All peptides / Muscle and bone
Follistatin-344
Myostatin-inhibiting peptide
Also Follistatin 344, FS-344, FST-344, Follistatin, FS344
- Evidence
- CAnimal and cell studies
- FDA status
- Not approvedas of 2026-10-27
- Route
- Injection
- Studies cited
- 2 human1 animal or cell
Sold to bodybuilders to build muscle past normal limits. In people it has only been tested as gene therapy in 12 patients, never as an injected protein.
Sold for
Follistatin-344 is sold to bodybuilders as a myostatin blocker that unlocks muscle growth beyond natural limits.
Studies say
Two six-patient gene therapy trials and myostatin-knockout mice are the whole evidence base, and none of it involves injecting the protein, so the grade is C.
Why grade C. For muscle growth, the use follistatin-344 is sold for, the human evidence is two open-label gene therapy trials of six patients each, in Becker muscular dystrophy and inclusion body myositis, with no placebo arm. The dramatic muscle phenotypes come from mice lacking myostatin, not from giving follistatin to anyone. No study has tested the injected peptide in humans. 145
FDA. Not approved by the FDA for any use and sold as a research chemical. Follistatin has been given to people only as a gene therapy, first in two small academic trials of the follistatin-344 gene and since 2022 in a few small company-run early-phase studies. The injectable protein sold online has never been tested in people.
Written from 5 cited sources. Last refreshed 2026-10-27. Not physician reviewed.
What this means for you
- What people take it for
- Building muscle.
- Can a US doctor prescribe it?
- No. It is not approved and is sold as a research chemical.
- Properly tested in people?
- Not properly: animal or lab studies, or small uncontrolled human reports. Two gene therapy studies of six patients each, no comparison group.
Evidence checklist
- Randomised controlled trial Two open-label trials, no placebono
- Human study over 100 people Six patients in each trialno
- More than one research group Gene therapy trials from related academic groupsno
- Clinical outcomes, not surrogate markers Walk distance and muscle biopsiesno
- Human safety data beyond case reports Gene therapy only; injected protein untestedno
Drawn from the cited studies below. A dash means the sources give no information.
What it is
Follistatin is a natural protein. It binds and blocks several members of the TGF-beta family, including activin and myostatin. Myostatin is the body's brake on muscle growth. Animals and rare humans born without it develop about double the normal muscle mass. This was shown in 1997, when mice lacking the myostatin gene were created. 1
Follistatin-344 is one of two spliced forms of the human protein. It is a 344-amino-acid precursor that the body trims to a 315-amino-acid form in the blood. Researchers chose it for gene therapy. Unlike the 317 isoform, it binds less tightly to cell surfaces. They thought it less likely to suppress pituitary hormones. What is sold online as 'follistatin-344' is a recombinant protein. Its source is usually unverified. It has never been through any clinical trial. 24
How it works
Myostatin signals through activin receptors on muscle fibers to hold back their growth. Follistatin binds myostatin and activin in the blood and tissue. It stops them reaching the receptor. Remove the brake and muscle fibers get larger and regrow more easily. Follistatin also suppresses release of follicle-stimulating hormone from the pituitary. That is where its name comes from. Binding activin also affects reproduction, inflammation and tissue repair. So body-wide exposure raises more questions than a muscle-only effect would. 12
What human research shows
In the first trial, six men had Becker muscular dystrophy. They received a gene therapy vector (AAV1) carrying the follistatin-344 gene. It was injected directly into the thigh muscles of both legs. Over a year, four of the six walked farther in a six-minute walk test, by 29 to 125 meters. Two did not improve. Muscle biopsies showed less fibrosis and larger fibers. There was no placebo group. 24
A follow-up report at two years found the walking gains held in the responders. 3
The second trial gave the same gene therapy to six patients with sporadic inclusion body myositis. This is an inflammatory muscle disease. Patients also followed an exercise program. Treated patients gained an annualized 56 meters on the walk test. Eight matched untreated patients lost 26 meters. Four of six treated patients improved a lot, and two barely at all. 5
None of this involves injecting follistatin protein. Gene therapy makes muscle cells produce follistatin on the spot and without a break. An injected peptide would clear within hours. It would also spread through the whole body. 45
| Study | Model | Finding |
|---|---|---|
| Myostatin knockout mice 1997 | Mice, genetic deletion of myostatin | Animals lacking myostatin had two to three times the normal muscle mass, establishing myostatin as the brake on muscle growth. 1 |
| Becker muscular dystrophy gene therapy | Humans, n=6, open-label phase 1/2a, intramuscular AAV1 carrying follistatin-344 | Four of six patients walked 29 to 125 meters farther at one year; two did not improve; no adverse events. 4 |
| Two-year follow-up | Humans, same six Becker patients | Walking improvements in the responders were maintained at two years. 3 |
| Inclusion body myositis gene therapy | Humans, n=6 treated vs 8 matched untreated, open-label, with exercise program | Treated patients gained 56 meters per year on the walk test while untreated patients declined 26 meters. 5 |
Safety
The gene therapy trials reported no serious adverse events in twelve patients. That says nothing about injecting the protein into the body. Follistatin throughout the body raises theoretical concerns. It could suppress FSH and reproductive hormones. It could affect activin-dependent processes such as wound healing and inflammation. Its effects on the heart are unknown, since myostatin also regulates heart muscle. Products sold online have unverified identity and purity. 245
Questions for your clinician
- Is there any controlled human trial of injected follistatin?
- How could it affect my hormones, heart or wound healing?
- Is a gene therapy trial a better route for my muscle disease?
Questions people ask
Is follistatin-344 FDA-approved?
No. It has never been submitted for approval. The only human use has been in two academic gene therapy trials of six patients each. 45
Has injected follistatin-344 been studied in humans?
No. The human trials put the follistatin gene into muscle with a viral vector. The muscle then made its own follistatin. Injecting the protein is a different step with no human data. 45
Does blocking myostatin build muscle in people?
Genetically, yes. People and animals born without myostatin are heavily muscled. Drug trials of myostatin blockers in healthy adults and in muscular dystrophy have been disappointing. The follistatin gene therapy trials showed modest walking gains in some patients. They did not show bodybuilding-level growth. 14
Citations
- McPherron 1997, Nature: regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member (myostatin) doi.org
- Mendell 2015, Molecular Therapy: a phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy (DOI record) doi.org
- Al-Zaidy 2015, Journal of Neuromuscular Diseases: follistatin gene therapy improves ambulation in Becker muscular dystrophy doi.org
- Mendell 2015, Molecular Therapy: a phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy pubmed.ncbi.nlm.nih.gov
- Mendell 2017, Molecular Therapy: follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes pubmed.ncbi.nlm.nih.gov