What Is IGF-1 LR3?
IGF-1 LR3 (“Long R3 IGF-1”) is a synthetic analog of insulin-like growth factor 1 (IGF-1) engineered with an arginine-for-glutamate substitution at position 3 and a 13-amino-acid N-terminal extension, which sharply reduce its binding to IGF-binding proteins and greatly extend its half-life. Because it evades the binding proteins that normally sequester IGF-1, it is substantially more biologically potent and longer-acting than native IGF-1. It is manufactured mainly as a cell-culture research reagent and is diverted for anabolic (muscle-building) use. It is not an approved drug or dietary supplement, and there are no human efficacy trials for muscle growth.
Evidence
IGF-1 is the principal mediator of growth hormone’s anabolic effects, signalling through the IGF-1 receptor and the PI3K/Akt/mTOR pathway to drive protein synthesis and muscle growth — this is well-established biology, but it is not the same as evidence that injecting IGF-1 LR3 safely builds muscle in healthy people. The specific analog has been studied in animals: infusion of Long R3 IGF-I stimulated organ growth in guinea pigs while lowering circulating IGF-I and IGF-binding-protein concentrations 1Reference 1Animalhttps://pubmed.ncbi.nlm.nih.gov/7561636/View study →. I found no human clinical trial of IGF-1 LR3 for muscle growth, body composition, or performance in this pass — the bodybuilding use is entirely off-label/anecdotal and rests on mechanism and animal data, not on human outcome trials. Claims of potent, reliable muscle gain in humans are therefore unproven here.
Dosage & Safety
There is no established human dose for IGF-1 LR3; regimens circulating online are not from controlled human trials, and this page makes no dosing recommendation. As a potent growth factor it carries real theoretical and pharmacological risks — including hypoglycaemia (via cross-reactivity at the insulin receptor) and concern that broadly stimulating cell proliferation could promote growth of existing tumours — none of which are quantified for this compound in this pass. It is not an approved drug or supplement and is prohibited in sport. Human safety, long-term effects, drug interactions, and pregnancy/lactation safety are not established. This entry is informational only and is not an endorsement of use.
References
- Long R3 insulin-like growth factor-I (IGF-I) infusion stimulates organ growth but reduces plasma IGF-I, IGF-II and IGF binding protein concentrations in the guinea pig — animal (in vivo) study. J Endocrinol (1995). https://pubmed.ncbi.nlm.nih.gov/7561636/