Follistatin-315 — Safety, Research, Risks & Legal Status

Not medical advice. Follistatin-315 is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	FST-315, FS-315, human follistatin serum isoform, follistatin long isoform
Category	Peptide (research chemical)
Isoform origin	315-amino-acid serum isoform of follistatin, produced by alternative splicing of the FST gene; predominant circulating form in humans
Primary targets	Myostatin (GDF-8) and activin A; also binds activin B and several other TGF-beta superfamily ligands with varying affinity
Gene therapy status	AAV1-FS344 completed Phase 1/2a for Becker muscular dystrophy (NCT01519349); no approved product; additional trials ongoing (NCT07285629, NCT06411366)
ACE-083 outcome	Acceleron's follistatin-Fc fusion increased FSHD muscle volume in Phase 2 but showed no functional benefit; development halted for that indication in 2022
WADA status	Prohibited substance in sport; WADA has published detection methodology for urine and blood and has conducted black-market product studies
Engineered variants	FST-315-Fc and heparin-binding-deficient variants show approximately 90-fold half-life improvement over native FST-315 in mouse models; remain experimental
US legal status	Not FDA-approved for any indication as of July 2026. No compounding pathway exists. Follistatin gene therapy (AAV1-FS344) has been studied under IND/clinical trial (NCT01519349) and is only available via formal enrollment. Research-grade FST-315 peptide is sold by unregulated vendors as a research chemical not for human consumption; these products carry no verified purity, lot consistency, or sterility. WADA has flagged follistatin as a prohibited substance in sport and has published detection studies for doping control. The regulatory status of vendor-sold peptide is a gray zone — there is no explicit FDA scheduling, but systemic administration in humans is unsupported by any approval.

What is Follistatin-315?

Follistatin-315 is the 315-amino-acid serum isoform of endogenous human follistatin, produced via alternative splicing of the FST gene. It is the predominant circulating form of follistatin and carries an additional acidic domain that distinguishes it from the shorter tissue-retained isoform, follistatin-288. As an endogenous glycoprotein rather than a synthetic small molecule, it occupies an unusual position: it is a naturally occurring human protein being explored as a biotherapeutic through both recombinant protein administration and gene delivery approaches. It is not a drug in the conventional sense, and no pharmaceutical-grade product is commercially approved.

How it works

FST-315 functions as a high-affinity extracellular antagonist of the TGF-beta superfamily, with primary targets including myostatin (GDF-8) and activin A. It sequesters these ligands before they can engage their receptors, thereby relieving Smad2/3-mediated transcriptional suppression downstream. In skeletal muscle, this blockade disinhibits AKT/mTOR signaling pathways associated with protein synthesis and hypertrophy, and reduces atrophy-gene expression driven by FoxO transcription factors. Structural studies confirm that follistatin wraps around and sterically occludes the receptor-binding surfaces of activin and myostatin. The 315 isoform also binds heparan sulfate proteoglycans on cell surfaces, which extends its local tissue retention compared to engineered heparin-binding-deficient variants but may contribute to off-target pituitary and gonadal effects.

Background & history

Follistatin was identified in the late 1980s as an activin-binding protein isolated from ovarian follicular fluid. The 315-amino-acid serum isoform was characterized through cloning and alternative splicing analysis in the early 1990s. Interest in muscle applications grew substantially after myostatin knockout mouse experiments in the late 1990s demonstrated dramatic muscle hypertrophy. Gene therapy work using an AAV1 vector carrying the FS344 cDNA began at Nationwide Children's Hospital in the 2000s, culminating in a Phase 1/2a trial (NCT01519349) reported in 2014-2017. Concurrently, Acceleron Pharma developed ACE-083, a locally acting follistatin-Fc fusion protein, which entered Phase 2 trials for facioscapulohumeral muscular dystrophy (FSHD) before development for that indication was halted in 2022 when the trial showed increased muscle mass without functional benefit. WADA added follistatin to its prohibited list and has published formal detection methodology. An unregulated market for research-grade FST-315 peptide emerged alongside these clinical developments.

What the research says

Evidence base is preclinical-dominant with limited and narrow human data. In animal models, subcutaneous or intramuscular follistatin administration reliably increases muscle mass and lean tissue; engineered FST-315-Fc and heparin-binding-deficient variants show 90-fold half-life improvements in mice versus native protein. Human evidence comes from two narrow streams: (1) The Phase 1/2a AAV1-FS344 gene therapy trial (6 Becker muscular dystrophy patients) showed 4 of 6 patients improved 6-minute walk test performance by 29-125 meters, with muscle hypertrophy on biopsy and no serious adverse events, though the sample size precludes efficacy conclusions; (2) The ACE-083 Phase 2 FSHD trial demonstrated increased muscle volume but no functional improvement, leading Acceleron to halt that indication. There are no published controlled trials of injected FST-315 protein in healthy humans for any purpose. Safety data for repeated systemic self-injection in humans is essentially absent. Long-term endocrine effects, immunogenicity, and oncogenic risk are uncharacterized in humans.

Reported effects

Myostatin and activin A sequestration, reducing TGF-beta superfamily signaling (established mechanistically in vitro and in animal models)
Skeletal muscle hypertrophy in animal models via disinhibition of AKT/mTOR pathways (preclinical)
Improved ambulation metrics in a small Phase 1/2a gene therapy trial for Becker muscular dystrophy (very limited human data, n=6, not generalizable)
Increased muscle volume without functional improvement in ACE-083 Phase 2 FSHD trial (human, but development halted for this indication)
Potential off-target effects on pituitary FSH regulation and gonadal axis due to activin binding (preclinical concern; attenuated in heparin-binding-deficient engineered variants)
No efficacy data for performance enhancement in healthy humans; extrapolation from disease models is unsupported

Dosing & administration (informational)

Published literature describes FST-315 or follistatin variants only in the context of gene therapy (single intramuscular injection of AAV1 vector, doses measured in vector genomes per kilogram) and the ACE-083 Fc fusion protein (intramuscular, disease-specific trial dosing). No peer-reviewed human pharmacokinetic or dose-ranging studies exist for injected recombinant FST-315 protein. Figures cited in unregulated vendor and community contexts (typically 50-200 mcg ranges) have no clinical trial basis and originate from uncontrolled self-experimentation reports. This entry does not provide a dosing protocol. Anyone considering follistatin for a therapeutic indication should consult a physician and, where applicable, pursue access through a registered clinical trial.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

Controlled gene therapy context (Phase 1/2a, n=6): well-tolerated with no serious adverse events; approximately 10% incidence of rash or urticaria
Uncontrolled research peptide use: injection site reactions reported in 25-35% of subjects at 500-600 mcg doses per gray-market observational data; 800 mcg trials reportedly terminated early due to 40% adverse event rate including severe injection site reactions requiring medical intervention (these figures are from unverified sources, not peer-reviewed trials)
No long-term safety data exist for repeated systemic administration of FST-315 protein in humans
Potential pituitary/gonadal axis disruption via activin blockade; activin regulates FSH secretion and reproductive function
Immunogenicity risk from repeated exogenous protein exposure is uncharacterized
Vendor-sourced peptides carry unquantified risk from unverified purity, sterility, and identity; contamination and mislabeling are documented concerns in research chemical markets
WADA-prohibited substance; carries anti-doping consequences in regulated sport

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Who should avoid it

Use in individuals with hormone-sensitive conditions given uncharacterized activin/FSH axis effects
Use in individuals with active malignancy; TGF-beta superfamily members have tumor-suppressive roles in some contexts and the oncogenic implications of systemic blockade are unstudied in humans
Combining with other investigational TGF-beta or myostatin pathway inhibitors outside a supervised trial setting
Use in pregnancy or lactation; activin plays documented roles in placentation and embryogenesis
Procurement from unregulated vendors where identity, purity, and sterility cannot be verified
Use in competitive sport under WADA or similar anti-doping jurisdictions; follistatin is a prohibited substance

How it is commonly combined

No clinical stacking data exist. In preclinical models, follistatin is sometimes studied alongside IGF-1 or anabolic agents given overlapping mTOR pathway effects, but this represents bench research, not human protocols. Community-reported combinations with peptides such as BPC-157, IGF-1 LR3, or PEG-MGF appear in gray-market forums and carry no clinical evidence basis. Theoretical additive risk of endocrine disruption increases with combinations involving gonadotropin-affecting compounds.

Legal & regulatory. Not FDA-approved for any indication as of July 2026. No compounding pathway exists. Follistatin gene therapy (AAV1-FS344) has been studied under IND/clinical trial (NCT01519349) and is only available via formal enrollment. Research-grade FST-315 peptide is sold by unregulated vendors as a research chemical not for human consumption; these products carry no verified purity, lot consistency, or sterility. WADA has flagged follistatin as a prohibited substance in sport and has published detection studies for doping control. The regulatory status of vendor-sold peptide is a gray zone — there is no explicit FDA scheduling, but systemic administration in humans is unsupported by any approval.

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Frequently asked questions

Is Follistatin-315 approved for human use?

No. No FDA-approved follistatin product exists as of July 2026. The only human exposure in a regulated context has been through clinical trials of follistatin gene therapy for rare muscular diseases. Vendor-sold FST-315 peptide is an unapproved research chemical with no verified identity or safety profile.

What does the human evidence actually show?

It is narrow and disease-specific. A Phase 1/2a gene therapy trial in 6 patients with Becker muscular dystrophy showed some improvement in walking distance and muscle hypertrophy on biopsy. A Phase 2 trial of ACE-083 (a follistatin-Fc fusion, not native FST-315) increased muscle volume in FSHD patients but produced no functional benefit, and development for that indication was discontinued. There are no controlled trials of injected FST-315 protein in healthy humans.

What are the main safety concerns?

The controlled gene therapy data are relatively reassuring for short-term use in a supervised setting (no serious adverse events, ~10% minor rash). However, uncontrolled self-injection reports suggest high rates of injection site reactions at doses commonly cited in gray-market contexts, with one observational report citing early termination at 800 mcg due to a 40% adverse event rate. Long-term risks - including endocrine disruption from activin blockade, immunogenicity from repeated protein exposure, and potential oncogenic effects - are uncharacterized in humans.

What is the difference between FST-315 and FST-288?

Both are follistatin isoforms produced from the same FST gene by alternative splicing. FST-288 (315 minus an acidic C-terminal domain) is the shorter, tissue-retained form; it binds heparan sulfate proteoglycans tightly and remains near the cell surface. FST-315 carries the extra domain and circulates systemically, making it the predominant serum isoform. Most exogenous myostatin inhibition research relevant to systemic effects involves FST-315 or engineered variants based on it.

What is the recommended dose?

No peer-reviewed clinical trial has established a dosing protocol for injected FST-315 protein in humans. This entry does not provide dosing guidance. Figures circulating in research chemical communities lack clinical trial support. Anyone exploring follistatin for a medical condition should speak with a physician and inquire about access through a registered clinical trial.

Is follistatin detectable in anti-doping tests?

Yes. WADA has funded and published methodology for detecting follistatin administration in both urine and blood, and has conducted studies of black-market products. Follistatin is a prohibited substance under the WADA Prohibited List. Athletes subject to anti-doping rules face sanctions if found to have used it.

References & further reading

PubMed: follistatin gene therapy Becker muscular dystrophy Phase 1 2a (search terms reflect PMID 25322757 and PMC5240576)
PubMed: structural basis inhibition activin signalling follistatin (PMC1409725)
PubMed: regulation muscle mass follistatin activins (PMID 20810712)
ClinicalTrials.gov: NCT01519349 follistatin gene transfer Becker muscular dystrophy sporadic inclusion body myositis; NCT07285629 Klotho follistatin gene therapy; NCT06411366 Phase I follistatin plasmid gene therapy
WADA scientific research: administration study black market follistatins FS-344 FS-315; detection follistatin doping urine blood (wada-ama.org/resources/scientific-research)