Also known as: Long R3 IGF-1, Long Arg3 IGF-1, IGF-I LR3
Half-life: ~20-30 hours
Last reviewed: · Published:
IGF-1 LR3 (Long Arg3 IGF-1) is a synthetic analog of native insulin-like growth factor 1, modified at two positions: an arginine substitution at position 3 (which dramatically reduces affinity for IGF binding proteins) and an N-terminal 13-amino-acid extension (which further protects against IGFBP sequestration and extends circulating half-life). Together, these modifications transform the molecule from a tightly bound, short-acting hormone into a long-acting agent that signals freely at the IGF-1 receptor on muscle and other tissues for hours rather than minutes.
Native IGF-1 has a circulating half-life of around 10-12 minutes when administered exogenously, because 98% of it is bound in ternary complexes with IGFBP-3 and the acid-labile subunit. IGF-1 LR3 sidesteps this regulation entirely, producing a half-life of roughly 20-30 hours and an unbound signal at the IGF-1 receptor far in excess of physiological IGF-1 levels. This makes it dramatically more potent as a research tool and a peptide of interest in muscle growth contexts — but also accounts for its more pronounced side-effect profile compared to native IGF-1 or recombinant rhIGF-1 (mecasermin/Increlex).
IGF-1 LR3 was developed for cell-culture research (where IGFBP contamination of serum complicates use of native IGF-1) and is widely sold for that purpose. Its use in self-experimentation is widespread in bodybuilding circles, where it is typically run at 20-50 mcg per day for 30-50 day cycles, often stacked with growth hormone secretagogues. The combination of long half-life and unbound activity means hypoglycemia is a real risk and care must be taken with timing around meals.
IGF-1 LR3 was developed in the 1980s by GroPep Limited (Adelaide, Australia) as a research reagent. The two modifications (Arg3 substitution and N-terminal 13-aa extension) were specifically designed to circumvent IGFBP binding and produce a long-acting variant suitable for sustained-release research applications. It became widely available as a research-grade reagent in the 1990s and entered the bodybuilding peptide market in the early-to-mid 2000s. The native recombinant version of IGF-1 (mecasermin) was later FDA-approved as Increlex for severe primary IGF-1 deficiency in children — but IGF-1 LR3 itself has never been approved for any indication.
IGF-1 LR3 has the most significant acute side-effect potential of any commonly used muscle-growth peptide because of its sustained insulin-like activity and ability to drop blood glucose. Hypoglycemia is a genuine risk and users should eat a substantial carbohydrate-containing meal within 30 minutes of injection. Joint pain and carpal-tunnel symptoms are common at higher doses. Long-term safety in humans is unknown, and the theoretical concern about IGF-1-driven proliferation of pre-existing neoplastic cells should be taken seriously by anyone with a personal or family history of cancer.
Dose Range
20-50 mcg
Frequency
Once daily (SubQ or IM), pre- or post-workout
Duration
30-50 days per cycle
Dose Range
20-40 mcg per site
Frequency
Into trained muscle, post-workout
Duration
30-50 days per cycle
Dosing information is for educational purposes only. Consult a healthcare professional before using any peptide.
Typical Vial Size
1 mg
Water Type
Bacteriostatic water (BAC water) or 0.1M acetic acid for higher stability
Mixing Volume
1 mL
Half-Life
~20-30 hours
Molecular Weight
9,111 Da
Store reconstituted vial refrigerated at 2-8°C. Use within 14-21 days. Always inject with a known carbohydrate meal within 30 minutes to mitigate hypoglycemia risk. Never use without ready access to fast-acting glucose.
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FDA Status
Not FDA approved. Native recombinant rhIGF-1 (mecasermin/Increlex) is FDA approved for severe primary IGF-1 deficiency in children, but IGF-1 LR3 itself is not.
Legal Status
Unregulated research chemical. Prohibited by WADA.
USA
Not approvedResearch-only; mecasermin (native rhIGF-1) approved separately
EU
Not approvedNot authorized as medicinal product
UK
Not approvedClassified as research chemical
Australia
Not approvedDespite local origin (GroPep), not authorized for human use
Canada
Not approvedNot authorized for human use
Tomas FM, Knowles SE, Owens PC, Read LC, Chandler CS, Gargosky SE, Ballard FJ
Biochemical Journal (1993)
Direct comparison of IGF-1 LR3 against native IGF-1 in rat models showing markedly increased potency for muscle growth, consistent with the longer half-life and reduced IGFBP binding of the analog.
View Study →Russell-Jones DL, Bates AT, Umpleby AM, Hennessy TR, Bowes SB, Hopkins KD, Jackson N, Kelly J, Shojaee-Moradie F, Jones RH, Sönksen PH
American Journal of Physiology (1995)
Human study establishing that exogenous IGF-1 produces both growth-hormone-like (anabolic) and insulin-like (hypoglycemic) effects on muscle metabolism, framing the dual benefit-and-risk profile of all IGF-1 analogs including LR3.
View Study →Pollak MN
Nature Reviews Cancer (2008)
Comprehensive review of IGF-1 receptor signaling in malignancy, providing the basis for caution about IGF-1 LR3's sustained, supraphysiological receptor activation in users with cancer risk factors.
View Study →Truncated IGF-1 variant — site-specific, short-acting muscle growth signal.
A locally-acting IGF-1 splice variant released by muscle in response to mechanical load.
Endogenous activin/myostatin binding protein — the gene-therapy myostatin inhibitor.
Long-acting growth hormone releasing hormone analog for sustained GH and IGF-1 elevation.
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