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Follistatin-344 (FS-344)

Activin-binding glycoprotein isoform

Also known as FS-344, Myostatin inhibitor

A myostatin-binding protein with dramatic animal results and essentially no human data for the administered protein. The gap between the two is the whole story.

What doses have been studied? What each trial gave, with the population it was given to.

Guide updated Oct 22, 2026Literature checked Oct 21, 20261 curated study24 literature recordsBaseline editorial

This entry is mechanistic only

There is no completed human trial reporting results for this compound. Everything below describes what has been observed in animals or in cell culture, and the transfer rate from those findings to humans is poor. Read the limitations on each study, not just the findings.

Overview

In shortA natural protein that blocks myostatin, the main brake on muscle growth. The animal results are genuinely spectacular: making animals produce more of it gives large increases in muscle. But the human work that exists used gene therapy to make muscle produce it locally, not an injection of the protein. Confusing those two is the central error in how this is discussed.

Follistatin is an endogenous glycoprotein that binds and neutralizes members of the TGF-beta superfamily, most notably myostatin, the principal negative regulator of skeletal muscle mass. FS-344 is one isoform.

The animal results are genuinely spectacular: follistatin overexpression produces large increases in muscle mass in mice and non-human primates. The human data for administering the protein is close to non-existent, and the small amount of human work that exists used gene therapy to induce local expression, not injection of a protein.

Conflating those two is the central error in how this compound is discussed.

Mechanism

In shortIt binds myostatin and a related signal tightly, taking the brake off muscle growth. Two practical problems sit between that and any effect from an injection. A protein this size does not last long in the blood or reach tissue well. And the signal it blocks does far more than control muscle, including suppressing tumors in some tissues, so blocking it body-wide is not a local intervention.

Follistatin binds myostatin and activin with high affinity, preventing receptor engagement and removing the brake on muscle differentiation and growth.

Two practical problems sit between that and any effect from an injection. First, an injected protein of this size has a short circulating half-life and poor tissue distribution. Second, activin signaling is involved in far more than muscle, including reproductive endocrinology, inflammation and, importantly, tumor suppression in some tissues. Systemic myostatin and activin blockade is not a localized intervention.

Evidence

In shortLab and animal work. Animal studies using genetic methods report large increases in muscle mass and strength. Human work is limited to small gene therapy trials in muscular dystrophies, delivering it locally to muscle, with modest changes in very small numbers. Nobody has even shown that an injected protein would reach muscle in a meaningful amount.

Mechanistic and preclinical. Animal work with genetic overexpression and with gene-therapy delivery reports substantial increases in muscle mass and strength.

Human work is limited to small gene-therapy trials in muscular dystrophies, delivering a follistatin construct locally to muscle, with modest reported functional changes in very small numbers.

There is no controlled human evidence for administering follistatin protein for any purpose. Material sold as follistatin-344 is a research reagent of unverified identity, and the pharmacokinetic argument that an injected protein would even reach muscle in a meaningful concentration has not been made, let alone tested.

Safety

In shortNo human safety data for the injected protein at all. The concerns follow directly from the mechanism: the signal it blocks helps suppress tumors in several tissues, and permanently blocking a growth-restraining pathway is exactly the kind of thing that should have long-term cancer safety data first. It has none. Effects on reproductive hormones are also plausible and uncharacterised.

No human safety data for the administered protein at all.

The theoretical concerns follow directly from the mechanism. Activin signaling has tumor-suppressive roles in several tissues, and sustained systemic blockade of a growth-restraining pathway is exactly the kind of intervention that should have long-term oncological safety data before anyone considers it. It has none.

Effects on reproductive endocrinology are also plausible and uncharacterised.

Regulatory status

In shortNot approved anywhere, and sold as a research reagent. Myostatin-blocking drugs from real pharmaceutical developers have repeatedly failed to show functional benefit in trials despite increasing muscle mass, which is its own warning about the premise.

Not approved anywhere. Research reagent. Myostatin-pathway drugs from pharmaceutical developers have repeatedly failed to show functional benefit in clinical trials despite increasing muscle mass, which is its own caution about the premise.

The literature

1 study indexed. Findings and limitations get equal weight, on purpose.

Always check the primary source

Members area

The Follistatin-344 literature needs an account

Everything above is free to read. What an account opens is the evidence underneath it: 1 study with their limitations, and the regimen each trial actually reported.

  • Every study, with what it found and what it cannot show
  • The limitations, given the same weight as the findings
  • The dose each named trial administered, where recorded
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The wider literature

24 indexed papers and 0 registered trials mentioning Follistatin-344, straight from PubMed and ClinicalTrials.gov. These have not been read by us. Unlike the studies above, they carry no findings and no limitations written by anyone here, because writing either would mean claiming a reading nobody has done. They are a starting point for yours.

Papers (24)

  1. Proteomic effects of short-term liraglutide vs. placebo in a blinded crossover RCT: Implications for efficacy, safety, and comparison with semaglutide.

    Stefanakis K, et al.Metabolism: clinical and experimental2026PMID 41513169

    journal articlerandomized controlled trialcomparative studyresearch support, non-u.s. gov't

  2. Safety and Efficacy of Approved and Unapproved Peptide Therapies for Musculoskeletal Injuries and Athletic Performance.

    Mendias CL, et al.Sports medicine (Auckland, N.Z.)2026PMID 41966639

    journal articlereview

  3. Risk Factors, Diagnostic Challenges, and Emerging Therapeutic Strategies for ICU-Acquired Weakness: A Brief Review.

    Zhang X, et al.Journal of multidisciplinary healthcare2025PMID 41340877

    journal articlereview

  4. Taldefgrobep Alfa and the Phase 3 RESILIENT Trial in Spinal Muscular Atrophy.

    Servais L, et al.International journal of molecular sciences2024PMID 39408601

    journal articlereview

  5. Myostatin inhibitory peptides in sports drug testing.

    Walpurgis K, et al.Drug testing and analysis2023PMID 36946003

    journal article

  6. The m(6)A methyltransferase METTL3 regulates muscle maintenance and growth in mice.

    Petrosino JM, et al.Nature communications2022PMID 35013323

    journal articleresearch support, n.i.h., extramuralresearch support, non-u.s. gov'tresearch support, u.s. gov't, non-p.h.s.

  7. Electrophoretic detection of black market myostatin propeptide.

    Reichel C, et al.Drug testing and analysis2022PMID 36336354

    journal article

  8. Impact of muscle mass on blood glucose level.

    Taha M, et al.Journal of basic and clinical physiology and pharmacology2022PMID 34856088

    journal article

  9. Myostatin inhibitor YK11 as a preventative health supplement for bacterial sepsis.

    Lee SJ, et al.Biochemical and biophysical research communications2021PMID 33588136

    journal articleresearch support, non-u.s. gov't

  10. Blocking the activin IIB receptor with bimagrumab (BYM338) increases walking performance: A meta-analysis.

    Spitz RW, et al.Geriatrics & gerontology international2021PMID 34405505

    journal articlemeta-analysis

  11. Fortetropin inhibits disuse muscle atrophy in dogs after tibial plateau leveling osteotomy.

    White DA, et al.PloS one2020PMID 32271840

    journal articlerandomized controlled trial, veterinaryresearch support, non-u.s. gov't

  12. Detection of black market follistatin 344.

    Reichel C, et al.Drug testing and analysis2020PMID 33460286

    published erratum

  13. Alterations in activin A-myostatin-follistatin system associate with disease activity in inflammatory myopathies.

    Vernerová L, et al.Rheumatology (Oxford, England)2020PMID 31990347

    journal articleresearch support, non-u.s. gov't

  14. Detection of black market follistatin 344.

    Reichel C, et al.Drug testing and analysis2019PMID 31758732

    journal article

  15. The Vicious Cycle of Myostatin Signaling in Sarcopenic Obesity: Myostatin Role in Skeletal Muscle Growth, Insulin Signaling and Implications for Clinical Trials.

    Consitt LA, et al.The Journal of frailty & aging2018PMID 29412438

    journal articleresearch support, n.i.h., extramuralreview

  16. The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs.

    Chang F, et al.Transgenic research2017PMID 27787698

    journal articleresearch support, non-u.s. gov't

  17. Matrix Metalloproteinase Responsive Delivery of Myostatin Inhibitors.

    Braun AC, et al.Pharmaceutical research2017PMID 27628627

    journal articleresearch support, non-u.s. gov't

  18. Comparative Proteomic and Transcriptomic Analysis of Follistatin-Induced Skeletal Muscle Hypertrophy.

    Barbé C, et al.Journal of proteome research2017PMID 28810121

    journal articleresearch support, non-u.s. gov't

  19. The role of myokines in muscle health and disease.

    Lightfoot AP, et al.Current opinion in rheumatology2016PMID 27548653

    journal articlereview

  20. An antibody blocking activin type II receptors induces strong skeletal muscle hypertrophy and protects from atrophy.

    Lach-Trifilieff E, et al.Molecular and cellular biology2014PMID 24298022

    journal article

  21. Micro-dystrophin and follistatin co-delivery restores muscle function in aged DMD model.

    Rodino-Klapac LR, et al.Human molecular genetics2013PMID 23863459

    journal articleresearch support, n.i.h., extramural

  22. Cellular mechanisms and local progenitor activation to regulate skeletal muscle mass.

    Cassano M, et al.Journal of muscle research and cell motility2009PMID 20195710

    journal articleresearch support, non-u.s. gov'treview

  23. Doping in the recombinant era: strategies and counterstrategies.

    Azzazy HM, et al.Clinical biochemistry2005PMID 16286094

    journal articlereview

  24. Muscle regeneration through myostatin inhibition.

    Wagner KRCurrent opinion in rheumatology2005PMID 16224249

    journal articleresearch support, n.i.h., extramuralresearch support, non-u.s. gov'tresearch support, u.s. gov't, p.h.s.review

Discussion

Follistatin-344 has its own forum, and sits in Training & Performance. Every post says what kind of claim it makes.

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For Research & Educational Discussion Only. Not Medical Advice

This guide is educational reference material compiled from published literature. It is not medical advice, not a recommendation, and not a substitute for a clinician who knows your history. Nothing here should be used to make a decision about your own health without one.

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