Follistatin (FST-344): Mechanism, Dosage and Side Effects
Follistatin (FST-344): the common dosing protocol, what its human gene-therapy and animal research actually shows, and its side effects and legal status.
The short version
Follistatin is a protein your body already makes to hold myostatin and activin in check, two signals that otherwise limit muscle growth. Vendors sell an injectable version, often labelled "FST-344," as a muscle-building research chemical. The only human trials of raised follistatin used one-time gene therapy in muscle-wasting disease, not repeated protein injections in a healthy person, so that evidence does not transfer to the vial being sold.
- It blocks myostatin and activin in animal studies; that is well established.
- No human trial has tested the injected protein sold as "FST-344."
- The only human data comes from gene therapy for muscle disease, a different product.
- FDA has named Follistatin in a compounding warning letter.
- It is a named, banned substance for any WADA-tested athlete, at all times.
Follistatin at a glance
- What it is
- Naturally occurring single-chain glycoprotein; sold to consumers as a recombinant splice-variant fragment labelled "FST-344"
- Main use
- The body's own brake on activin and myostatin signalling, studied academically as a gene-therapy payload for muscle-wasting disease. Sold separately, as an injected research-chemical protein, for grey-market muscle-building use that its own human trials have never tested.
- Also called
- FST, Follistatin-344, FST-344, Activin-Binding Protein
- Size
- 344 amino acids Details
- Evidence
- Animal / lab only
- US status
- Research use only
- Half-life
- Not established in humans
- How it is taken
- Subcutaneous injection
- Common protocol
- 100-500 mcg, 2 to 4 times a week, for 8-16 weeks (common practice, not trial-established) Dosage chart
Dosing: what the human gene-therapy and animal studies used.
Follistatin is a protein your body already makes, in muscle, the gonads and elsewhere, to keep activin and myostatin signalling in check. Raising it builds muscle in animals, and gene-therapy trials have tested it in people with muscle disease. What is sold online as a “research peptide”, often labelled FST-344, is a recombinant version made by an unregulated vendor, and no trial has tested it.
Studied in cells or animals. No controlled trials in humans. No human trial has tested the injected recombinant protein sold online. The only human data on raised follistatin comes from a one-time gene therapy delivered into diseased muscle in small trials for Becker muscular dystrophy and inclusion body myositis: a different product, route and population, so it does not transfer to the vial being sold.
What is Follistatin?
Follistatin is a single-chain glycoprotein that binds and blocks activin and myostatin (also called GDF-8), two signalling proteins in the TGF-beta superfamily. In animal and cell studies, myostatin limits how much muscle a body builds. Follistatin binds it, along with activin, and stops both reaching their receptor on the cell surface. Remove that brake in an animal and muscle mass increases; the mechanism is well characterised in cells and animals.
The body makes more than one form:
- FST315, the main form in the blood, has an acidic tail that keeps it circulating rather than sticking to tissue.
- FST288 lacks that tail. It binds tightly to proteoglycans on cell surfaces and stays in the tissue that made it.
Vendors selling “Follistatin-344” are naming the longer precursor that is processed into the circulating form. The name says nothing about whether a vial contains it, correctly folded, at the stated purity, and no regulator checks that it does.
Follistatin structure
- Length
- 344 amino acids
- Modifications
- FST-344 is the 344-residue precursor (UniProt isoform 1); a 29-residue signal peptide is removed, leaving the 315-residue mature protein (FS315). Glycoprotein.
Natural human protein, too large for a 2D diagram. What grey-market vials actually contain is not verifiable.
Sequence checked against UniProt.
Follistatin benefits: what the research shows
Follistatin has been tested in people, as gene therapy. Researchers at Nationwide Children’s Hospital ran small trials in adults with Becker muscular dystrophy and, separately, sporadic inclusion body myositis. A viral vector, injected once into muscle, made the patient’s own muscle cells produce a follistatin isoform.
The published reports describe larger muscles on imaging and, in the inclusion body myositis group, a longer walking distance. Other researchers disputed the walking result in the same journal. They argued that the published primary outcome differed from the one registered before the trial, and that the statistical method was unreliable. The dispute is about how the walking result was analysed, not the muscle-size finding or safety, and no larger trial has settled it.
What is not established
No study has measured what a healthy person’s body does with injected recombinant follistatin. There is no human pharmacokinetic data, no known half-life and no dosing schedule with evidence behind it. I don’t publish one, and I would treat any schedule online as invented.
Follistatin is also not a muscle-only switch. It binds activin broadly, and activin has established roles in reproductive hormone regulation and other tissues outside skeletal muscle. Suppressing that signal body-wide, in a healthy adult, by repeated injection, has not been studied in humans. Off-target effects have not been ruled out because no one has looked for them.
Follistatin dosage
The common Follistatin dose is 100-500 mcg, 2 to 4 times a week. From a 1 mg vial mixed with 1 mL of bacteriostatic water, 300 mcg is 30 units (0.3 mL) on a U-100 insulin syringe.
The protocol most often used in practice is 100-500 mcg injected under the skin, 2 to 4 times a week, for roughly 8-16 weeks. That is common practice, not a tested dose: no published trial has dosed the injected protein sold online. The only trial dosing that exists is for different products: a one-time viral-vector gene therapy in two small human trials, plus a mouse study of a different follistatin isoform. Full detail on all three, and the reconstitution calculator: Follistatin dosage.
Follistatin side effects
No published human trial has given anyone the injected protein sold as FST-344. The only human data come from the two small gene-therapy trials, which delivered a different, one-time product into diseased muscle.
Reported in studies
- Becker muscular dystrophy: no adverse effects across its six patients Mendell 2015 - Mendell JR, Sahenk Z, Malik V et al. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther. 2015;23(1):192-201., in the references below.
- Sporadic inclusion body myositis: the available abstract gives no verifiable adverse-event tally Mendell 2017 - Mendell JR, Sahenk Z, Al-Zaidy S et al. Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes. Mol Ther. 2017;25(4):870-879., in the references below. The later dispute over this trial concerned how its walking result was analysed, not safety.
Neither trial says anything about repeated injections of the recombinant protein in a healthy person. The activin question under what is not established, possible effects outside muscle, also applies here and remains unstudied.
Follistatin is usually sold for injection under the skin. Redness, itching or swelling at the injection site is covered in general at injection-site reactions; no study has looked at this for follistatin itself.
Is Follistatin legal?
US legal status - Research use only
Not FDA-approved for any use, and not a dietary supplement. FDA named Follistatin directly in an April 2020 warning letter to a compounding pharmacy, stating it failed the conditions for the section 503A compounding exemption - no applicable USP/NF monograph, not a component of an FDA-approved drug, and not on the 503A Bulks List.
The same warning letter records that the pharmacy told FDA, in 2018, that it had “decided to cease selling Follistatin and have removed it from [its] formulary”. The supplier itself had dropped it two years before FDA put its finding in writing.
Follistatin is also named on the WADA 2026 Prohibited List, section S4.3 (Agents Preventing Activin Receptor IIB Activation), as a myostatin-binding protein. It is banned at all times, in and out of competition, for any athlete subject to testing.
Related
Regulatory changes for this compound and others, as they happen: US peptide regulation tracker.
How Follistatin compares
Growth hormone and growth factors. Doses for unapproved compounds are common practice, not trial-established. All growth hormone and muscle peptides →
| Compound | Main use | Evidence | Usual dose | US status |
|---|---|---|---|---|
| HGH 191AA | Marketed to consumers for body recomposition, recovery and anti-aging in adults who do not have a diagnosed growth hormone deficiency. The identical molecule, somatropin, is FDA-approved by prescription for defined deficiency and wasting conditions - see the "Where approved somatropin fits" section on the main guide for that distinction. | Limited human data | 333-1333 mcg, Once daily (split into a morning and evening dose above roughly 3 IU/day), subcutaneous | Research use only |
| IGF-1 LR3 | Originally engineered as a laboratory reagent - an IGF-1 variant with reduced binding to IGF-binding proteins, used to study IGF-1 signalling in cell culture. Sold to consumers as a research chemical for muscle growth, a use with no controlled human trial behind it. | Animal / lab only | 20-50 mcg, once daily | Research use only |
| IGF-1 DES | Originally isolated and studied as a naturally occurring IGF-1 fragment in laboratory pharmacology, used to probe IGF-1 receptor signalling with reduced interference from binding proteins. Sold to consumers as a research chemical for localised, site-specific muscle growth, a use with no controlled human trial behind it. | Animal / lab only | 20-120 mcg, on training days only, about 4-5 times a week, not on rest days | Research use only |
| MGF | Marketed to consumers as a peptide injected directly into a trained muscle immediately after exercise, for local muscle repair and growth. No completed human trial has tested it for this or any outcome. | Animal / lab only | 100-400 mcg, Immediately post-workout, 2-3 times a week (training days only) | Research use only |
| PEG-MGF | Marketed to consumers for muscle repair and growth after resistance training. Never approved for human use; FDA states it has identified no human exposure data on it by any route. | Animal / lab only | 200-400 mcg, 2-3 times a week, post-workout | Research use only |
| Follistatin | The body's own brake on activin and myostatin signalling, studied academically as a gene-therapy payload for muscle-wasting disease. Sold separately, as an injected research-chemical protein, for grey-market muscle-building use that its own human trials have never tested. | Animal / lab only | 100-500 mcg, 2 to 4 times a week | Research use only |
Where to buy Follistatin for research
Each vendor is checked against my vendor checklist.
I don't list a vendor for Follistatin yet. Before buying from anyone, work through the vendor checklist.
Common questions
What is the usual Follistatin dose?
The protocol most often used in practice is 100-500 mcg injected under the skin, 2 to 4 times a week, for roughly 8-16 weeks. That is common practice, not a tested dose: no published trial has dosed the injected protein sold online. Full detail, including the gene-therapy and mouse doses that do exist: [Follistatin dosage](/follistatin-dosage/).
What are the side effects of Follistatin?
No published human trial has given anyone the injected protein sold online. The only human trials of raised follistatin were two small gene-therapy studies, a different one-time product delivered into diseased muscle. The Becker muscular dystrophy trial reported no adverse effects; the inclusion body myositis trial gives no verifiable adverse-event tally. Neither says anything about repeated protein injections in a healthy person.
What does follistatin actually do?
It binds and blocks activin and myostatin (GDF-8), two proteins in the TGF-beta family that normally restrain muscle growth. In animal studies, removing that restraint increases muscle mass. The mechanism is well characterised in cells and animals.
Has follistatin been tested in humans?
Yes, but not the product sold online. Researchers at Nationwide Children's Hospital ran small gene-therapy trials in adults with Becker muscular dystrophy and sporadic inclusion body myositis. A viral vector, injected once into muscle, made the muscle produce follistatin itself. That is single-dose gene therapy in diseased muscle, not repeated protein injections in a healthy person, and it has not become an approved product.
Is "FST-344" the same thing as the follistatin studied in gene-therapy trials?
They share a name, not a verified product. The trials delivered a gene that tells the patient's own cells to make a specific follistatin isoform. A vial bought online is a recombinant protein, made and tested by an unregulated vendor, and no regulator checks that it matches its label.
Is follistatin FDA-approved?
No, it has never been approved for anything. FDA named Follistatin in an April 2020 warning letter to a compounding pharmacy, saying it did not meet the conditions to be lawfully compounded under section 503A.
Is follistatin banned in sport?
Yes. WADA's 2026 Prohibited List names follistatin under section S4.3 as a myostatin-binding protein. It is banned at all times, in and out of competition, for any athlete subject to testing.
Is it safe to take follistatin?
It has not been tested, so no one knows. No published human trial has given a healthy person repeated injections of recombinant follistatin: there is no human safety record, no known side-effect profile and no established dose. The only human data comes from one-time gene therapy in diseased muscle, which doesn't answer the question for a protein injected repeatedly.
Does follistatin burn fat?
There is no human evidence either way. Its studied mechanism is blocking myostatin and activin to increase muscle, not a fat-metabolism pathway. No published trial has measured body fat after follistatin exposure in humans, so any fat-loss claim for the injected product is unverified.
Are follistatin gains permanent?
That has not been studied. The gene-therapy trials did not report long-term follow-up after treatment ended. No study has tracked what happens after stopping the injected protein either, because that product has never been tested in a trial.
Does follistatin build muscle?
In animals, yes: mice engineered to make more follistatin, or injected with it daily, gained muscle mass. In people, only one-time gene therapy in diseased muscle has been tested, and small trials reported larger muscle fibres on biopsy. No human trial has tested whether the injected protein sold as FST-344 builds muscle in a healthy person.
References (4 sources)
Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.
- 1.Mendell JR, Sahenk Z, Malik V et al. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther. 2015;23(1):192-201.Clinical trialPMID 25322757full text
- 2.Mendell JR, Sahenk Z, Al-Zaidy S et al. Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes. Mol Ther. 2017;25(4):870-879.StudyPMID 28279643full text
- 3.FDA warning letter to Tailor Made Compounding LLC (594743), naming Follistatin among bulk drug substances that fail the section 503A compounding conditions, and quoting the firm's own 2018 statement that it had stopped selling it- US Food and Drug Administrationregulatory
- 4.WADA 2026 Prohibited List (International Standard), section S4.3, Agents Preventing Activin Receptor IIB Activation - lists follistatin by name as a myostatin-binding protein, under substances prohibited at all times- World Anti-Doping Agencyregulatory