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IGF-1 LR3

IGF-1 LR3 (Long R3 IGF-1) is a lab-made version of insulin-like growth factor 1 (IGF-1), the hormone that carries out many of growth hormone's effects. Two changes make it bind poorly to the proteins that normally hold IGF-1 in the blood, so it is more potent than natural IGF-1 in cell and animal studies. It was designed as a research reagent. There are no published human studies of it.

Not approved · research onlyWADA Prohibited at all times (S2.3, IGF-1 and its analogues)Checked October 2026
Open the IGF-1 LR3 calculatorSources

Approval status

Not approved as a medicine anywhere; it is marketed as a research compound. A 2026 clinical review found no peer-reviewed human studies. Its approved relative is mecasermin (Increlex), which has the natural IGF-1 sequence and is FDA-approved only for growth failure in children with severe primary IGF-1 deficiency, or with a growth hormone gene deletion and antibodies to growth hormone.392

What it’s used or studied for

  • Animal & lab only

    In rats given a muscle-wasting steroid, it reduced weight loss and improved nitrogen balance more potently than natural IGF-1.4

  • Animal & lab only

    In food-restricted rats it kept body weight higher but did not preserve muscle protein.5

  • Animal & lab only

    Increased the size of the gut and the growth of the intestinal lining in rats.6

  • Animal & lab only

    Given into the nose of mice with an Alzheimer's-like disease, it changed some brain plaque features but did not protect memory.7

  • Animal & lab only

    Used in lab studies on cells as a more potent stand-in for IGF-1.1

  • No studies

    Promoted online for muscle growth and body recomposition; there are no human studies of this.3

Each label shows how strong the evidence is: an approved use, human studies, animal or lab studies only, or a popular claim with no published studies. Being studied is not the same as being proven safe or effective.

Side effects

There are no published human safety studies. In pigs, LR3 and similar IGF-1 analogs lowered blood sugar more strongly and for longer than natural IGF-1.38

Cautions

  • The label of mecasermin, the approved relative (not LR3 itself), warns of low blood sugar, severe allergic reactions, raised pressure inside the skull, enlarged tonsils and a risk of tumors.9
  • Mecasermin's label rules out its use in children with active or past cancer. This comes from the approved relative, not from studies of LR3.9
  • A 2026 clinical review calls cancer risk theoretical but relevant: higher IGF-1 levels are linked to several cancers, and there are no long-term safety data for IGF-1 analogs.3
  • A doping lab found a black-market vial containing a His-tagged LR3 made for lab work, whose effects in people are unknown.10

Sources

  1. Francis GL et al. Novel recombinant fusion protein analogues of IGF-I (J Mol Endocrinol, 1992)
  2. WADA 2026 Prohibited List
  3. Dominikowski A et al. Performance-enhancing peptides modulating the GH-IGF1 axis: clinical evidence vs self-administration (Front Endocrinol, 2026)
  4. Tomas FM et al. IGF-I and especially IGF-I variants are anabolic in dexamethasone-treated rats (Biochem J, 1992)
  5. Tomas FM. LR3IGF-I ameliorates the loss of body weight but not of skeletal muscle during food restriction (Growth Horm IGF Res, 2001)
  6. Steeb CB et al. Prolonged administration of IGF peptides enhances growth of gastrointestinal tissues in normal rats (Am J Physiol, 1994)
  7. Engel MG et al. Intranasal long R3 IGF-1 in 5XFAD mice (J Alzheimers Dis, 2025)
  8. Tomas FM et al. IGF-I variants show more potent and prolonged hypoglycaemic action in pigs and marmoset monkeys (J Endocrinol, 1997)
  9. FDA label: Increlex (mecasermin) injection, revised 07/2025
  10. Kohler M et al. Detection of His-tagged Long-R3-IGF-I in a black market product (Growth Horm IGF Res, 2010)

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