IGF-1 LR3 (Long R3 IGF-1) is a lab-engineered version of insulin-like growth factor 1, the hormone that carries out many of growth hormone’s effects. It was designed in the early 1990s to escape the binding proteins that normally hold IGF-1 in check, which makes it more potent in cell experiments.[2] It is used to grow cells in biomanufacturing[4] and is sold online for muscle growth, but it has never been tested in humans.[1] It is not approved for any use, and WADA bans it.[16]

This profile covers what IGF-1 LR3 is, what animal and cell studies found, how it compares with approved IGF-1 (mecasermin) and growth hormone, risks, dosing claims and its legal status.

What is IGF-1 LR3?

Natural IGF-1 is a 70-amino-acid protein. In the blood, more than 80% of it is bound to IGF-binding protein 3 and a partner protein, and lower IGFBP-3 levels speed up its clearance.[7]

In 1992, Australian researchers made several IGF-1 “fusion” analogs. Long [Arg3]-IGF-I swaps glutamic acid for arginine at position 3 and adds a 13-amino-acid extension (the first 11 amino acids of pig growth hormone plus two linker amino acids) to the front.[2] Added to the 70 amino acids of IGF-1, that makes 83 amino acids in all.[7] In rat muscle cells, it was more potent than IGF-1 at stimulating protein and DNA synthesis. In cells that do not release IGF-binding proteins, it was less potent than IGF-1. The authors concluded that its extra potency comes mainly from changes in how it interacts with binding proteins.[2]

A 2026 review explains that IGF-1 LR3 works through the same IGF-1 receptor as natural IGF-1, switching on growth and cell-survival pathways.[1]

IGF-1 LR3 benefits: what the research shows

There are no human trials. Everything known comes from cells and animals:

  • Diabetic rats: given by continuous infusion for 7 days, LR3-IGF-I was 2.5 to 3 times more potent than IGF-1 at restoring growth, and it raised muscle protein synthesis. Unlike insulin, it did not lower sugar loss in the urine.[3]
  • Cell culture: in two lines of Chinese hamster ovary (CHO) cells used to make protein drugs, LongR3 kept cells alive under production conditions better than insulin.[4]
  • Rats in a doping-test study: after one injection into muscle, intact LongR3-IGF-I disappeared from the blood within about 4 hours, while breakdown products were detectable for up to 16 hours.[5]

The 2026 review placed IGF-1 LR3 in its lowest evidence tier (D), alongside PEG-MGF, with no peer-reviewed human studies. It noted that even natural IGF-1 has limited evidence for improving muscle mass or strength in healthy people.[1]

IGF-1 LR3 vs mecasermin (Increlex) and HGH

Mecasermin (Increlex) is recombinant IGF-1 with the natural human sequence, first approved by FDA on August 30, 2005, and now marketed in the U.S. by Eton Pharmaceuticals.[8][7] It treats growth failure in children aged 2 and older with severe primary IGF-1 deficiency, or with growth hormone gene deletion and antibodies to growth hormone. It is not a substitute for growth hormone in approved growth hormone uses.[7] In a long-term study of 76 children, growth rose from 2.8 to 8.0 cm a year in the first year of treatment.[6]

Increlex’s label gives a real picture of what IGF-1 drugs can do:[7]

  • Low blood sugar: 42% of 71 children in trials had it at least once; 5 had severe episodes and 4 had seizures or passed out. It must be taken within 20 minutes of a meal or snack.
  • Enlarged tonsils: seen in 15%, and 7 children needed surgery.
  • Raised pressure in the skull in 3 children, plus allergic reactions, including anaphylaxis.
  • Cancer warning: postmarketing reports of malignant tumors in children, more often at higher-than-recommended doses or with IGF-1 levels above normal. Increlex is not used in children with cancer or a history of cancer.
  • Half-life: about 5.8 hours in young people with severe primary IGF-1 deficiency.

HGH works upstream: in children with growth hormone deficiency, IGF-1 levels are usually low and tend to rise with somatropin treatment.[9] In adults, doctors adjust HGH doses partly by IGF-1 blood levels and lower the dose when IGF-1 goes above the normal range.[9] IGF-1 LR3 skips that feedback entirely. Peptides such as tesamorelin and MK-677 instead prompt the body to release its own growth hormone. See all growth hormone peptides.

IGF-1 LR3 dosage

There is no tested or approved human dose.[1] The 2026 review warned that dosing schedules circulating online come from bodybuilding forums and sellers and must not be read as validated or safe.[1] For comparison, the approved IGF-1 drug Increlex starts at 0.04 to 0.08 mg/kg twice daily in children, with blood sugar checks before meals.[7]

For the math of mixing and measuring any vial, see our peptide calculator. That tool does not make an untested dose safe.

IGF-1 LR3 side effects and risks

  • No human safety data: side effects have never been measured in a trial.[1]
  • Low blood sugar: IGF-1 has insulin-like effects, and natural IGF-1 often causes hypoglycemia.[7] The 2026 review notes that off-label users frequently describe symptoms suggesting low blood sugar, though many of those reports come from non-clinical sources.[1]
  • Black-market quality: doping labs have found unpurified long-R3-IGF-1 among confiscated products,[11] an injection vial containing a His-tagged version normally made for lab research whose effects in humans are unknown,[10] and many oxidized, lower-quality peptide forms in black-market IGF-1 analog products.[5]
  • Theoretical cancer concern: this is not proven for IGF-1 LR3. In pooled studies, women in the highest fifth of natural IGF-1 levels had 28% higher odds of breast cancer than those in the lowest fifth,[17] and men had 29% higher odds of prostate cancer.[18] These are associations with the body’s own IGF-1, not evidence that any IGF-1 drug causes cancer. A 2021 Russian review from the National Medical Research Centre for Oncology described the IGF system as often decisive for tumor cell survival.[19] The 2026 review calls the risk from IGF-1 analogs “theoretical but clinically relevant.”[1]
  • Not approved: IGF-1 LR3 has no approval for any use.[1]
  • Compounding: it does not appear on FDA’s 503A lists of nominated bulk substances (updated May 2026),[13] or on FDA’s list of substances that raise significant safety risks.[12] FDA’s rule defines a protein as any amino acid chain of more than 40 amino acids,[14] and FDA says licensable biological products are not eligible for the compounding exemptions under sections 503A and 503B.[15] In our reading, IGF-1 LR3 (83 amino acids) would therefore be treated as a biological product that pharmacies cannot legally compound. This is our interpretation; FDA has not addressed IGF-1 LR3 by name.
  • Banned in sport: WADA’s 2026 list names “Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues” under S2.3, prohibited at all times.[16] A French anti-doping lab has validated a test for LongR3-IGF-I.[5]

This is general information, not legal advice.

What users report

We did not find trustworthy, non-vendor user reports about IGF-1 LR3, so this profile does not summarize user experiences. The 2026 review notes that what is known about off-label use comes from forums and seller sites rather than research.[1]