IGF-1 LR3 vs MGF: both are lab-made peptides linked to insulin-like growth factor 1 (IGF-1) and sold online for muscle growth, but neither has ever been tested in humans.[3] IGF-1 LR3 is a long, engineered version of mature IGF-1 designed to dodge the proteins that normally hold IGF-1 in check; it was more potent than IGF-1 in rat cells.[1] MGF (mechano growth factor) is a short peptide copied from the tail of an IGF-1 variant that muscle makes after strain; its key muscle-cell findings could not be reproduced by an independent team.[2][6] Neither is approved, and WADA bans both.[13]
See the IGF-1 LR3 and PEG-MGF profiles, or the muscle and metabolism peptides hub.
IGF-1 LR3 vs MGF at a glance
| IGF-1 LR3 | MGF / PEG-MGF | |
|---|---|---|
| What it is | Engineered mature IGF-1 with one amino acid swapped[1] | Synthetic copy of the E-domain tail of IGF-1Ec[2] |
| Size | 83 amino acids (70 of IGF-1 plus a 13-amino-acid extension)[1] | About 24 amino acids (PEG chains added in PEG-MGF)[6] |
| Proposed action | Activates the IGF-1 receptor, freed from binding proteins[1] | Proposed separate action on muscle precursor cells[2] |
| Best evidence | Rat cells and diabetic rats[1][4] | Cell studies, some not reproduced[2][6] |
| Human studies | None[3] | None[3] |
| Main concerns | Low blood sugar, theoretical cancer risk[3][8] | Immune reactions, impurities, theoretical cancer risk[9][16] |
| FDA compounding | Not on nomination lists; likely a biologic[10][11] | MGF category 3; PEG-MGF nominated then withdrawn[10][9] |
| Banned in sport | Yes[13] | Yes[13] |
How each one works
IGF-1 LR3. In the blood, most natural IGF-1 is bound to IGF-binding proteins, which limit how much reaches tissues. In 1992, Australian researchers built “Long R3 IGF-I” by swapping the amino acid at position 3 and adding an extension to the front. In rat muscle cells it was more potent than IGF-1 at stimulating protein and DNA synthesis, but in cells that do not make binding proteins it was less potent. The authors concluded its extra strength comes mainly from evading binding proteins.[1]
MGF. The IGF-1 gene can be spliced several ways. One version, IGF-1Ec in humans, has a different tail section called the E domain. Researchers in London found this version rises in muscle after mechanical stress and called it mechano growth factor.[2] The MGF sold online is a synthetic peptide matching the end of that tail, not mature IGF-1. “PEG-MGF” has polyethylene glycol attached, which sellers say slows breakdown, but its half-life has not been reported.[3]
Evidence quality: cells and rodents only
IGF-1 LR3
- In diabetic rats given continuous infusions for 7 days, it was 2.5 to 3 times more potent than IGF-1 at restoring growth and raised muscle protein synthesis.[4]
- No human study has tested it for any purpose.[3]
MGF
- In an early lab study, the MGF E peptide increased muscle cell growth and delayed their maturation.[2]
- In people (with no drug given), heavy leg exercise raised natural MGF messenger RNA in young adults but not in people aged 70 to 82.[5]
- Replication problem: in 2014, scientists at two drug companies found that MGF peptide at up to 500 ng/mL did not increase growth of mouse or human muscle cells or stem cells, while mature IGF-1 worked as expected.[6] A 2010 review noted that no natural MGF peptide like the synthetic one had been found in tissues or body fluids.[7]
Side effects and risks
Because neither has been given to people in a trial, side effects are unknown. What we can say:
- Low blood sugar (IGF-1 LR3): IGF-1 has insulin-like effects. The approved IGF-1 drug, Increlex (mecasermin), caused low blood sugar at least once in 42% of children in trials and must be taken with food.[8] The 2026 review notes that users frequently describe symptoms suggesting low blood sugar.[3]
- Cancer concern (both): Increlex carries a warning about tumors reported after approval.[8] Higher natural IGF-1 levels are associated with higher breast cancer odds in pooled studies, which shows an association, not that any drug causes cancer.[17] In lab work, prostate cancer tissue made more MGF, and a synthetic MGF peptide stimulated prostate cancer cell growth.[16]
- Immune reactions (PEG-MGF): FDA says compounded PEG-MGF may carry immune-reaction risks and impurity problems.[9]
- Wrong product: anti-doping labs have found a His-tagged lab-research version of LR3 in a black-market vial[14] and modified MGF analogs rather than human MGF in black-market products.[15]
Legal status and access
- Not approved: neither IGF-1 LR3 nor MGF has FDA approval. The only approved IGF-1 drug is Increlex, for severe primary IGF-1 deficiency in children.[8]
- IGF-1 LR3 and compounding: it is not on FDA’s 503A nomination list.[10] FDA defines a protein as a chain of more than 40 amino acids, and biological products are not eligible for compounding exemptions, so in our reading the 83-amino-acid IGF-1 LR3 cannot legally be compounded. FDA has not addressed it by name.[11][12]
- MGF: plain MGF is in category 3 (nominated without adequate support) on FDA’s 503A list; PEG-MGF was placed in category 2 before its nomination was withdrawn.[10][9]
- Sport: WADA bans IGF-1 analogs and mechano growth factors at all times.[13]
There is no legitimate, regulated source of either for human use. See research-use-only peptides. This is general information, not legal advice.
Questions to discuss with a clinician
- What is driving your interest: muscle size, recovery or an injury? Approved, tested options exist for many of these goals; see peptides vs SARMs and peptides vs TRT for how other options compare.
- Do you have diabetes, low blood sugar episodes or a personal or family history of cancer? IGF-1-type compounds raise specific concerns here.[8]
- Do you compete in tested sport? Both are prohibited.[13]
The verdict
IGF-1 LR3 has a clearer mechanism and more consistent lab results, but those results are from rat cells and rats, and the approved IGF-1 drug shows real risks such as low blood sugar.[1][8] MGF has an intriguing biology story, but its core findings failed an independent replication.[6] What is known: neither has human data. What is not known: whether either helps muscle in people or what harm it causes. Compare with Fragment 176-191 vs AOD-9604 and peptides vs prohormones.
