Follistatin (FST-344) Compound Monograph

Follistatin (FST-344) at a Glance

Follistatin is an endogenous glycoprotein that binds and neutralises activin and myostatin, the signal that limits how much muscle the body will build. Blocking myostatin produces dramatic muscle growth in animals, which is why follistatin attracts interest. The critical qualifier is delivery: those results come from gene therapy — viral vectors making the body produce follistatin continuously — not from injecting the protein, which clears rapidly. It is not approved, and the injectable products sold do not reproduce the gene-delivery experiments.

History & Development

Follistatin is the compound on this hub with the widest gap between what is sold and what has ever been tested. No human being has been given injected follistatin protein in a trial.


First described	Not established in the sources consulted for this section
Discovered / synthesised by	Not established in the sources consulted for this section
Institution	Not established in the sources consulted for this section
Origin	Endogenous secreted glycoprotein; binds and neutralises activin and myostatin
Key patent	Not established in the sources consulted for this section
Assignee	Not established in the sources consulted for this section
Development status	Experimental - human experience is gene therapy only; never approved anywhere

Discovery

Follistatin’s appeal is straightforward: it binds and neutralises myostatin, the signal that limits muscle growth. Animals lacking myostatin are famously, visibly overmuscled, so a molecule that blocks it is an obvious candidate for muscle-wasting disease - and an obvious target for the physique market.

It is an endogenous secreted glycoprotein, not a peptide, and it also binds activin, which is why its biology reaches beyond muscle into inflammation and reproduction.

Development & ownership

The legitimate human work on follistatin is gene therapy - delivering the gene so that tissue produces the protein continuously, trialled in muscular dystrophies. That is a fundamentally different intervention from injecting the protein, and that distinction is the most important thing to carry.

There has never been a human trial of injected follistatin protein. The reason is pharmacokinetic rather than regulatory: the protein clears rapidly from circulation, so injecting it cannot reproduce the sustained local exposure that gene therapy produces. The delivery problem is the obstacle, and it has not been solved.

How it reached the market

Follistatin has never been approved by any regulator, anywhere, for any indication, and it is not a lawful dietary-supplement ingredient in the US, UK, EU or Australia. It is nonetheless sold as an injectable for muscle growth - a protein with no human injection data, no established dose, and a known clearance problem that makes the marketed route the one least likely to work.

What It's Used For

“Follistatin” names four different things, and the gray market treats them as one. Getting them apart is the whole page. (a) Endogenous follistatin is a real human secreted glycoprotein — roughly 35 kDa, 288 or 315 amino acids in its mature forms, heavily disulfide-bonded and N-glycosylated — that binds and neutralizes TGF-β-superfamily ligands, above all myostatin (GDF8) and the activins, releasing a brake on skeletal-muscle growth. The muscle field usually works with FS344, the 344-residue precursor that is processed to the circulating FS315 form; the shorter FS288 binds cell-surface heparan sulfate and acts locally. (b) Follistatin gene therapy — an AAV viral vector or plasmid carrying the FST344 gene, injected into muscle so the muscle itself manufactures the protein continuously — is a genuine clinical research program with real, published human trials. (c) “Follistatin 344” sold as an injectable research-chemical peptide is what consumers actually buy, and it has no clinical evidence of its own — not one trial, in any population. (d) Short synthetic fragments are a further step removed again.

Evidence from (a) and (b) does not transfer to (c), and the pharmacology explains why. Injected follistatin protein is cleared from the circulation in minutes — an intravenous rat study measured an initial plasma half-life of about four minutes, with the dose accumulating in liver ^{12Reference 12Kogure K et al. · 1996AnimalIntravenous administration of follistatin: delivery to the liver and effect on liver regeneration after partial hepatectomy — animal (rat, IV pharmacokinetics)View study →} — and a follistatin-Fc fusion engineered specifically to survive longer still produced growth only in the muscle it was injected into, with no systemic effect ^{11Reference 11Castonguay R et al. · 2019AnimalFollistatin-288-Fc fusion protein promotes localized growth of skeletal muscle — animal (mouse, administered fusion protein)View study →}.

That is the single most decision-relevant fact here: every positive muscle result in this literature comes from a transgene or a gene vector producing protein continuously inside the tissue, and a subcutaneous bolus of protein is not a slow way to do the same thing. It is a different intervention with no demonstrated pharmacokinetic basis. This site treats the parent-versus-derivative problem the same way for TB-500 and thymosin β4; here the split is between a gene and a protein, and it is wider.

Follistatin has never been approved by any regulator, anywhere, for any indication.

Best-supported — and it is gene therapy, not a peptide:two small open-label trials at Nationwide Children’s Hospital delivered AAV1-FS344 by direct bilateral intramuscular quadriceps injection to six Becker-muscular-dystrophy patients and six sporadic-inclusion-body-myositis patients, reporting improved six-minute-walk distance and reduced fibrosis on biopsy ^{7,8Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →Reference 8Mendell JR et al. · 2017Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes — human (open-label, n=6 vs 8 matched untreated, intramuscular AAV1-FS344 gene therapy)View study →}. Twelve people, no randomization, and the myositis efficacy claim was formally challenged in print as unsupported by the design ^{9Reference 9Greenberg SA · 2017Unfounded claims of improved functional outcomes attributed to follistatin gene therapy in inclusion body myositis — published critique / letterView study →}.

Strong animal biology, entirely by genetic delivery:follistatin overexpression reliably increases muscle mass and force in mice, aged mice and pigs, via combined myostatin + activin inhibition and Smad3/mTOR signaling ^{1,2,3,4,5,6Reference 1Lee SJ · 2007AnimalQuadrupling muscle mass in mice by targeting TGF-beta signaling pathways — animal (mouse, transgenic)View study →Reference 2Gilson H et al. · 2009AnimalFollistatin induces muscle hypertrophy through satellite cell proliferation and inhibition of both myostatin and activin — animal (mouse)View study →Reference 3Winbanks CE et al. · 2012AnimalFollistatin-mediated skeletal muscle hypertrophy is regulated by Smad3 and mTOR independently of myostatin — animal (mouse)View study →Reference 4Iyer CC et al. · 2021AnimalFollistatin-induced muscle hypertrophy in aged mice improves neuromuscular junction innervation and function — animal (aged mouse, scAAV9 gene delivery)View study →Reference 5Giesige CR et al. · 2018AnimalAAV-mediated follistatin gene therapy improves functional outcomes in the TIC-DUX4 mouse model of FSHD — animal (mouse, AAV gene therapy)View study →Reference 6Chang F et al. · 2017The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs — animal (pig, transgenic)View study →}. Every one of these is a transgene or a viral vector.

The class has a poor human track record — this is not merely untested:four separate myostatin/activin-pathway drugs from major pharma (bimagrumab, domagrozumab, landogrozumab) added lean mass and missed their functional primary endpoints in randomized trials ^{13,14,15,16Reference 13Hanna MG et al. · 2019RCTSafety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial — human (RCT; primary endpoint not met)View study →Reference 14Rooks D et al. · 2020RCTBimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial — human (RCT, n=180; +7% lean mass, primary endpoint not met)View study →Reference 15Wagner KR et al. · 2020RCTRandomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy — human (RCT, n=120; no significant treatment effect on function)View study →Reference 16Woodhouse L et al. · 2016RCTA phase 2 randomized study investigating the efficacy and safety of myostatin antibody LY2495655 versus placebo in patients undergoing elective total hip arthroplasty — human (RCT; lean mass increased, primary objective not reached)View study →}. Muscle mass is not the same outcome as muscle function, and the pathway has repeatedly delivered the first without the second.

Popular but entirely unstudied — un-scored:injectable “follistatin-344” for muscle gain, fat loss and “epigenetic age reversal” in healthy people. There is no trial of an injected follistatin protein in any human, ever. The registered studies that touch healthy adults are offshore plasmid gene therapy ventures in Honduras, not peptide studies. Marketing claims are not evidence and are not scored here.

The honest read:a real protein with real physiology, a real gene-therapy program, and a consumer product that borrows the credibility of both while sharing the pharmacology of neither.

Evidence by IndicationStrength of support · Beta

Support is an experimental score I’m building — a composite weighted by study type (human > animal > in vitro > review) and study volume. It’s a beta: a fast way to rank strength of evidence at a glance, not a validated metric. Each application links down to its write-up.

Read the low scores carefully — two different things are going on. The gene-therapy row is low because the human studies are tiny and uncontrolled. The muscle-growth and sarcopenia rows are low for a harder reason: the pathway has been tested in humans by other drugs and repeatedly failed its functional endpoints ^{13,14,15,16Reference 13Hanna MG et al. · 2019RCTSafety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial — human (RCT; primary endpoint not met)View study →Reference 14Rooks D et al. · 2020RCTBimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial — human (RCT, n=180; +7% lean mass, primary endpoint not met)View study →Reference 15Wagner KR et al. · 2020RCTRandomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy — human (RCT, n=120; no significant treatment effect on function)View study →Reference 16Woodhouse L et al. · 2016RCTA phase 2 randomized study investigating the efficacy and safety of myostatin antibody LY2495655 versus placebo in patients undergoing elective total hip arthroplasty — human (RCT; lean mass increased, primary objective not reached)View study →}. That is a negative result, not an absence of one. And no row scores the product people actually buy — an injected follistatin peptide has never been given to a human subject in a study of any kind, so its own evidence is not “weak,” it is zero.

This is the real human evidence, and it is gene therapy — read every sentence of it that way. Both trials ran at Nationwide Children’s Hospital and both delivered rAAV1.CMV.huFollistatin344, an adeno-associated viral vector carrying the FST344 gene, by direct bilateral intramuscular injection into the quadriceps. The muscle then produces follistatin continuously and locally. That is about as far from a subcutaneous peptide injection as a delivery route can get: different agent (DNA in a capsid, not protein), different site (into the target muscle, not into fat), different duration (sustained expression, not a bolus).

In Becker muscular dystrophy (n=6, Phase 1/2a; cohort 1 at 3 × 10¹¹ vg/kg/leg, cohort 2 at 6 × 10¹¹), four of six patients improved on the six-minute walk test — by 58, 125, 108 and 29 meters — while two showed no change. Biopsies showed reduced endomysial fibrosis and muscle hypertrophy, more pronounced at the higher dose, and no adverse effects were reported ^{7Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →}.

In sporadic inclusion-body myositis (n=6, 6 × 10¹¹ vg/kg to both quadriceps), treated subjects gained a median +56.0 m/year against −25.8 m/year in eight age-, sex- and baseline-matched untreated subjects (p = 0.01); four improved 58–153 m and two only marginally ^{8Reference 8Mendell JR et al. · 2017Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes — human (open-label, n=6 vs 8 matched untreated, intramuscular AAV1-FS344 gene therapy)View study →}.

Both trials also included an exercise regimen.

The myositis result was publicly disputed. A formal critique in the same journal argued that the claimed functional improvement was not supported — the comparison was against non-randomized matched controls rather than a randomized placebo arm, in a disease with variable natural history ^{9Reference 9Greenberg SA · 2017Unfounded claims of improved functional outcomes attributed to follistatin gene therapy in inclusion body myositis — published critique / letterView study →}. That objection is not a technicality; it is the difference between a finding and an observation. Animal work does support the direction across mdx, DUX4/FSHD and other dystrophy models by the same genetic route ^{5Reference 5Giesige CR et al. · 2018AnimalAAV-mediated follistatin gene therapy improves functional outcomes in the TIC-DUX4 mouse model of FSHD — animal (mouse, AAV gene therapy)View study →}.

Gap: Twelve patients across two diseases, open-label, non-randomized, one efficacy claim contested in print, and no randomized controlled trial in either indication. Nothing here tests an injected peptide, and nothing here involves healthy people.

The muscle-building reputation is built on animal overexpression and nothing else. Transgenic follistatin roughly doubles muscle mass in mice, and follistatin expressed on a myostatin-null background quadruples it — which is itself an important mechanistic result, because it proves follistatin acts on ligands beyond myostatin (chiefly the activins) ^{1Reference 1Lee SJ · 2007AnimalQuadrupling muscle mass in mice by targeting TGF-beta signaling pathways — animal (mouse, transgenic)View study →}.

Follistatin drives hypertrophy through satellite-cell proliferation and combined myostatin + activin inhibition ^{2Reference 2Gilson H et al. · 2009AnimalFollistatin induces muscle hypertrophy through satellite cell proliferation and inhibition of both myostatin and activin — animal (mouse)View study →}, and through Smad3/Akt/mTOR/S6K signaling independently of myostatin ^{3Reference 3Winbanks CE et al. · 2012AnimalFollistatin-mediated skeletal muscle hypertrophy is regulated by Smad3 and mTOR independently of myostatin — animal (mouse)View study →}.

It generalizes across species: transgenic pigs carrying human FST-344 gained skeletal muscle mass ^{6Reference 6Chang F et al. · 2017The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs — animal (pig, transgenic)View study →}.

Every one of those studies delivers follistatin as a transgene. None administers protein.

No trial has ever given an injected follistatin product to a healthy adult for hypertrophy — not the gray-market peptide, not recombinant protein, not anything. So the honest statement is not “weak evidence”; it is no evidence.

The class context makes this worse rather than neutral. Myostatin-pathway inhibition has been tested properly in humans by pharmaceutical programs, and the pattern is consistent and discouraging: the anti-myostatin antibody domagrozumab in Duchenne muscular dystrophy (randomized Phase 2, n=120) produced no significant between-group difference on any clinical functional measure ^{15Reference 15Wagner KR et al. · 2020RCTRandomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy — human (RCT, n=120; no significant treatment effect on function)View study →}; landogrozumab (LY2495655) in patients undergoing elective hip arthroplasty produced dose-dependent, statistically significant increases in appendicular lean mass but did not reach its primary objective ^{16Reference 16Woodhouse L et al. · 2016RCTA phase 2 randomized study investigating the efficacy and safety of myostatin antibody LY2495655 versus placebo in patients undergoing elective total hip arthroplasty — human (RCT; lean mass increased, primary objective not reached)View study →}; and the activin-receptor blocker bimagrumab followed the same shape in both sarcopenia ^{14Reference 14Rooks D et al. · 2020RCTBimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial — human (RCT, n=180; +7% lean mass, primary endpoint not met)View study →} and inclusion-body myositis ^{13Reference 13Hanna MG et al. · 2019RCTSafety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial — human (RCT; primary endpoint not met)View study →}.

A review of the field by one of its originators frames precisely this as the central unsolved challenge — muscle mass gained, function not delivered ^{17Reference 17Lee SJ · 2023ReviewChallenges and future prospects of targeting myostatin/activin A signaling to treat diseases of muscle loss and metabolic dysfunction — reviewView study →}.

Whatever the “Belgian Blue cattle” imagery suggests, in humans this pathway has a track record, and it is not a good one.

Gap: Species-spanning animal data, all by genetic overexpression; zero human trials of any follistatin product in healthy adults; and independent human evidence that inhibiting this pathway adds lean mass without improving function.

Mechanisms

Target / pathway	Effect	Relevant to
Myostatin (GDF8)	Bound and neutralized → growth brake released	Muscle growthdystrophysarcopenia
Activin A / B	Neutralized — high affinity, especially FS288	Hypertrophy beyond myostatin; the reason FST outperforms pure anti-myostatin agents in mice
GDF11	Bound and neutralized	Muscle; speculative aging claims
Smad2/3 signaling	↓ phosphorylation	Downstream of ligand sequestration
Akt / mTOR / S6K1	Activated → ↑ protein synthesis	Hypertrophy independent of myostatin
Satellite cells	↑ proliferation	Fiber growth and regeneration
Pituitary activin–inhibin axis	Activin inhibition can suppress FSH	Off-target reproductive-axis concern — the stated reason the trials chose FS344/FS315 over the higher-activin-affinity FS288
Insulin signaling (AKT, TBC1D1/4, PAK1)	↑ insulin-stimulated glucose uptake (mouse)	Metabolic
Heparan-sulfate binding (FS288 only)	Anchors the protein to the cell surface	Why FS288 acts locally and does not circulate

The activin row is the one most often left out of the marketing. Follistatin is not a selective myostatin blocker — it is a broad TGF-β-superfamily sink. That breadth is why it outperforms anti-myostatin antibodies in mice, and it is also the source of the off-target concerns below.

Pharmacokinetics

This section decides the page. Follistatin is not a small peptide that could be handled like semaglutide or BPC-157 — it is a ~35 kDa glycoprotein of 288 or 315 residues, with multiple disulfide-bonded domains and N-linked glycans that are required for correct structure and activity.

Circulating follistatin is cleared very fast. When radiolabeled follistatin was given intravenously to rats, serum radioactivity fell biexponentially with an initial half-life of 4.0 minutes and a terminal half-life of about 131 minutes, with the protein accumulating in the liver ^{12Reference 12Kogure K et al. · 1996AnimalIntravenous administration of follistatin: delivery to the liver and effect on liver regeneration after partial hepatectomy — animal (rat, IV pharmacokinetics)View study →}. Protein engineering has been thrown at this problem and has not solved it for systemic use: a follistatin-288-Fc fusion — an Fc domain is the standard trick for extending a protein’s half-life — grew muscle only locally, in the muscle it was injected into, with no systemic muscle effect ^{11Reference 11Castonguay R et al. · 2019AnimalFollistatin-288-Fc fusion protein promotes localized growth of skeletal muscle — animal (mouse, administered fusion protein)View study →}. FS288 in any case binds heparan sulfate on cell surfaces and does not meaningfully circulate; FS315 circulates but is short-lived.

That is why the entire efficacious literature uses gene delivery or a transgene: continuous local synthesis is the only demonstrated way to keep tissue follistatin elevated. There is no published human pharmacokinetic study of injected follistatin protein, no bioavailability figure for a subcutaneous route, and no half-life for any gray-market “follistatin-344” product. Oral products are pharmacologically inert — a 35 kDa glycoprotein is digested, not absorbed. A vendor claiming a subcutaneous peptide reproduces the animal overexpression results is claiming something no published pharmacology supports.

Clinical & regulatory status

Follistatin is an unapproved experimental biologic. No follistatin product holds marketing authorization from the FDA, EMA, TGA, PMDA or any other major regulator, for any indication, and it is not a lawful dietary-supplement ingredient anywhere. It has never been an approved drug, so no withdrawn-approval history exists either.

Every registered human study is gene therapy. Two are conventional academic trials, both sponsored through Nationwide Children’s Hospital and both intramuscular AAV1-FS344: NCT01519349 (Phase 1, completed, enrollment 15) is the record covering follistatin gene transfer in both Becker muscular dystrophy and sporadic inclusion-body myositis — the registry record for the two published trials above ^{7,8Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →Reference 8Mendell JR et al. · 2017Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes — human (open-label, n=6 vs 8 matched untreated, intramuscular AAV1-FS344 gene therapy)View study →}. NCT02354781 (Phase 1/2, completed, enrollment 3) is a separate, smaller record for the same vector in Duchenne muscular dystrophy, with no published result located in this pass. (Note for anyone checking sources: the myositis trial is frequently mis-cited to NCT02354781; I verified both records against the ClinicalTrials.gov v2 API in July 2026, and NCT02354781 is the Duchenne record.)

The remainder are offshore self-experimentation ventures, and they are the registered home of the “longevity” marketing — not conventional regulated development programs. NCT06411366 (Phase 1, completed, n=43, sponsor Minicircle, conducted in Honduras, started 2022) gave an injectable FST344 plasmid gene therapy to participants with DXA and epigenetic-panel read-outs; NCT07285629 (Early Phase 1, recruiting, n=30, Minicircle, started December 2025) is a combination klotho + follistatin plasmid study in healthy adults; and NCT07443826 (Phase 1/2a, recruiting, n=12, Unlimited Biotechnology, Honduras, started June 2026) combines intramuscular AAV9-follistatin with a VEGF plasmid for age-related muscle loss. All three are open-label, non-randomized, and offshore. None has published results, and — decisively — none of them administers a peptide. They are gene therapies too.

There is no registered trial, anywhere, of an injected follistatin protein or peptide product.

Completed	Ongoing	Terminated	Preclinical
3(all gene therapy: BMD/sIBM, DMD, offshore plasmid)	2(both offshore gene therapy, healthy/aging adults)	0	Dozens(multi-species overexpression)

Last checked: July 2026.

Sourcing, Route & Legal Status

Regulatory status. Follistatin has never been approved by any regulator for any indication, and it is not a legal dietary-supplement ingredient in the US, UK, EU or Australia. Its entire legitimate human track record is investigational gene therapy under trial protocols. Injectable “follistatin-344” sold to consumers is an unapproved research chemical, labeled “for research use only / not for human consumption,” which is the legally accurate frame for anyone acquiring it. It is not a scheduled substance in the US, but “not scheduled” is not “approved” — marketing it for human use is unlawful in most Western jurisdictions. As a myostatin-pathway agent it also falls within the World Anti-Doping Agency’s prohibited categories, so it is a sanctionable substance for any tested athlete.

How it’s supplied & administered. There is no consumer-legitimate dosage form. In research, follistatin is delivered as a gene therapy — an AAV vector or plasmid injected directly into muscle — which produces protein continuously in the tissue. Gray-market vendors sell lyophilized “follistatin-344” powder for reconstitution with bacteriostatic water and subcutaneous or intramuscular self-injection. This format has no demonstrated pharmacokinetic basis for reproducing the sustained local overexpression that drives every positive result in the literature (see Pharmacokinetics). Oral forms cannot deliver intact protein at all.

Sourcing reality — and there is a problem here specific to this molecule. The usual gray-market hazards apply in full: unregulated manufacture with no GMP oversight, no assurance of identity, actual content, sterility or freedom from bacterial endotoxin, and documented mislabeling and under- or over-concentration across the research-peptide market. All of it is amplified because the product is injected.

But follistatin adds a failure mode that a certificate of analysis cannot address. This is a large, disulfide-rich, glycosylated protein whose activity depends on correct three-dimensional folding and correct post-translational glycosylation. A typical COA reports mass and chromatographic purity — it says nothing about whether the protein is correctly folded, correctly glycosylated, or aggregated. Material expressed in bacteria cannot glycosylate at all. So a vial can be “99% pure” by HPLC and still contain biologically inert protein, and no test a gray-market vendor offers can demonstrate otherwise. No independent assay of marketed follistatin product was located in this pass. This page does not recommend, rank, or link any vendor.

Safety

No injected follistatin product has ever been tested in a human, so its safety profile is unknown rather than reassuring — and the load-bearing concern is that follistatin is a broad TGF-β-superfamily sink, not a selective myostatin blocker.

Safety at a Glance

Toxicity unknown

Never tested in a human as an injected product. “Unknown” is not “well tolerated” — there is no study here to have produced a finding.
Off-target activin inhibition can suppress FSH. Reproductive-axis consequences are plausible, and are why the trials chose a lower-activin-affinity isoform.
Not assessed in pregnancy or breastfeeding. Reproductive effects are biologically plausible here, not merely theoretical — see the box below.
Current or prior cancer, or a strong family history. Sustained antagonism of this pathway removes a growth brake in tissues other than muscle.
The human trial data belong to gene therapy, not to this. Twelve patients given a locally-acting vector in hospital cannot characterize repeated systemic self-injection.
Large glycoproteins cannot be verified by vendor certificate. Identity, purity, folding and glycosylation are all unprovable, on top of the usual injection risks.

Full Safety & Interactions Detail

No injected follistatin product has ever been tested in a human, so its safety profile is unknown — not “well tolerated.” That distinction is the whole point: “well tolerated” is a finding, and there is no study here to have produced one.

What human safety data exist belong to gene therapy. In the intramuscular AAV1-FS344 trials, no adverse effects were reported ^{7,8Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →Reference 8Mendell JR et al. · 2017Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes — human (open-label, n=6 vs 8 matched untreated, intramuscular AAV1-FS344 gene therapy)View study →} — but that covers twelve patients receiving a locally-acting vector into the quadriceps, monitored in a hospital trial setting. It is not transferable to repeated systemic self-injection of protein from an unregulated source, and twelve patients cannot characterize anything but the most common events.

The load-bearing mechanistic concern is off-target activin inhibition. Follistatin is a broad TGF-β-superfamily sink, not a selective myostatin blocker. Through the pituitary activin–inhibin axis, activin inhibition can suppress follicle-stimulating hormone, with plausible reproductive-axis consequences — this is exactly why the trials chose the FS344/FS315 isoform, which has lower activin affinity, over FS288 ^{7Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →}. Beyond that, activins and GDF11 restrain proliferation in many tissues, so sustained systemic antagonism raises a theoretical concern about removing a growth brake somewhere other than muscle. None of this has been characterized in humans, in either direction.

Class context matters for expectations, not just risk. Across four randomized human trials of myostatin/activin-pathway drugs, the agents were generally safe and well tolerated — and still failed their functional endpoints ^{13,14,15,16Reference 13Hanna MG et al. · 2019RCTSafety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial — human (RCT; primary endpoint not met)View study →Reference 14Rooks D et al. · 2020RCTBimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial — human (RCT, n=180; +7% lean mass, primary endpoint not met)View study →Reference 15Wagner KR et al. · 2020RCTRandomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy — human (RCT, n=120; no significant treatment effect on function)View study →Reference 16Woodhouse L et al. · 2016RCTA phase 2 randomized study investigating the efficacy and safety of myostatin antibody LY2495655 versus placebo in patients undergoing elective total hip arthroplasty — human (RCT; lean mass increased, primary objective not reached)View study →}. A reader weighing risk should note that the demonstrated human benefit of inhibiting this pathway is, so far, cosmetic body-composition change rather than improved function ^{17Reference 17Lee SJ · 2023ReviewChallenges and future prospects of targeting myostatin/activin A signaling to treat diseases of muscle loss and metabolic dysfunction — reviewView study →}.

Injection-specific risks. Reconstituted powder from unregulated sources carries injection-site reactions, infection from non-sterile technique, abscess, endotoxin exposure, and dosing error from self-mixed concentrations of unverified potency.

Interactions. Not assessed. No human interaction study of any kind exists. At mechanism level, co-use with other myostatin/activin-pathway agents (ActRIIB-Fc-class drugs, anti-myostatin antibodies) would be expected to overlap, but there is no controlled basis for specific guidance.

Who should avoid it. There is no established safe-use population, so the honest answer is everyone — but the specific flags are: anyone with a current or prior cancer diagnosis or strong family history; anyone with a reproductive-hormone condition or trying to conceive; anyone pregnant or breastfeeding; children and adolescents; anyone subject to anti-doping testing; and anyone unwilling to inject a product whose identity, folding and sterility cannot be verified.

Scope disclosure (honesty, not a claim):

Human data? None for an injected follistatin product. The human trials cited here are AAV or plasmid gene therapy — a different intervention, by a different route, producing protein continuously inside the muscle.
Interactions assessed? No — none, for any drug class.
Pregnancy/lactation assessed? No — no human or animal reproductive data located.
Long-term safety established? No. There is no long-term human safety data for any route; the absence of adverse reports reflects the absence of study, not evidence of safety.

Never imply safety from silence. Where follistatin has not been studied — which, for the product people actually buy, is everywhere — the correct statement is “not assessed,” not “safe.”

Pregnancy & Lactation

Not established in pregnancy Not established in lactation

Not assessed / not established. There are no human pregnancy or lactation data and no reproductive-toxicity data for administered follistatin in the accessible literature. Follistatin’s role in the activin/FSH axis makes reproductive effects biologically plausible rather than merely theoretical. Treat as contraindicated by default in pregnancy and breastfeeding.

Dosage

There is no established human dose for follistatin, and none is derivable. This is not a case of a dose existing but being unvalidated — there is no unit in which to express one for the consumer product, because no human has ever been given injected follistatin protein in a study.

The only human exposures on record are vector doses, not protein doses, and they belong to gene therapy: AAV1-FS344 at 3–6 × 10¹¹ vg/kg per leg by direct bilateral intramuscular quadriceps injection ^{7,8Reference 7Mendell JR et al. · 2015A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy — human (Phase 1/2a, open-label, n=6, intramuscular AAV1-FS344 gene therapy)View study →Reference 8Mendell JR et al. · 2017Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes — human (open-label, n=6 vs 8 matched untreated, intramuscular AAV1-FS344 gene therapy)View study →}, plus undisclosed plasmid doses in the offshore studies. A vector dose describes how much DNA-carrying capsid was injected into a muscle; it cannot be converted into micrograms of a peptide in a syringe, and anyone presenting it as a dosing guide has misunderstood the intervention.

Milligram “follistatin-344” protocols circulate in vendor copy and forum posts. Those are not dose data — they have no experimental basis, no pharmacokinetic anchor (the protein’s initial plasma half-life in rats is about four minutes ^{12Reference 12Kogure K et al. · 1996AnimalIntravenous administration of follistatin: delivery to the liver and effect on liver regeneration after partial hepatectomy — animal (rat, IV pharmacokinetics)View study →}), and reproducing them here would launder marketing as evidence, so they are not given. Oral products cannot deliver intact protein at all. This entry is informational only and is not an endorsement of use.

References

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