Follistatin
FLGR242, modified follistatin fragment
Follistatin (FLGR242, modified follistatin fragment) is a gh & growth research compound. Follistatin binds and neutralizes myostatin, the brake on muscle growth. Full-length follistatin also binds ACTIVIN, and activin blockade is where several myostatin-pathway drug programs ran into vascular and bleeding side effects. FLGR242 is sold as a fragment engineered NOT to bind activin — the intent being myostatin inhibition without the activin liability.
Follistatin quick facts
| Reported research dose | 100-300mcg per administration |
| Route | Subcutaneous |
| Frequency | Undefined |
| Half-life | Unpublished for this construct |
| Forms | Injectable |
| Evidence level | Theoretical — no published trial of this construct in any species |
This is not Follistatin 344 and should not be dosed as if it were — the Vault carries that separately. The selectivity claim is the entire premise of the molecule and it comes from the manufacturer, not from a published paper. Worth knowing before anyone reasons from FST-344 data.
How Follistatin works
Follistatin binds and neutralizes myostatin, the brake on muscle growth. Full-length follistatin also binds ACTIVIN, and activin blockade is where several myostatin-pathway drug programs ran into vascular and bleeding side effects. FLGR242 is sold as a fragment engineered NOT to bind activin — the intent being myostatin inhibition without the activin liability.
Proposed benefits
Myostatin inhibition for muscle growth, engineered to skip the activin blockade that derailed earlier drugs in this class.
Where to get Follistatin
Buy Follistatin at Biolongevity Labs →Bacteriostatic water is the diluent — sterile water with 0.9% benzyl alcohol, which is what lets a vial be drawn from more than once. It does not come with the vial, and unlike the compound it is bought again every time.
Need bacteriostatic water? Get it at AminoWell USA (my company) → Code CAMERON.
The evidence for Follistatin
Graded by what exists behind each claim.
Human clinical evidence
- No completed randomized human trials yet — human work is underway, so this is a statement about the calendar rather than about the molecule.
📊 Correlative data
- Follistatin biology in humans comes almost entirely from the full-length protein and from gene-therapy work, not from this construct. A small follistatin gene-therapy study in Becker muscular dystrophy reported improved walking distance in a handful of patients — an uncontrolled trial, a different molecule, and a different delivery route.
- Do not import Follistatin 344 data onto this. The Vault carries FST-344 separately for exactly that reason.
🧪 How the mechanism reads
- Myostatin restrains muscle growth; follistatin binds and neutralizes it. Humans and cattle with loss-of-function myostatin mutations are visibly, heritably more muscular, so the target is real and the direction is not in doubt.
- The selling point of this construct is what it does NOT bind. Full-length follistatin also neutralizes activin, and activin blockade is where several pharmaceutical myostatin programs ran into trouble — including bleeding and vascular events that ended at least one late-stage program. A fragment that spares activin would, in principle, keep the muscle effect and drop that liability.
- That selectivity claim originates with the manufacturer. There is no published characterization of FLGR242's binding profile, potency, or half-life to check it against — so the central premise of the molecule is currently an assertion, not a finding.
Why an empty tier is not a verdict →
How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →
What Follistatin actually does
This product is defined by a negative. It is sold as a follistatin fragment engineered not to bind activin, and everything the buyer is paying for sits inside that claim. So the useful question is not what follistatin does — the sibling page covers that — but whether the specific piece of engineering being claimed is chemically achievable, who has checked it, and what would check it.
First, what is on the public record about this construct: nothing. There are 0 indexed publications under the designation FLGR242, in any species, in any journal. No sequence, no mass, no binding constant, no cell assay. The designation is a manufacturer's product code and nothing published says what the digits refer to. They do not correspond to a feature boundary in the human follistatin entry, which runs to 344 residues with a signal peptide at 1 to 29 and a mature chain of 315 UniProt Consortium 2026. So the mechanism below is the mechanism of the claim, which is the only thing available to reason about.
Why separating myostatin binding from activin binding is a hard problem rather than a formality. Follistatin's ligand grip and its tissue positioning are not built out of separate parts. The heparin-binding sequence sits at residues 75 to 86, inside the first of the three follistatin domains, and mutational dissection produced a result that should stop anybody claiming an easy separation: replacing that sequence, or the whole of domain 1, abolished cell-surface binding and simultaneously enhanced activin binding 4- to 8-fold, while mutating the lysine pair at 75 and 76 reduced activin binding and pituitary bioactivity 5-fold Sidis 2005. Edit the region that controls where the protein sits and you change what it grabs, in whichever direction the edit happens to fall. The two properties are entangled at the level of the same residues.
Second reason it is hard: the two ligands are built to look alike. Myostatin and the activins are transforming growth factor beta superfamily members that signal through the same activin type II receptors, and blocking that shared receptor with follistatin, with the myostatin propeptide, or with a dominant-negative receptor all produce the same muscle phenotype in transgenic mice Lee 2001. A trap that discriminates between myostatin and activin A has to resolve a difference that the receptor itself does not resolve.
And third, the empirical record runs the other way. The best-characterized follistatin-based trap in existence is ACE-083, built by a pharmaceutical company with structural work behind it and profiled by surface plasmon resonance and cell-based assay. It binds and potently neutralizes myostatin, activin A, activin B and growth differentiation factor 11 Pearsall 2019. Four ligands. A funded program with the assays in-house produced a promiscuous trap and solved the off-target problem a different way, by making the protein stick to the extracellular matrix of the muscle it was injected into rather than by making it selective. The claim attached to this vial is that a research-market fragment achieved the selectivity that program did not attempt.
That is not proof the claim is false. It is a statement about where the burden of evidence sits, and about how cheap the experiment that would settle it is.
Cell, rodent, human — and where it stops
Say the count first, because it is the honest headline for this product. Cell studies of this construct: 0. Rodent studies: 0. Large-animal studies: 0. Human studies: 0. Published binding data: 0. There is no translation chain here, because nothing has been translated. What exists is a chain belonging to other molecules, and the reason it cannot be borrowed is the same claim the product is sold on.
What the parent protein has. Muscle-specific follistatin transgenic mice reach muscle mass comparable to myostatin knockouts Lee 2001. Transgenic pigs carrying human follistatin-344 in muscle raised lean meat from 69.18% to 72.95%, with N equal to 16 and P less than 0.05, alongside reduced Smad2 phosphorylation and increased Akt phosphorylation Chang 2017. Both results come from full-length follistatin, expressed from a gene, for the life of the animal.
What the engineered traps have. ACE-083, injected intramuscularly in randomized placebo-controlled phase 2 trials, increased muscle volume by 13.5% to 16.4% and produced no consistent functional improvement Pearsall 2019. ACE-031, a soluble activin receptor trap given subcutaneously to boys with Duchenne muscular dystrophy, was stopped after its second dosing regimen because of epistaxis and telangiectasias Campbell 2017. That second result is the reason this product's selectivity claim exists at all: the vascular events are attributed to blocking the broader activin arm, and a construct that avoided the activins would in principle avoid them.
The obstacle, and it is unusually clean. Every one of those results belongs to a molecule whose identity is published. The claim made for this vial is that it is pharmacologically different from all of them. Both halves of that claim cannot be used at once: you cannot cite full-length follistatin's muscle data as evidence this works while citing its selectivity difference as evidence it is safer. If it is the same molecule, the trials transfer and so does the activin liability. If it is a different molecule, neither transfers. The one thing that cannot be true is the version on the sales page, where the efficacy comes from follistatin and the safety comes from not being follistatin.
The second obstacle is that this is the part of the field with the worst translation record in the first place. An independent 2021 review covering 20 years of antimyostatin treatment is subtitled the story of great expectations and limited success, and its argument is that preclinical results in this pathway have repeatedly failed to reach patients Nielsen 2021. A construct with no preclinical results at all is not exempt from that record; it simply has not started.
Follistatin pharmacokinetics — how much of it actually gets in
The data card says the half-life is unpublished for this construct. That is the correct entry, and here is what can still be reasoned from the chemistry.
The fork the product literature does not resolve. Follistatin's own pharmacokinetic behavior is decided by a single structural feature: an acidic 26-residue C-terminal tail. FS315 carries it, folds it over its own heparin-binding site, and circulates; FS288 lacks it, binds cell-surface proteoglycans, and stays in tissue — the ranking of proteoglycan binding is FS288 above FS303 above FS315, and an isoform-specific assay confirmed FS315 is the form found in serum Schneyer 2004. A fragment may or may not include that tail, and the two possibilities have opposite pharmacokinetics: one is a circulating trap with systemic reach, the other is a matrix-bound depot that stays near the needle. Nothing published says which one this is, and the difference decides the dose, the route, the injection site and the entire safety argument.
What clears it, on either branch. If the construct retains the C-terminal tail, it inherits the behavior worked out on the structure of the FS315-activin complex: capture a ligand, lose the tail's coverage of the heparin site, acquire FS288-like heparin binding, and be cleared as a complex Lerch 2007. That makes the trap stoichiometric — consumed by doing its job — so the dose is set by how much ligand there is rather than by potency. If it does not retain the tail, it is sticky from the moment it is injected and most of it never leaves the injection site. Either way it is a protein, degraded by cellular uptake and lysosomal proteases rather than filtered out intact.
The oral barrier. Absolute, and for the ordinary reason: a follistatin-domain protein swallowed intact meets gastric acid, pancreatic proteases and brush-border peptidases, and hepatic first-pass extraction would take anything that survived. Oral bioavailability is effectively 0 and no oral preparation exists.
The route and the numbers that do exist. The market sells this for subcutaneous injection at 100 to 300 micrograms. For scale, the only follistatin-based protein ever injected into humans under a protocol was dosed at 240 milligrams per muscle, intramuscularly, every 3 weeks Pearsall 2019. That is a difference of roughly a thousand-fold in delivered protein, by a different route, into a different compartment. Nothing published reconciles the two, and no vendor page acknowledges the gap.
What would have to be true, and how you would know it was not
Three predictions. The first is the test of the claim this product is entirely built on, and it costs one blood draw.
1. If the selectivity claim is true, FSH must not move. Follistatin's classical bioassay is suppression of pituitary follicle-stimulating hormone secretion, driven by activin binding, and mutants with reduced activin binding lose that bioactivity in proportion Sidis 2005. So a construct that genuinely does not bind activin should leave FSH untouched at any dose that reaches the circulation. Draw FSH and luteinizing hormone at baseline and at 6 weeks. A fall in FSH falsifies the selectivity claim outright, and it is the only assay a buyer can actually run on that claim. Here is what will fool you: no change is not proof of selectivity, because a construct that never reaches the circulation, or that was degraded in the vial, also produces no change. The test is informative in one direction only, and saying so is part of using it.
2. The prediction that cuts against the compound: nothing will change in 8 weeks. There is no published evidence that this construct does anything in a cell, a rodent, a pig or a person. The closest published relative, injected as a protein into a muscle in randomized trials at roughly a thousand times the dose, produced measurable hypertrophy and no consistent functional gain Pearsall 2019. So the honest expectation for a microgram-scale subcutaneous dose is no measurable change in lean mass on DEXA, no change in strength, and no change in IGF-1 (Insulin-like Growth Factor 1), which this pathway does not run through in any case.
3. The safety readout is visual, not hematological. The adverse events that stopped the systemic trap in this family were nosebleeds and telangiectasias Campbell 2017 — small dilated vessels, typically on skin and mucosa. A CBC is worth having as a baseline but it will not detect them; they are found by looking. Anyone running this should note new nosebleeds, gum bleeding, or new small red spots on the face and trunk, and treat their appearance as a reason to stop rather than as a curiosity, because in the trial that saw them they were sufficient to end the program.
What nobody has tested yet
Four experiments. The first two are ordinary laboratory work that would take a week and have never been done.
1. Nobody has published a sequence or a mass for this construct. One tandem mass spectrometry run on one vial would produce the first structural datum this product has ever had, and would answer the question the pharmacokinetics section could not: whether the acidic C-terminal tail that decides between a circulating trap and a tissue-bound depot is present Schneyer 2004. Nothing about that run is difficult. It has simply never been reported by anyone.
2. Nobody has run the selectivity assay the entire product depends on. Surface plasmon resonance against myostatin and against activin A, side by side, is exactly the assay used to characterize ACE-083 Pearsall 2019. It is standard, it is commercially available as a service, and it would settle in one afternoon a claim that has been repeated on sales pages for years. That the assay has never been run and published is the single most informative fact about this compound.
3. Nobody has compared the two follistatin products sold side by side. This site carries both this construct and full-length follistatin-344. Run one lot of each through the same mass spectrometer and the same binding assay. If they are the same material, buyers are paying two prices for one molecule. If they differ, that is the first published compositional distinction between any two follistatin preparations in this market.
4. Nobody has asked what an engineered follistatin does to the endogenous one. Follistatin is a normal human protein with a regulatory job in the reproductive axis and in tissue repair. A modified version carries non-native residues by definition, which is the classic setting for an antibody response that can cross-react with the endogenous protein. No immunogenicity data exists for this construct because no study of it exists, and for an engineered variant of a self-protein that blank is larger than it would be for a synthetic peptide with no human counterpart.
Follistatin — its own safety story, not its class's
The shared block on this page is about myostatin and activin inhibition in general. This product's own safety story is narrower and stranger: its safety argument and its efficacy argument contradict each other, and no amount of class doctrine addresses that.
1. An unverifiable selectivity claim is an unusable safety claim. The reason to prefer an activin-sparing construct is concrete: a soluble activin receptor trap given systemically to children was stopped for epistaxis and telangiectasias Campbell 2017. If this construct genuinely spares activin, that specific hazard is reduced and the product has a real advantage. If it does not, the buyer is using a follistatin-type trap with none of the monitoring that trial had. There is no published binding data either way Pearsall 2019, so the safety margin being sold is a statement of intent.
2. If the claim is true, that is its own unknown. A protein that neutralizes myostatin while leaving activin signaling intact has never been given to a human being. Every clinical experience in this family involves broader blockade Pearsall 2019 Campbell 2017. Selectivity is not the same as safety; it means the compound's effects have no precedent at all, in either direction, and there is no trial in which an unexpected consequence could have been observed.
3. Immunogenicity is the hazard that is specific to an engineered self-protein. Full-length follistatin injected into a person is a native human sequence. A deliberately modified follistatin fragment is not, at the positions that were modified, and repeated subcutaneous dosing of a non-native protein variant is the textbook setting for anti-drug antibodies. The consequence that matters is not loss of effect, it is cross-reaction with the endogenous protein, which regulates the reproductive axis and tissue repair. No antibody data exists for this construct, and the risk is a direct consequence of the very feature it is sold on.
4. What zero means here. There are 0 published human studies of this construct, 0 animal studies, 0 cell studies and therefore 0 recorded adverse events. No setting has ever existed in which an adverse event could have been written down. The safest reading is that the tolerability reports circulating for this product are reports about an injection, not about a molecule.
Sources read for this page
- Sidis Y, et al. Heparin and activin-binding determinants in follistatin and FSTL3. Endocrinology 2005 · PMID 15471966
- Lerch TF, et al. Structural and biophysical coupling of heparin and activin binding to follistatin isoform functions. Journal of Biological Chemistry 2007 · PMID 17409095
- Schneyer AL, et al. Differential distribution of follistatin isoforms: application of a new FS315-specific immunoassay. Journal of Clinical Endocrinology and Metabolism 2004 · PMID 15472207
- UniProt Consortium. UniProtKB P19883 (FST_HUMAN) - Follistatin: signal peptide 1-29, chain 30-344, sequence length 344, isoform 1 named FS315. UniProtKB, entry retrieved from rest.uniprot.org on 4 September 2026
- Pearsall RS, Davies MV, Cannell M, Li J, Widrick J, Mulivor AW, Wallner S, Troy ME, Spaits M, Liharska K, et al. Follistatin-based ligand trap ACE-083 induces localized hypertrophy of skeletal muscle with functional improvement in models of neuromuscular disease. Scientific Reports 2019 · PMID 31388039
- Campbell C, 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 · PMID 27462804
- Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth. Proceedings of the National Academy of Sciences 2001 · PMID 11459935
- Chang F, Fang R, Wang M, Zhao X, Chang W, Zhang Z, Li N, Meng Q. The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs. Transgenic Research 2017 · PMID 27787698
- Nielsen TL, et al. Antimyostatin Treatment in Health and Disease: The Story of Great Expectations and Limited Success. Cells 2021 · PMID 33802348
Follistatin — safety, predicted from mechanism
Predicted from mechanism, not from a human safety trial. How that reasoning works →
What the mechanism predicts
Derived from the molecule, not a trial.
- These inhibit myostatin or activin signaling — the brake on muscle growth. Removing a brake the body installed deliberately is the whole premise, and the predicted problems follow from that.
- Myostatin and activin are not muscle-only signals. The same TGF-β family regulates tendon and connective tissue, and the concern that follows is muscle gaining force capacity faster than tendon adapts — predicting tendon and joint injury rather than muscle injury.
- Activin signaling also has roles in reproduction and inflammation, so systemic inhibition has predicted consequences well beyond the muscle the user is aiming at.
What has actually been reported
- Trials of myostatin-pathway drugs have repeatedly shown the dissociation that matters: muscle mass increases without a proportional increase in strength or function. That result recurs across programs and is the main reason several were discontinued.
- Bimagrumab produced muscle gain and fat loss in trials alongside diarrhea and muscle spasms.
- YK-11 has no human trial data at all. It is frequently described as a myostatin inhibitor on the strength of a single cell-culture study.
How to reduce the risk
Same mechanism as the prediction.
- If tendon lags muscle, the answer is loading tendon deliberately — slow heavy resistance work — and not adding load as fast as the new muscle allows.
- Treat rapid strength gain as a reason to be more conservative with progression, not less.
What it does to your bloodwork
A fact about the assay.
- Liver function, especially for the oral compounds in this group. Creatine kinase if you are getting unusual soreness or spasm.
Don't run this if
- You have a history of tendon injury, or you are progressing loading aggressively already.
The honest unknown
- Almost everything. No compound in this group has established long-term human safety, and the consistent trial finding is that mass gained this way has not translated into function.
Not medical advice. If you take prescription medication or have a diagnosed condition, check this with a pharmacist or doctor.
Follistatin — interference & stacking
Predicted from mechanism, not from an interaction study. How mechanism-predicted claims are made →
What Follistatin moves on your bloodwork
Expected direction, not a measured one.
- IGF-1 (Insulin-like Growth Factor 1) — ↑ expected to rise
This is the point. IGF-1 rising is the compound doing its job — it is the stable downstream readout of a GH pulse.
What to do: Test it before you start and again at 6–8 weeks. It is the only number that tells you whether the product was real and the dose was enough. - Growth Hormone, Serum — ✕ unreliable here
A random GH level is close to meaningless here. GH is secreted in pulses during deep sleep and sits undetectable between them, so a daytime draw catches a trough almost every time — including when the compound is working perfectly.
What to do: Do not use GH to judge a secretagogue. Read IGF-1 instead. - Fasting Insulin — ↑ expected to rise
GH is a counter-regulatory hormone: it opposes insulin. Fasting insulin and glucose drifting up is the predicted trade-off, not a surprise.
What to do: Check fasting insulin and HbA1c at baseline and again at 8–12 weeks. This is the marker that decides whether you keep running it. - HbA1c (Hemoglobin A1c) — ↑ expected to rise
Same mechanism, longer window — a slow drift rather than a jump.
What to do: Pair it with fasting insulin; either alone can mislead. - Free T4 (Thyroxine) — ↓ expected to fall
GH accelerates the peripheral conversion of T4 to T3, so free T4 can fall while free T3 holds or rises. Read alone it looks like new hypothyroidism, and it usually isn't.
What to do: Run a full thyroid panel rather than TSH alone before concluding anything.
Everything above follows from one fact: these raise GH and therefore IGF-1. Nothing here needs a trial of the specific molecule.
- How to work up to it, and when not to
- When to take it, and why that window
- Cycle length
- Time off between cycles
- Fasted or fed, and when in the day
- Needle gauge and injection site
- Coach Cam's personal notes
- Which compounds push the same lever, and why the dose adds up faster than people count
- What blunts it — the stacks that waste your money
- What compounds the risk, so a side effect arrives sooner than any one of them suggests
- Coach Cam's read on running it alongside the rest of your protocol
Everything above is free and stays free. Skool is where it becomes a plan — Follistatin in an order, with the rest of what you're running.
Unlock in Skool — $10/mo →Bloodwork to run alongside Follistatin
Baseline first, then again at 8–12 weeks.
| Marker | What it’s watching for |
|---|---|
| IGF-1 (Insulin-like Growth Factor 1) | The number that actually tracks your GH exposure — dose by this, not by feel |
| Fasting Insulin | GH raises insulin resistance; this moves before glucose does |
| HbA1c (Hemoglobin A1c) | The slower confirmation that the insulin change is real |
| Comprehensive Metabolic Panel (CMP) | Fasting glucose, and liver and kidney at baseline |
The “Running GH Peptides or MK-677” panel covers these in one order — 9 markers, $132.30 with the discount applied.
Check results you already have → · All 103 markers A–Z
Follistatin — frequently asked questions
What is Follistatin?
Follistatin (FLGR242, modified follistatin fragment) is a gh & growth research compound. Follistatin binds and neutralizes myostatin, the brake on muscle growth. Full-length follistatin also binds ACTIVIN, and activin blockade is where several myostatin-pathway drug programs ran into vascular and bleeding side effects. FLGR242 is sold as a fragment engineered NOT to bind activin — the intent being myostatin inhibition without the activin liability.
Is the full Follistatin protocol on this page?
The reported research dose is on this page, along with how Follistatin works and the evidence behind it. The protocol — how to work up to it, frequency, cycle length, time off, what not to stack it with and Coach Cam's notes — is inside Skool.
What is the half-life of Follistatin?
Follistatin has an approximate half-life of Unpublished for this construct, which is part of what determines how often it's dosed.
What's the evidence behind Follistatin?
Current evidence level: Theoretical — no published trial of this construct in any species. Follistatin is offered for research purposes only and is not an approved medicine.
What Follistatin is used for
Follistatin appears under 2 goals in the goal router.
Related GH & Growth compounds
Where this goes next
The pages here are the frameworks. The protocols — the dosing, the order to correct things in, the week-by-week schedule and what to retest — are inside Skool.