Fitness & muscle
IGF-1 LR3
A modified, long-acting version of IGF-1, the growth factor downstream of growth hormone that directly drives muscle cell proliferation. Used in bodybuilding circles for localized and systemic muscle growth.
Research chemical (acute risk) Fitness & muscle
- Full name
- Long R3 Insulin-like Growth Factor 1 (a modified, longer-acting analog of IGF-1)
- Also known as
- IGF-1 LR3, Long R3 IGF-1
- Regulatory status
- Research chemical, not FDA-approved for any indication. Native IGF-1 (mecasermin) is an approved drug for a rare pediatric growth-failure condition, but the modified LR3 analog discussed here has no approval of its own.
Origin
Native IGF-1 mediates most of growth hormone's downstream tissue-building effects, but has a very short half-life (minutes) because it's rapidly bound and cleared by IGF-binding proteins. LR3 carries an amino acid substitution and an added 13-amino-acid extension that dramatically reduces binding to those clearance proteins, extending the functional half-life to many hours.
Mechanism
Binds the IGF-1 receptor, directly stimulating satellite cell proliferation and differentiation, the process by which muscle fibers gain new nuclei and grow. Unlike GH secretagogues, which work upstream by increasing the body's own hormone release, IGF-1 LR3 works downstream, delivering the growth signal directly. Some bodybuilders inject it locally near a specific muscle group aiming for localized hypertrophy, though whether meaningful localization occurs versus rapid systemic distribution is disputed.
Research summary
Evidence
Native IGF-1's biology is extremely well studied in growth hormone deficiency and general physiology, but the modified LR3 analog has essentially no dedicated human clinical trial data. It exists almost entirely as a bodybuilding/self-experimentation compound, not a researched pharmaceutical candidate. Unlike tesamorelin, where the underlying biology and the product in use are both studied, here the biology (IGF-1's role in muscle growth) is established but the specific product and dosing patterns are not tested in controlled human trials.
Evidence tier: Research chemical (acute risk) — see the methodology note for how this is assessed.
Citations
- Revolutionary decellularized Alstroemeria stem-based nerve conduit integrated with GelMA and controlled IGF-1 LR3 release for enhanced rat sciatic nerve regeneration — Yavuz et al. 2025
- Provisional Treatment of Volumetric Muscle Loss With Insulin-like Growth Factor 1 Releasing Muscle Void Fillers — Clark et al. 2025
- Insulin-like growth factor-I and analogues increase growth in artificially-reared neonatal pigs — Dunshea et al. 2002
Reported benefits
- Reported increases in muscle fullness/pump during use, sometimes described as more pronounced and immediate than GH secretagogues
- Interest for localized muscle growth via site-specific injection, though this specific claim is more folklore than established pharmacology
- Sometimes used short-term around intense training blocks rather than continuously
Dosing protocols reported in the literature & community
These are protocols reported by compounding pharmacies, published trials, or self-experimentation communities — not a prescription. Start low, especially for anything new.
| Route | Reported protocol |
|---|---|
| Subcutaneous or intramuscular injection | Community-reported doses are typically very low — commonly 20-50 mcg per day — reflecting IGF-1's high potency; doses are often kept short-term (a few weeks) rather than continuous, partly due to concerns about hypoglycemia and receptor downregulation with longer use. |
Side effects
- Hypoglycemia (low blood sugar) is a well-documented risk given IGF-1's insulin-like receptor cross-activity — this is one of the more acutely dangerous side effects discussed in this wiki and requires blood glucose awareness, especially combined with fasting or low-carb dieting
- Injection site reactions, and with intramuscular use, more significant local soreness
- Theoretical organ growth (organomegaly) concern with prolonged high-dose use, extrapolated from IGF-1's known growth-promoting physiology
- Theoretical cancer-growth-promotion concern given IGF-1's role in cell proliferation signalling — a reason it's generally considered contraindicated for anyone with active or recent cancer
Safety notes
Community & reddit notes (anecdotal — not clinical evidence)
Popular in bodybuilding-adjacent corners of the peptide community for its rapid, visible pump and fullness effect, generally used in short cycles rather than continuously. Discussion centers on hypoglycemia risk management, keeping fast-acting carbohydrate on hand, avoiding fasted use, more than for most other compounds here, reflecting real awareness of this specific danger.
Related peptides
Mixing compatibility
Pulled from a community-charted mixing-compatibility reference (anecdotal, clinic-use, and community-reported signals) — not a safety guarantee. Verify independently before combining anything.