FLGR-242, also written FLGR242 and sold as "Follistatin (FLGR242)," is a product marketed as a modified form of Follistatin 344. Sellers say it blocks myostatin, a protein that limits muscle growth, without blocking activin, and that it lasts for weeks after one shot.
No study of FLGR-242 has been published. Its sequence is not public, there are no clinical trials, and every claim about it comes from the people selling it. It is sold as a research chemical and is not FDA approved.
- FLGR-242 is a brand-name follistatin product launched in late 2025 by BioLongevity Labs, a company co-founded by Jay Campbell.
- Sellers say it is a shortened Follistatin 344 that skips activin and carries an albumin-binding tail for a longer half-life. None of this has been published or independently tested.
- The muscle results quoted for it come from myostatin knockout mice and follistatin gene therapy, not from FLGR-242.
- Painful swelling at the injection site is the most common user complaint.
- It is not FDA approved, and WADA bans follistatin at all times.
- Seller protocols are 5 mg every two weeks or 2.5 mg a week. No study has tested any dose.
| What it is | Proprietary follistatin analog, sequence not published |
| Claimed size | About 40 kDa (a protein, not a short peptide) |
| Common vial | 5 mg, often sold as two vials |
| Route | Injection under the skin (seller instructions) |
| Published studies of FLGR-242 | None |
| FDA status | Not approved; sold for research use only |
| In sport | Prohibited at all times (WADA S4.3) |
What Is FLGR-242?
FLGR-242 is a product name, not a molecule described in the scientific literature. Searches of PubMed, Europe PMC and ClinicalTrials.gov return nothing for it. BioLongevity Labs began selling it in late 2025, and resellers now carry it under names like "Follistatin (FLGR242)" and "Recombinant Follistatin."
Sellers make three claims about it:
- A shortened Follistatin 344. Follistatin is a protein your body makes. The follistatin gene makes 344- and 317-amino-acid precursors,[4] which are trimmed into the forms known as FST-315 and FST-288. FLGR-242 is described as a "fragmented, modified" version of the 344 form.
- Built to skip activin. Sellers say the part of follistatin that grabs activin was removed, so it only blocks myostatin. They point to a real research molecule called FS I-I, which bound myostatin more than 1,000 times more tightly than activin in a 2008 study.[6] FS I-I is a different molecule. Nobody has shown FLGR-242 behaves the same way.
- Longer lasting. Natural follistatin clears from the blood within hours. Sellers say FLGR-242 carries a short tail that sticks to albumin, the main protein in your blood, giving it a half-life of 19 days.[21] Albumin-binding tails are a real way to make proteins last longer, and the tail's core comes from an albumin-binding peptide first described in 2002.[7] The full tail is covered by a 2025 patent, US 12,377,158, filed by Michael Farber, the developer the sellers name. The patent uses a different protein (Klotho) as its worked example. It says proteins carrying the tail have a half-life of about 19 days in people, which is albumin's own half-life, and it reports a follistatin version that lasted 7 days in dogs.[18] It contains no FLGR-242 sequence and no measurements in people.
The product descriptions don't agree with each other. Some sellers call it a recombinant glycoprotein of about 40 kDa, and others call it a synthetic peptide. A protein of that size is not something you can confirm with the simple purity test printed on most certificates of analysis, especially when the sequence is secret.
How Follistatin and Myostatin Work
Myostatin is a growth brake for muscle. Mice bred without it grow muscles two to three times heavier than normal mice.[1] Follistatin binds myostatin tightly and stops it from working,[3] and mice engineered to make extra follistatin gain muscle on a scale similar to mice with no myostatin at all.[2]
Follistatin also binds activin, a hormone that helps control the reproductive system, including the release of follicle-stimulating hormone (FSH).[13] That's why sellers stress that FLGR-242 skips activin: blocking activin could affect fertility and hormone levels.
In monkeys, though, blocking myostatin alone produced much smaller muscle gains than in mice, and researchers found that activin A plays a larger role in regulating muscle mass in primates.[12] So a follistatin built to ignore activin may also give up some of the muscle effect. No one has tested FLGR-242 to find out either way.
What People Use FLGR-242 For
FLGR-242 is sold for four things. The evidence behind each:
- Muscle growth. This is the main reason people buy it. The evidence is from animals bred without myostatin[1][2] and from follistatin gene therapy (see below). There is no published muscle data for FLGR-242. Seller claims such as "up to 1.5 lbs of muscle a week" and sponsored athletes' before-and-after results are marketing.
- Strength. More muscle doesn't always mean stronger muscle. In mice without myostatin, the muscles were bigger, but each gram of muscle produced 36% less force and tired faster.[16]
- Fat loss and body composition. Myostatin research in animals touches on fat and metabolism, but there are no human data for any injected follistatin product.
- Age-related muscle loss (sarcopenia). This is a real research goal for drugs that block this pathway. No follistatin product is approved for it.
The Research Behind FLGR-242
No one has published a study of FLGR-242. The studies sellers cite are about other follistatin forms or about follistatin gene therapy:
| Study | What was tested | What it found | Does it apply to FLGR-242? |
|---|---|---|---|
| Myostatin knockout mice[1] | Mice born without myostatin | Muscles 2 to 3 times heavier | Indirectly: shows the target matters |
| FS I-I in mice[6] | A follistatin variant that spares activin | Bound myostatin far more tightly than activin; more muscle in mice | No: a different, published molecule |
| Gene therapy in monkeys[8] | A virus carrying the FS344 gene, injected into muscle | Larger, stronger muscles; no organ damage found | No: gene therapy, not an injected protein |
| Becker muscular dystrophy trial[9] | FS344 gene therapy in 6 men | Two patients walked 58 and 125 meters farther in 6 minutes | No: gene therapy |
| Inclusion body myositis trial[10] | FS344 gene therapy in 6 patients | Walking distance improved vs untreated patients | No: gene therapy, and the result was disputed[11] |
Gene therapy is not the same thing. In the human follistatin trials, doctors injected a virus carrying the follistatin gene into the patient's muscles, so the muscle made its own follistatin for months.[9][10] That's a different treatment from injecting a protein under your skin. The inclusion body myositis results were also challenged by another researcher, who wrote that the claimed improvements were unfounded.[11]
Drug companies tried this and stopped. ACE-083, a follistatin-based drug from Acceleron injected straight into muscle, increased the size of treated muscles. In a phase 2 trial in facioscapulohumeral muscular dystrophy, it did not improve muscle function, and development was dropped.[15] Read our ACE-031 page for a related drug that was stopped for safety reasons.
Is FLGR-242 Legal?
FLGR-242 is not FDA approved for any use. It is sold online as a research chemical labeled not for human or veterinary use. It is not a legal dietary supplement, and it is not on the list of substances that compounding pharmacies may use, so a clinic can't legally prescribe a compounded version.[19]
In sports, it's banned. The WADA Prohibited List bans "myostatin-binding proteins (e.g. follistatin, myostatin propeptide)" under section S4.3, at all times, in and out of competition.[20] Anti-doping labs have published tests for black-market follistatin.[17]
If you want muscle or hormone treatment under medical care, you can find a peptide clinic near you.
FLGR-242 vs Follistatin 344 and 315
Follistatin 344 and 315 are real, well-studied forms of a natural protein. FLGR-242 is a brand name for an undisclosed construct.
| Follistatin 344 | Follistatin 315 | FLGR-242 | |
|---|---|---|---|
| What it is | Natural follistatin form, 344 amino acids before processing[4] | Natural circulating form | Brand-name product, sequence not published |
| Published research | Decades, including gene therapy trials | Decades | None |
| Binds activin | Yes | Yes[5] | Seller says no; not tested |
| Half-life | Hours | Hours | Seller says 19 days; not measured |
| What's in vials sold online | Often uncertain: in one test, only 9 of 17 black-market products contained follistatin[17] | Rarely sold | Can't be confirmed without a public sequence |
Can I dose FLGR-242 like Follistatin 344? No. Sellers give doses in milligrams for FLGR-242 and micrograms for Follistatin 344, because they claim very different half-lives. Neither dose has been tested. Read more on our Follistatin 344 page and in Follistatin 344 side effects.
FLGR-242 Dosage
These are seller protocols, not tested doses
No study has tested FLGR-242 in people, so there is no proven dose. The numbers below come from the company that sells it. Talk to a licensed clinician before you inject anything.
The seller's published schedules:[21]
- Standard: 5 mg under the skin once every two weeks, or 2.5 mg once a week
- "Advanced": 5 mg once a week
- Starting dose in a later article: 1.25 to 2.5 mg a week[22]
- Cycle: the later article says 12 to 24 weeks on, then 6 to 12 weeks off.[22] The earlier page says there's no need to cycle.[21] The seller's own advice isn't consistent.
The seller says not to inject it into muscle, and recommends thin 31 or 32 gauge insulin needles to reduce the pain at the injection site.
FLGR-242 dosage chart (5 mg vial)
Most sellers say to mix each 5 mg vial with 1 mL of bacteriostatic (BAC) water. That gives 5 mg per mL, so each unit on a U-100 insulin syringe holds 0.05 mg.
| Dose | Units with 1 mL BAC water | Units with 2 mL BAC water |
|---|---|---|
| 1.25 mg | 25 units | 50 units |
| 2.5 mg | 50 units | 100 units |
| 5 mg | 100 units (the whole vial) | Two full syringes |
At 2.5 mg a week, one 5 mg vial lasts 2 weeks. At 5 mg every two weeks, it is one vial per shot. Our peptide calculator works out other volumes, and the reconstitution guide walks through mixing step by step. For the related natural forms, see the follistatin dosage guide.
FLGR-242 Side Effects
FLGR-242 has never been studied for safety, so there's no side effect list from trials.
What users report:
- Pain, swelling, heat and redness at the injection site. This is the most common complaint by far, and some users stop because of it. The seller recommends thinner 31 or 32 gauge needles to reduce the swelling and pain.[21]
- Soreness lasting a day or more after injecting
Risks from the pathway itself:
- Bleeding and blood vessel changes. ACE-031, a drug that blocks the same receptor system, was stopped in a trial of boys with Duchenne muscular dystrophy because of nosebleeds and small dilated blood vessels on the skin.[14]
- Weaker, more tired muscle. Muscle grown by removing myostatin produced less force per gram and tired faster in mice.[16]
- Hormone effects. Natural follistatin lowers FSH by blocking activin.[13] The claim that FLGR-242 avoids this hasn't been tested.
The bigger unknowns:
- What is actually in the vial. The sequence is secret, there is no reference standard, and black-market follistatin has a poor track record.[17]
- Long-term effects of a protein said to last weeks. If you react badly, you can't stop it quickly.
- Whether your body makes antibodies against it, which is a known issue with engineered proteins.
Who should avoid it: anyone pregnant, breastfeeding or trying to conceive, anyone under 18, anyone with cancer or a history of it, people with bleeding disorders, and athletes who are drug tested.
Frequently Asked Questions
What is FLGR-242? A brand-name product sold as a modified Follistatin 344 that blocks myostatin but not activin. No study of it has been published.
What is the difference between FLGR242 and follistatin 344? Follistatin 344 is a natural, well-studied protein. FLGR-242 is said to be a shortened version with an albumin-binding tail, but its sequence and claimed differences are unpublished.
Is FLGR242 a peptide? Not really. Sellers describe a protein of about 40 kDa, much larger than typical research peptides.
How much FLGR-242 should I take? No dose has been tested. Seller schedules are 5 mg every two weeks or 2.5 mg weekly.
How many units is 2.5 mg of FLGR-242? With a 5 mg vial and 1 mL of BAC water, 2.5 mg is 50 units. With 2 mL, it's 100 units.
Is taking follistatin safe? No safety data exist for FLGR-242. Injection-site pain and swelling are common, and a related drug, ACE-031, was stopped for bleeding problems.
Is follistatin legal to buy? It's sold as a research chemical. It's not FDA approved, not a legal supplement and banned by WADA.
Is FLGR242 available for sale? Yes, from BioLongevity Labs and resellers, usually in 5 mg vials. No outside lab can confirm the contents.
Does FLGR-242 build muscle? No published study shows it. The cited results come from knockout mice and gene therapy.
Is FLGR242 banned in sports? Yes. WADA bans follistatin and other myostatin-binding proteins at all times.
Should FLGR-242 be injected into the muscle? The seller says under the skin, with 31 or 32 gauge needles. No study has compared routes.
References
- McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature. 1997;387(6628):83-90. PMID: 9139826
- Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth. Proc Natl Acad Sci U S A. 2001;98(16):9306-9311. PMID: 11459935
- Amthor H, Nicholas G, McKinnell I, et al. Follistatin complexes Myostatin and antagonises Myostatin-mediated inhibition of myogenesis. Dev Biol. 2004;270(1):19-30. PMID: 15136138
- Shimasaki S, Koga M, Esch F, et al. Primary structure of the human follistatin precursor and its genomic organization. Proc Natl Acad Sci U S A. 1988;85(12):4218-4222. PMID: 3380788
- Sidis Y, Mukherjee A, Keutmann H, et al. Biological activity of follistatin isoforms and follistatin-like-3 is dependent on differential cell surface binding and specificity for activin, myostatin, and bone morphogenetic proteins. Endocrinology. 2006;147(7):3586-3597. PMID: 16627583
- Nakatani M, Takehara Y, Sugino H, et al. Transgenic expression of a myostatin inhibitor derived from follistatin increases skeletal muscle mass and ameliorates dystrophic pathology in mdx mice. FASEB J. 2008;22(2):477-487. PMID: 17893249
- Dennis MS, Zhang M, Meng YG, et al. Albumin binding as a general strategy for improving the pharmacokinetics of proteins. J Biol Chem. 2002;277(38):35035-35043. PMID: 12119302
- Kota J, Handy CR, Haidet AM, et al. Follistatin gene delivery enhances muscle growth and strength in nonhuman primates. Sci Transl Med. 2009;1(6):6ra15. PMID: 20368179
- Mendell JR, Sahenk Z, Malik V, et al. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther. 2015;23(1):192-201. PMID: 25322757
- Mendell JR, Sahenk Z, Al-Zaidy S, et al. Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes. Mol Ther. 2017;25(4):870-879. PMID: 28279643
- Greenberg SA. Unfounded Claims of Improved Functional Outcomes Attributed to Follistatin Gene Therapy in Inclusion Body Myositis. Mol Ther. 2017;25(10):2235-2236. PMID: 28927986
- Latres E, Mastaitis J, Fury W, et al. Activin A more prominently regulates muscle mass in primates than does GDF8. Nat Commun. 2017;8:15153. PMID: 28452368
- Al-Zaidy SA, Sahenk Z, Rodino-Klapac LR, Kaspar B, Mendell JR. Follistatin Gene Therapy Improves Ambulation in Becker Muscular Dystrophy. J Neuromuscul Dis. 2015;2(3):185-192. PMID: 27858738
- Campbell C, McMillan HJ, Mah JK, et al. Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial. Muscle Nerve. 2017;55(4):458-464. PMID: 27462804
- Statland JM, Campbell C, Desai U, et al. Randomized phase 2 study of ACE-083, a muscle-promoting agent, in facioscapulohumeral muscular dystrophy. Muscle Nerve. 2022;66(1):50-62. PMID: 35428982
- Giannesini B, Vilmen C, Amthor H, Bernard M, Bendahan D. Lack of myostatin impairs mechanical performance and ATP cost of contraction in exercising mouse gastrocnemius muscle in vivo. Am J Physiol Endocrinol Metab. 2013;305(1):E33-E40. PMID: 23632633
- Reichel C, Gmeiner G, Thevis M. Detection of black market follistatin 344. Drug Test Anal. 2019;11(11-12):1675-1697. PMID: 31758732
- Farber M. Enhanced peptide constructs for albumin binding. US Patent 12,377,158 B1. Granted August 5, 2025. patents.google.com
- U.S. Food and Drug Administration. Bulk drug substances used in compounding under section 503A of the FD&C Act. fda.gov
- World Anti-Doping Agency. The 2026 Prohibited List, section S4.3. wada-ama.org
- Campbell J. Recombinant follistatin: the future of true muscle growth. jaycampbell.com. Updated November 22, 2025. jaycampbell.com
- Campbell J. FLGR-242 vs Follistatin 344: albumin binding changes everything. jaycampbell.com. September 2, 2026. jaycampbell.com
This information is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting any peptide therapy. Regulatory status may change.
