecasermin) is dispensed only by paediatric endocrinologists and requires food intake around every dose.
06 Side effects & safety
IGF-1 LR3 has one of the most concerning safety profiles of any peptide in this directory, with three distinct categories of risk. First, acute hypoglycaemia. IGF-1 and insulin receptors are structurally related, and IGF-1 has measurable affinity for the insulin receptor. The LR3 modifications produce a longer-acting, IGFBP-resistant analogue that can drive blood glucose down, sometimes hours after an injection, sometimes overnight, often unexpectedly. Severe hypoglycaemia is a real acute risk and the most common reason users present to hospital. Even the approved native IGF-1 drug (mecasermin) requires patients to eat shortly before or after every dose, and lists hypoglycaemia as a primary adverse reaction. Second, a biologically plausible cancer signal. Large epidemiology, Renehan et al. 2004 Lancet meta-analysis, and follow-up work synthesised by Pollak in Nature Reviews Cancer, has repeatedly associated higher circulating IGF-1 with increased risk of prostate, premenopausal breast and colorectal cancers. Causation is not proven, but the mechanism (IGF-1R signalling promotes cell proliferation and inhibits apoptosis) is biologically plausible. Chronic, supraphysiological exposure from an IGFBP-resistant analogue is exactly the exposure pattern that concern was raised about. Third, acromegaly-like effects. Chronically elevated IGF-1, the downstream driver of growth hormone, produces the same constellation seen in acromegaly: jaw and brow bone overgrowth, soft-tissue thickening, organ enlargement, insulin resistance, and cardiovascular remodelling. These changes accumulate slowly and some are not reversible. Fourth, quality control is absent. Vials sold as "IGF-1 LR3" are unregulated research-chemical material. Identity, purity, sterility, dosing accuracy and endotoxin content are not guaranteed and have repeatedly failed independent testing in adjacent peptide markets. If you have a personal or family history of cancer, diabetes, pre-diabetes, or cardiovascular disease, or are pregnant, breastfeeding or under 25, IGF-1 LR3 is a particularly poor choice.
07 Where to buy (research use only)
Vetted on quality and transparency, not an endorsement to use.
08 Legal & regulatory status
- UKNot licensed as a medicine. The MHRA treats IGF-1 LR3 as an unlicensed medicinal product when sold for human use; sale or supply for human use is unlawful. Sold only as a research reagent for laboratory use.
- USNot FDA-approved as IGF-1 LR3. Not on the FDA's list of bulk substances permitted for compounding. The approved IGF-1 drug, mecasermin (Increlex), is a different molecule (recombinant native IGF-1), available only by prescription for severe primary IGF-1 deficiency.
- EUNo marketing authorisation in any EU member state for IGF-1 LR3. Sale for human use is unlawful; "research reagent for cell culture" framing is standard.
- AUS / CANProhibited for sale for human use in both jurisdictions. Available only as a laboratory research reagent.
- Sport (WADA)Prohibited at all times under S2.3, Growth Factors and Growth Factor Modulators. IGF-1 (mecasermin) and its analogues are explicitly named. IGF-1 LR3 is a textbook example of the analogues this section was written to cover.
09 Clinical studies & research
Primary sources. Read the science yourself.