Follistatin 344

DNot approved; research reagentGrowth hormone, IGF-1 and growth factors →FS344follistatinFST-344
In one paragraphFollistatin is a natural protein that binds and blocks myostatin, the signal that limits muscle growth; animals engineered to overexpress it become dramatically muscular. 'Follistatin 344' is the full-length form, sold as a lyophilised injectable for muscle. No human trial has ever tested injecting it. The only clinical data is a 2015 gene-therapy study in six men with Becker muscular dystrophy, in which a virus carrying the follistatin gene was injected into the thigh muscles — some walked further, some did not, and it is not the product being sold. It is prohibited in sport.
Schematic 3D structure of Follistatin 344
344 residues · glycoprotein with three follistatin domains · schematic
Overall gradeD No human evidence
Human trials0 for the injected protein; 1 gene-therapy trial (n=6)
Outcomes graded4
Last reviewed10 September 2026
EvidenceWhat it isHow it worksUnknownsMolecule & PKRegulationQuestionsReferences

What the human research shows

The gap between the animal pictures and the human evidence is about as wide as it gets on this site. Everything below the first row is preclinical.

MetabolicCardioMuscleHealingNeuroImmuneSexualSkinAgeing

Evidence profile0–5

  • Metabolic & weightD0.0
  • Cardiovascular—0.0
  • Muscle & growth—0.0
  • Healing & repair—0.0
  • Neuro & sleep—0.0
  • Immune & inflammation—0.0
  • Sexual & hormonal—0.0
  • Skin & appearance—0.0
  • Ageing & longevity—0.0
No benefit outcome for Follistatin 344 has both human evidence and a measurable effect, so the profile is empty. That is the finding, not a gap in the chart.
OutcomeGradeEffect sizeHuman studiesWhere the evidence comes from
Muscle growth / strength (injected protein)DNo human evidenceNot measurable0 controlledNo human study of any kind has given follistatin protein by injection. The grey-market product has no clinical file, and a 344-residue glycoprotein injected subcutaneously would be expected to be cleared quickly and to bind targets far beyond muscle.2
Muscle function in Becker muscular dystrophy (gene therapy)DNo human evidenceSmall1 open-label phase 1/2a (n=6)A follistatin gene delivered by AAV into the quadriceps of six patients: four walked further on the six-minute test, two did not change, with no control group. Encouraging as proof of principle; not evidence for the injected protein, and not evidence in healthy people.1
Fat lossDNo human evidenceNot measurableNoneClaimed on the basis of myostatin-knockout animals; no human data.
Safety in humansadverseDNo human evidenceNot measurableNone for the proteinFollistatin also neutralises activin, which regulates reproduction, inflammation and the pituitary. Systemic blockade in humans has never been characterised. The gene-therapy trial reported no adverse effects in six people over a limited follow-up — in muscle-restricted delivery.
AMultiple good human trials, consistentBSome human trials, mostly consistentCFew or weak human studiesDAnimal or anecdotal onlyeffect size · red = adverse

Literature searched to 10 September 2026. Human studies only; animal and cell work informs the mechanism section and nothing else.

Registered trials

ClinicalTrials.gov lists 4 interventional studies naming Follistatin 344 as an intervention — 3 completed, 1 recruiting, and none with results posted to the registry.

A registration is not a result. Anyone can register a study, and many never report — so this list shows what has been attempted, not what has been shown. The graded evidence above is the answer to whether something works.

StudyPhaseStatusParticipantsSponsorResults
Phase I: Safety and Efficacy of an Injectable Follistatin Plasmid Gene Therapy in HumansNCT06411366 · started Aug 2022Phase 1Completed43MinicircleNone
Follistatin Gene Transfer to Patients With Becker Muscular Dystrophy and Sporadic Inclusion Body MyositisNCT01519349 · started Jan 2012Phase 1Completed15Nationwide Children's HospitalNone
CALM-AF-AI: Counteracting Age-related Loss of Muscle With AAV-Follistatin Combined With Angiogenesis-Inducing VEGF Plasmid Gene TherapyNCT07443826 · started Jun 2026Phase 1Recruiting12*Unlimited Biotechnology LLCNone
Safety and Efficacy of Klotho and Follistatin Gene TherapyNCT07285629 · started Dec 2025EARLY_PHASE1Completed14MinicircleNone

Participant counts marked * are the sponsor's estimate, not a final enrolment. Source: ClinicalTrials.gov, read directly from the registry and refreshed weekly.

What it is

Follistatin is a glycoprotein made throughout the body that binds members of the TGF-β family — myostatin and activin above all — and stops them reaching their receptors. FS344 is the longer of its two natural splice forms. Mice, cattle and dogs with disabled myostatin are the 'double-muscled' animals whose photographs sell the product.

It is sold as a research-grade protein, usually as 1 mg vials, on the promise of those animals. It has never been in a human trial as an injected drug, and the pharmaceutical work on myostatin has gone into antibodies and receptor decoys instead — most of which have also failed to build clinically useful muscle in trials.

How it is thought to work

Myostatin restrains muscle growth by signalling through the activin receptor; follistatin sequesters myostatin so the brake is released. In the gene-therapy trial the gene was delivered directly into muscle so the protein was made locally and continuously. An injected protein under the skin is a different pharmacology entirely.

What is still unknown

  • Whether injected follistatin reaches muscle in any meaningful amount before it is cleared — no human data.
  • The consequences of blocking activin systemically — fertility, inflammation and pituitary effects are all plausible and unstudied.
  • What is actually in the vials sold as follistatin 344.

The molecule and how long it lasts

Identity

A 344-residue glycoprotein — no small-molecule record

Pharmacokinetics
Route
Injected as sold; no human study of the peptide
Half-life
No human pharmacokinetic data

No pharmacokinetic study in people.

Regulatory status

European Union
Unauthorised medicinal product

No marketing authorisation anywhere. Supplying it for human use is an offence under national medicines law.

United States
Not approved

Sold as a research reagent 'not for human use'; no clinical programme for the protein.

United Kingdom
Unlicensed medicine

No MHRA licence; supply for human use is an offence.

Australia
Prescription-only / unapproved

Not on the ARTG; imports are seized.

Canada
Not authorised

No Drug Identification Number.

Sport (WADA)
Prohibited at all times

Myostatin inhibitors are prohibited under the S4 class (agents preventing activin receptor IIB activation), in and out of competition.

Questions people actually ask

Does follistatin 344 build muscle?
In animals engineered to make more of it, dramatically. In humans, no trial has tested injecting it, so the honest answer is unknown — and the six-patient gene-therapy study people cite used a virus to make muscle produce it locally, which is a different intervention.
Is the gene-therapy trial evidence for the injection?
No. Local, continuous production inside the muscle from a delivered gene is not the same as a protein injected under the skin, which is cleared quickly and acts everywhere. The trial also had six patients and no control group.
Is follistatin safe?
Unknown. It blocks activin as well as myostatin, and activin does many things — including in reproduction. No one has measured what happens when it is blocked systemically in people.

One page per claimed effect

References

  1. 1
    Mendell JR, Sahenk Z, Malik V, et al. A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy.Mol Ther, 2015Six patients; AAV1.CMV.FS344 injected into the quadriceps; no control group.
  2. 2
    Searches of PubMed and trial registries for injected follistatin in humans.LYO literature search, September 2026No human trial of the injected protein identified.
  3. 3
    World Anti-Doping Agency. The Prohibited List — S4: hormone and metabolic modulators.WADA, current listMyostatin inhibitors / agents preventing activin receptor IIB activation.

Change log

  • 2026-09-10Initial publication. Grade D on absence of any human trial of the injected protein; the gene-therapy study is described and separated.