IGF-1 LR3

It is like taking an engine-tuning trick that car factories use to stress-test parts on a workshop bench and bolting it into your own car for the motorway. The part is real and powerful and you understand what it does, but no one has ever crash-tested this exact set-up with a person inside.

Full agonist at the IGF-1 receptor (a receptor tyrosine kinase), activating IRS-1/PI3K/Akt and Ras/MAPK signalling to drive cell proliferation, protein synthesis and survival. The engineered N-terminal extension and the Arg3 substitution sharply reduce binding to the IGF binding proteins (especially IGFBP-3), so the molecule escapes the normal carrier-protein 'sink', circulates as a higher free fraction, is more potent and lasts considerably longer than native IGF-1. Like native IGF-1 it has weak insulin-receptor cross-reactivity, which underlies its hypoglycaemic potential. Administered parenterally (by injection).

Research contexts. Not proven uses, and not recommendations.

None for IGF-1 LR3 specifically. The molecule was engineered as a cell-culture reagent and has no published human pharmacokinetic, efficacy or safety studies for muscle growth, performance, recovery or any therapeutic use. Every claim of human benefit is extrapolated from native IGF-1 biology and laboratory work. The only solid human IGF-1 data come from mecasermin (native recombinant IGF-1), approved narrowly for severe primary IGF-1 deficiency in children, and from small investigational studies such as a controlled trial in Rett syndrome. These involve a different molecule used for different purposes, and hypoglycaemia was the dominant adverse effect. There is no basis to assume those efficacy results, or that safety profile, apply to a more potent, longer-acting, binding-protein-evading analogue self-administered for bodybuilding.

• !No human trials of IGF-1 LR3 itself for any purpose (efficacy and safety in people are simply unknown).
• !Hypoglycaemia: IGF-1 cross-reacts with the insulin receptor and suppresses hepatic glucose output; a more potent, longer-acting analogue raises a real risk of prolonged low blood sugar.
• !Theoretical cancer/neoplasia concern: chronic supraphysiological IGF-1R (mitogenic, pro-survival) signalling is biologically the wrong direction for anyone with, or at risk of, malignancy; long-term consequences in healthy users are unstudied.
• !Other adverse signals seen with mecasermin include lymphoid tissue hyperplasia, intracranial hypertension, slipped capital femoral epiphysis and tissue overgrowth.
• !Quality/identity risk: grey-market 'research chemical' vials are unregulated and may be underdosed, mislabelled, degraded, non-sterile or not actually contain LR3-IGF-1.
• !Sold explicitly as 'not for human consumption': no pharmaceutical-grade manufacturing, no medical oversight, no informed-consent framework.
• !Banned in sport by WADA at all times; use carries doping sanctions.

IGF-1 LR3 is not a licensed medicine in the UK (the MHRA has authorised no IGF-1 LR3 product). The only licensed IGF-1 medicine is mecasermin (native rhIGF-1, brand Increlex), a prescription-only medicine for severe primary IGF-1 deficiency in children; it is designated an orphan medicine and is a different molecule for a different purpose. IGF-1 LR3 is sold online as an unlicensed 'research chemical' labelled 'not for human consumption', a label that sidesteps but does not satisfy medicines law: marketing or supplying it for human use would engage the Human Medicines Regulations 2012 as an unlicensed medicinal product. As an IGF-1 analogue it is also a WADA-prohibited substance (class S2) at all times in sport. There is no legitimate, regulated route to obtain it for human use in the UK.

1. 01
   IGF-1 LR3 (Long R3 IGF-1) — structure, IGFBP affinity, potency and half-life, Wikipedia (2024)

   Encyclopaedic summary of the 83-residue structure (N-terminal extension + Arg3 substitution), reduced IGFBP binding, increased potency and extended half-life. Tertiary source, not primary literature, included only for the structural description.

2. 02
   Mecasermin (recombinant human insulin-like growth factor I) — Keating GM, Drugs (2008)

   Review of the licensed native rhIGF-1 medicine, the best-characterised human IGF-1 pharmacology and safety profile (notably hypoglycaemia), used here as a proxy since IGF-1 LR3 has no human data of its own.

3. 03
   INCRELEX (mecasermin) US Prescribing Information — indication (severe primary IGF-1 deficiency), hypoglycaemia, lymphoid hyperplasia and intracranial hypertension warnings, FDA Drugs@FDA label, BLA 021839 (2019)

   Regulatory label for the approved native IGF-1 product; documents the class adverse-effect profile.

4. 04
   Safety, pharmacokinetics, and preliminary assessment of efficacy of mecasermin (recombinant human IGF-1) for the treatment of Rett syndrome — Khwaja OS et al., PNAS (2014)

   Small controlled human IGF-1 study that illustrates genuine human IGF-1 evidence exists only for native rhIGF-1, not for the LR3 analogue.