Compounds / Follistatin 344
Follistatin 344
Also known as: FST-344; FS344; follistatin (344-residue precursor isoform); activin-binding protein; FSH-suppressing protein
Primary research focus: Muscle-growth and muscle-wasting research (myostatin and activin antagonist)
Last updated July 2026 · Reviewed against FDA labeling and published research.
What is Follistatin 344?
Follistatin 344 (FST-344) is the 344-amino acid precursor isoform of follistatin, a naturally occurring glycoprotein that binds and neutralizes myostatin (GDF-8) and activin. By blocking these growth-limiting signals it can promote muscle growth. It has been studied in humans via AAV gene therapy for muscular dystrophy and preclinically as a recombinant protein. It is not FDA-approved, and no injectable follistatin protein protocol has been validated in humans.
Follistatin is a high-affinity binding protein and functional antagonist of TGF-beta superfamily ligands, most notably myostatin (GDF-8) and activin A and B. By sequestering these ligands extracellularly, it prevents them from engaging their type II receptors (ActRIIA/ActRIIB) and blocks the downstream SMAD2/3 signaling that normally restrains muscle protein synthesis and satellite-cell activation. The net effect is relief of myostatin-mediated inhibition, allowing muscle fiber hypertrophy. The FS344 precursor is processed to the mature FS315 (circulating) isoform, which was favored in gene-therapy work for its skeletal-muscle targeting and reduced effect on reproductive (FSH) signaling.
How it’s supplied
Not an FDA-approved or commercially available medicine. In clinical research it has been delivered as AAV gene therapy (AAV1.CMV.FS344) by intramuscular injection, not as a protein injection. It is separately sold as a research-grade recombinant lyophilized glycoprotein (commonly 1 mg vials) for laboratory use; these products are not FDA-reviewed for potency or sterility, and no validated injectable protein protocol exists.
Dosage Chart
Units shown are for a U-100 insulin syringe (100 units = 1 mL), calculated from the vial size and BAC water you select. Always confirm against your prescription and your syringe markings.
| Phase | Dose | Frequency | Notes |
|---|---|---|---|
| Days 1-30 | 100mcg | 1x Daily | — |
- Rotate injection sites to reduce irritation and lipohypertrophy
- Not for use during pregnancy or breastfeeding
- Do not exceed the recommended dose to try to speed results
Potential Benefits and Side Effects
Effects reported in published research. Not a promise of results, and not medical advice.
- Reported in gene-therapy trials and preclinical research; injectable-protein efficacy is not established.
- Potent inhibition of myostatin (GDF-8) and activin, relieving natural limits on muscle growth.
- Muscle hypertrophy and strength gains in animal models and non-human primates.
- Improved six-minute walk distance in some Becker muscular dystrophy patients receiving follistatin gene therapy.
- Reduced fibrosis and more normal muscle-fiber histology reported after gene therapy.
- In the follistatin gene-therapy trials, no serious adverse effects and no effects on pituitary hormones were reported at the doses used.
- For injectable or systemic myostatin inhibition more broadly, ocular events (including retinal detachment) have been reported in some contexts.
- Excessive or irreversible muscle hypertrophy and unknown off-target effects are concerns for unvalidated protein use.
- Human safety data for the recombinant protein are essentially absent.
Warnings and contraindications
Important safety information. This is not exhaustive — read the full prescribing information and consult your prescriber or pharmacist.
- No FDA-approved labeling exists; the following reflect trial observations and the compound's status, not established label warnings.
- The human data come from AAV gene therapy, not from injecting recombinant follistatin protein; injectable protein use is unvalidated.
- Injectable and gene-therapy myostatin inhibition has been associated with ocular adverse events, including retinal detachment, in some settings.
- Strong myostatin inhibition can cause substantial and potentially difficult-to-reverse muscle hypertrophy, with unknown long-term effects on tendons and joints.
- Because follistatin also binds activin and influences FSH, off-target endocrine and reproductive effects are a theoretical concern (minimized with the FS315 isoform in trials).
- Follistatin is prohibited in sport under the WADA Prohibited List.
- Research-grade material varies in identity and purity.
- Not established as an approved drug; no FDA labeling. Precautionary considerations include active or suspected malignancy (growth-promoting signaling), pregnancy (activin/FSH biology), and known hypersensitivity.
Reconstitution Steps
How to prepare the lyophilized vial. Confirm specifics with your pharmacist or prescriber.
- Confirm the amount of peptide in the vial and the volume of diluent you intend to use before starting.
- Wash your hands and gather supplies: the lyophilized vial, the diluent (typically bacteriostatic water), an alcohol swab, and a sterile syringe.
- Let the vial and diluent reach room temperature if they were refrigerated or frozen.
- Wipe the rubber stopper of both the peptide vial and the diluent vial with a fresh alcohol swab and let them air dry.
- Draw the intended diluent volume into the syringe.
- Insert the needle into the vial and slowly release the diluent down the inside wall rather than directly onto the powder, to avoid foaming.
- Do not shake. Gently swirl or roll the vial until fully dissolved.
- Inspect the solution: it should be clear and colorless with no visible particles. Do not use if cloudy or containing particles.
- Note the resulting concentration (amount divided by diluent volume) so it matches the reconstitution calculator on this page.
- Label the vial with the reconstitution date and store as directed under reconstituted storage.
US regulatory status
Classifications are public record but can change. For reference only, not legal advice.
Development: Follistatin was first isolated in 1987 from porcine ovarian follicular fluid as an inhibitor of FSH secretion, and later characterized as the high-affinity binding protein for activin and myostatin. Its muscle-growth potential is illustrated by the myostatin-knockout 'mighty mice.' In humans, follistatin has been studied as AAV gene therapy: a Phase 1/2a trial (Mendell et al., Molecular Therapy 2015) delivered AAV1.CMV.FS344 by intramuscular injection to six Becker muscular dystrophy patients, with some improvement in the six-minute walk test, supportive histology, and no serious adverse events, and the program was extended to sporadic inclusion body myositis. The FS344/FS315 isoform was chosen to minimize effects on reproductive hormones. The injectable recombinant protein remains preclinical and unvalidated in humans. There is no FDA-approved follistatin product as of mid-2026.
Notes: Follistatin 344 is not FDA-approved. As a recombinant glycoprotein and gene-therapy target rather than a small-molecule bulk substance, it is not on the FDA 503A bulk drug substances list and is not eligible for 503A or 503B compounding; gene therapy is regulated as a biologic. Products sold as research-grade recombinant follistatin are not FDA-reviewed for safety, potency, or sterility. Follistatin is prohibited in sport (WADA). It is not a controlled substance.
International status
Pharmacokinetics
Storage and handling
Important Notes
Practical considerations for consistency and safety.
- Follistatin 344 is a large glycoprotein (about 37 to 38 kDa), not a small peptide; ignore vendor listings citing roughly 3,780 Da.
- The human evidence is from AAV gene therapy (AAV1.CMV.FS344), not from injecting recombinant protein; injectable protein use is unvalidated.
- Myostatin inhibition has been linked to ocular risks (including retinal detachment) and can cause hard-to-reverse muscle hypertrophy.
- Follistatin is banned in sport (WADA).
- Research-grade material varies in identity and purity.
Lifestyle Factors
- Muscle-growth effects are studied alongside resistance training in the broader myostatin-inhibition literature.
- Human lifestyle interactions for the recombinant protein are not characterized.
References
Related compounds
Important disclaimer
This page is an educational reference, not medical advice, a diagnosis, or a treatment recommendation, and not a substitute for your prescriber or pharmacist. Approval and regulatory status varies by compound and can change. Always confirm against current prescribing information and consult a qualified healthcare professional before making any decision about a medication.