IGF-1 Long R3 (Long-Arg3 IGF-1)
Modified recombinant IGF-1 with extended half-life
At a glance
What it is: Modified recombinant IGF-1 with extended half-life.
Primary research applications:
- Research reagent (original purpose)
- Claimed: muscle hypertrophy in bodybuilding circles
Editorial summary: IGF-1 LR3 is a laboratory research reagent, not a therapeutic. Its popularity in bodybuilding rests on extrapolation from IGF-1 biology rather than RCT evidence. Systemic IGF-1 elevation carries meaningful long-term risk considerations.
What is IGF-1 LR3?
IGF-1 LR3 is a modified form of recombinant human IGF-1 with a 13-amino acid N-terminal extension and an arginine substitution at position 3. The changes reduce its binding to IGF-binding proteins, dramatically increasing its circulating half-life and bioavailability. It was developed as a research reagent for cell culture.[1]
Mechanism of action
Native IGF-1 drives the anabolic and growth-related effects downstream of GH. LR3 modification extends activity by evading IGFBPs. Effects on tissues include muscle hypertrophy, hyperplasia, and nutrient partitioning.
What the research shows
The peer-reviewed literature on IGF-1 LR3 is summarized below across two tiers: human research (the highest standard), and preclinical / emerging research (animal models and early-stage human work).
Claims and the evidence behind them
This table summarizes commonly discussed claims and how the published evidence weighs in. The aim is clarity — supported claims, claims that look promising but need more data, and claims that outrun the science.
| Claim | What the evidence shows | Verdict |
|---|---|---|
| Produces muscle hypertrophy | Animal and cell-culture evidence | Plausible |
| Safe for human use | Unsupported; long-term human safety not characterized | Unsupported |
| Has been tested in humans as a drug | No; LR3 is a research reagent | Unsupported |
Reported user experiences
How the research describes administration
Used by injection in grey-market settings. This is not a validated clinical use.
Editorial note
Administration details above describe how the peptide is given in published studies. We summarize this for educational completeness — these descriptions are not protocols, dosing recommendations, or instructions for personal use. Decisions about treatment require an appropriately licensed clinician.
Safety considerations and open questions
The takeaway
IGF-1 LR3 is a research reagent being used off-label in bodybuilding with no clinical trial evidence to support either efficacy or safety. Of the peptides on this site, it's one of the ones with the largest gap between use and evidence.
Frequently asked questions
Is IGF-1 LR3 a drug?
No. It was developed as a cell culture research reagent. No therapeutic approval exists.
Does IGF-1 LR3 cause cancer?
No direct evidence in humans. Epidemiologic data associating elevated IGF-1 with cancer risk raises concern about chronic pharmacologic exposure, but direct human studies do not exist.
Is IGF-1 LR3 banned in sport?
Yes — IGF-1 and its analogs are on the WADA prohibited list.
References
- Tomas FM, Knowles SE, Owens PC, Chandler CS, Francis GL, Read LC, Ballard FJ. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. Biochem J. 1992;282(Pt 1):91-7. https://pubmed.ncbi.nlm.nih.gov/1540151/
- Yakar S, Adamo ML. Insulin-like growth factor 1 physiology: lessons from mouse models. Endocrinol Metab Clin North Am. 2012;41(2):231-247. https://pubmed.ncbi.nlm.nih.gov/22682628/
- Pollak M. Insulin and insulin-like growth factor signalling in neoplasia. Nat Rev Cancer. 2008;8(12):915-928. (Important context for IGF-1 / cancer-risk discussion.) https://pubmed.ncbi.nlm.nih.gov/19029956/