Verified July 2026 · Cited to primary sources
Follistatin (FST-344 / FS344): Evidence Grade D. Animal studies only, unproven in humans.
The honest verdict
A compelling target with a misleading reputation. The muscle-growth story rests almost entirely on animal work plus a tiny AAV gene-therapy trial that is not the same product as the injectable peptide being sold. For follistatin as a peptide, the human efficacy evidence is basically Grade D, safety is uncharacterized, systemic myostatin blockade carries real theoretical risks, and it is banned in sport. Interesting biology, not a justified purchase.
Follistatin (FST-344 / FS344) at a glance
- Class
- Myostatin/activin-binding glycoprotein (muscle regulator)
- Mechanism
- Follistatin binds and neutralizes myostatin (GDF-8) and activins, removing a brake on skeletal-muscle growth. Blocking myostatin drives muscle-fiber hypertrophy, which is why it is attractive for muscle building. The catch is that follistatin is a large, short-lived glycoprotein, so a simple injected peptide does not durably produce the effect, which is why the credible human work used gene therapy to make muscle express it continuously.
- Also known as
- FST-344, FS344, follistatin-344, FST-315, activin-binding protein
- Research applications
- AAV gene-therapy vector (FS344) for Becker muscular dystrophy and inclusion body myositis (human Phase 1/2a)
- Preclinical muscle hypertrophy and anti-atrophy models
- Research into fibrosis reduction in dystrophic muscle
- Forms
- Injectable recombinant/research peptide (preclinical, gray-market), AAV gene-therapy construct (AAV1.CMV.FS344, experimental, clinical-trial only)
- Legal status
- Research-only
- WADA (anti-doping)
- Myostatin-inhibiting agents, including follistatin and follistatin gene therapy, are explicitly prohibited by WADA at all times (S4.4, agents affecting muscle growth). Treat as banned.
- Evidence grade
- Grade DAnimal studies only, unproven in humans
How we grade evidence
Every grade comes from a fixed A to F rubric: human-trial strength, not hype or affiliate status. Last verified July 6, 2026.
What is Follistatin (FST-344 / FS344)?
A natural myostatin blocker that in theory should unlock muscle growth. The reality: the injectable peptide is essentially preclinical, and the only human evidence is from AAV gene therapy, a completely different technology, in a handful of muscular-dystrophy patients.
Follistatin binds and neutralizes myostatin (GDF-8) and activins, removing a brake on skeletal-muscle growth. Blocking myostatin drives muscle-fiber hypertrophy, which is why it is attractive for muscle building. The catch is that follistatin is a large, short-lived glycoprotein, so a simple injected peptide does not durably produce the effect, which is why the credible human work used gene therapy to make muscle express it continuously.
How strong is the evidence for Follistatin (FST-344 / FS344)?
For the injectable follistatin peptide that people actually buy, human efficacy data is essentially absent; support is animal and mechanistic. The one legitimate human dataset, Mendell's Phase 1/2a trial, used AAV gene therapy delivering the FS344 gene by direct muscle injection in six patients, a fundamentally different modality from a peptide you inject. That is early human safety/signal data for gene therapy, not for the peptide. Since the peptide itself is preclinical and the human evidence belongs to a different technology, the honest grade for follistatin-as-a-peptide is D.
Primary sources (2)
Is Follistatin (FST-344 / FS344) legal? (Status July 2026)
No legal supervised US route.
What is Follistatin (FST-344 / FS344) used for?
Follistatin (FST-344 / FS344) is marketed for the goals below. See how it ranks against other peptides in each, by evidence, not hype.
What does Follistatin (FST-344 / FS344) cost, and how do you access it legally?
Typical cost
Gray-market research vials: roughly $60 to $200 each. Legitimate gene therapy is experimental and not commercially priced.
The price gap reflects two different things sold under one name: cheap research-labeled peptide of unproven bioactivity versus experimental gene therapy that exists only inside trials. Paying for the vials does not buy the gene-therapy evidence.
No legal supervised access route right now.
Follistatin (FST-344 / FS344) has no compliant US route today. Vials sold "for research use only" are a gray-market fig-leaf, not a legal loophole, so we don't link them. If you pursue Follistatin (FST-344 / FS344), do it with a licensed clinician, and re-check its legal status first.
Is Follistatin (FST-344 / FS344)safe? Side effects & risks
In the small gene-therapy trial, no serious adverse events occurred and liver, kidney, blood, and hormone measures stayed normal, but that is six patients with a gene-therapy construct, not evidence for repeated peptide injections. The injectable research peptide has no meaningful human safety data. Systemic myostatin blockade is not consequence-free in theory: myostatin and activin signaling touch tendon integrity, the heart, and reproductive and metabolic pathways, so broad long-term inhibition is a genuine unknown. Gray-market follistatin is unregulated for identity, purity, and dose, and given it is a large glycoprotein, much of what is sold may not even be bioactive. It is also a clear anti-doping violation.
Medical disclaimer: This page is independent editorial information, not medical advice, and Best Peptide For That is not a medical provider. We do not provide dosing. Talk to a licensed clinician before starting, stopping, or changing any peptide or medication. Full medical disclaimer.
FAQ
Follistatin (FST-344 / FS344) FAQ
Does injectable follistatin build muscle in humans?
There is no solid human efficacy data for the injectable peptide. The muscle-growth case comes from animal studies and from a small gene-therapy trial, which is a different technology, not a follistatin injection.
What is the difference between follistatin peptide and follistatin gene therapy?
The peptide is protein you inject, which is large and short-lived so its effect is fleeting. Gene therapy delivers the follistatin gene so muscle keeps producing it. The only real human data used gene therapy, not the peptide.
Is follistatin banned in sports?
Yes. WADA prohibits myostatin inhibitors, including follistatin and follistatin gene therapy, at all times. It is a clear anti-doping violation.
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