Growth Hormone
CAS
946870-92-4
Molecular Weight
9138
Da
A lab-engineered, long-acting version of IGF-1, the growth factor HGH works through. Native IGF-1 is a well-studied approved drug; this specific analog is a research reagent with essentially no human trials of its own.
Injectable
Intranasal Suitable
No
Research Compound
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IGF-1 LR3 carries a near-mythic reputation in bodybuilding as one of the most potent anabolics available, paired with equally loud warnings about its risk profile. The dominant practical debate is whether its systemic, long-acting action is worth it versus more localized or shorter-acting IGF approaches, with frequent caution about hypoglycemia and the unknown long-term consequences of driving IGF-1 signaling for extended periods. More careful voices repeatedly point out the conflation problem the score is built around: the impressive IGF-1 clinical literature is about the native hormone, not this engineered research analog.
IGF-1 LR3 (Long R3 IGF-1) is an 83-amino-acid analog of human insulin-like growth factor 1, which is itself 70 amino acids. Two engineered changes define it: a 13-residue extension added to the N-terminus ("Long") and a substitution at position 3 ("R3"). Together these cut its affinity for IGF-binding proteins by roughly 1,000-fold, which stops the protein from being sequestered in circulation and stretches its active half-life from minutes to roughly a day. It was designed as a research tool, and that is overwhelmingly how it is used: in cell culture and preclinical models, and in the gray-market anabolic scene.
IGF-1 LR3 binds and activates the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase on muscle, bone, and many other cell types, triggering the PI3K/Akt pathway (driving protein synthesis and cell survival) and the MAPK/ERK pathway (driving proliferation). It is the same receptor and the same downstream biology as native IGF-1; the modifications change pharmacokinetics, not the core signal. Because HGH exerts most of its effects through IGF-1, this compound sits one step downstream of growth hormone in the same axis.
Its legitimate use is as a research reagent for studying growth-factor signaling and cell proliferation, including stem-cell and tissue-engineering work. Off-label, it is used in bodybuilding for its anabolic and proliferative effects on muscle. There are no human clinical trials of the LR3 analog itself; the clinical IGF-1 evidence base belongs to native IGF-1 (mecasermin), a different, FDA-approved molecule.
No human safety data exist for the analog. Predictable risks from its mechanism include hypoglycemia (IGF-1 has insulin-like activity), and the broader, serious concern attached to sustained IGF-1R activation: IGF-1 signaling is implicated in tumor growth, so any compound that drives it potently and for a long duration carries a theoretical cancer-promotion risk that has not been studied for this analog. Tissue overgrowth and organ effects are additional theoretical concerns. It is explicitly not for human use.
Supplied as a lyophilized powder reconstituted in dilute acetic acid or bacteriostatic water and stored frozen; it aggregates in neutral solution. As an ~9 kDa protein it is delivered by injection in research and gray-market use, and there is no oral or intranasal route that delivers it intact.
IGF-1 LR3 is sold strictly "for research use only" and is not approved for human or veterinary use anywhere. It should not be confused with native IGF-1 (mecasermin, brand name Increlex), which is an FDA-approved prescription drug for severe primary IGF-1 deficiency. IGF-1 and its analogs are prohibited in sport under WADA. Distributing or administering the LR3 analog for human use is not permitted.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6589107/
https://www.ncbi.nlm.nih.gov/books/NBK596664/
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