Follistatin 344
FST-344
Follistatin 344 (FST-344) is the name of a naturally occurring human protein isoform and, separately, of a gray-market injectable "peptide" sold to people seeking muscle growth. This distinction is the single most important safety point: essentially all human data on FST-344 come from two small, non-randomized gene-therapy studies (Mendell et al., Nationwide Children's Hospital) in which an AAV viral vector encoding FS344 was injected directly into the quadriceps of patients with Becker muscular dystrophy or sporadic inclusion body myositis. There are no human trials of the injected recombinant follistatin peptide that consumers actually buy. Follistatin works by binding and neutralizing myostatin (GDF-8) and activins, TGF-beta-superfamily proteins that restrain muscle growth; inhibiting them increases muscle size. Because the marketed peptide has never been studied in humans, its pharmacokinetics, purity, dosing, and toxicity are unknown; systemic activin blockade has broad off-target biology (reproductive/pituitary FSH regulation, inflammation, fibrosis, vascular and possibly tumor-related pathways) that has never been safety-tested for chronic systemic use; and gray-market product carries contamination, mislabeling, and immunogenicity risks. Any reassurance from the gene-therapy trials (which reported no adverse events) does not transfer to a self-injected peptide of unknown quality. Overall evidence grade: D.
Mechanism of action
Pharmacokinetics
Unknown for the injected recombinant peptide in humans; no human PK study exists. Endogenous circulating follistatin is cleared rapidly (short, minutes-to-hours-scale biological persistence), which is a major reason the human trials used sustained gene expression from an AAV vector rather than repeated protein injection.
Not established for the peptide. In the gene-therapy trials, localized transgene expression and histologic/functional benefit persisted for months to years after a single intramuscular vector injection, but this reflects continuous in-situ production, not the duration of an injected peptide dose.
Human data exist only for direct intramuscular AAV vector injection into a target muscle. The gray-market 'FST-344 peptide' is marketed for subcutaneous/intramuscular self-injection, a route and formulation never characterized in humans.
As a protein, follistatin is expected to undergo proteolytic degradation and receptor/tissue uptake with renal and hepatic reticuloendothelial clearance of fragments; specific human clearance parameters for exogenous FST-344 are unknown. Washout of an injected peptide cannot be reliably estimated from available data.
For monitoring and washout planning, not drug-test evasion.
Physiological & performance effects
- In two small non-randomized gene-therapy trials (AAV-delivered FS344 injected intramuscularly), treated patients showed histologic muscle hypertrophy, increased fiber size, reduced endomysial fibrosis, and reduced central nucleation.
- In the Becker muscular dystrophy trial (n=6), 4 of 6 patients improved on the 6-minute walk test (ranging ~29-125 meters) while others showed no change.
- In the sporadic inclusion body myositis trial (n=6), treated subjects improved a median +56 m/year on the 6-minute walk test versus -25.8 m/year decline in matched untreated subjects (p=0.01); 4 of 6 improved meaningfully, 2 minimally.
- Mechanistically the intended effect is increased skeletal muscle mass/strength via myostatin and activin inhibition.
- No efficacy or effect data exist for the injectable recombinant FST-344 peptide sold to consumers; claimed muscle-building effects in that context are unproven in humans.
Adverse effects by system
No human cardiovascular safety data for the injected peptide. The gene-therapy trials did not report cardiac adverse events but were tiny, localized to limb muscle, and not designed to detect cardiovascular effects. Activin/TGF-beta signaling participates in vascular and cardiac remodeling, so chronic systemic blockade is a theoretical unknown. Treat cardiovascular risk as uncharacterized.
No evidence of direct hepatotoxicity and no human liver-toxicity data for the peptide. As a protein it is not expected to cause the hepatic strain seen with 17-alpha-alkylated oral steroids, but gray-market product contaminants and immune reactions could secondarily affect the liver. Data absent.
Mechanistically concerning but not studied in humans for this product. Follistatin suppresses pituitary FSH by neutralizing activin; systemic elevation could perturb FSH/LH regulation and the hypothalamic-pituitary-gonadal axis. Broader activin blockade may affect other endocrine tissues. No human hormonal data exist for injected FST-344.
Mechanistically the highest-concern non-muscle system: follistatin's defining action is activin/FSH suppression, and activins are central to spermatogenesis, ovarian folliculogenesis, and gonadal function. Systemic follistatin excess could theoretically disrupt fertility and gonadal hormone signaling. No human reproductive safety data for the peptide.
No known psychiatric effects and no human neuropsychiatric data. Not characterized.
No human renal safety data. No specific renal toxicity is described in the trials. Activin A has roles in kidney fibrosis/injury biology, so systemic modulation is a theoretical unknown. Data absent.
No human hematologic data. Activin signaling influences inflammation and iron/erythropoiesis-related pathways (e.g., activin-hepcidin axis), so systemic blockade is a theoretical unknown. Immune responses/neutralizing antibodies to an exogenous protein are a hematologic/immunologic risk. Not studied.
No systematic dermatologic data. Injection-site reactions, local pain, swelling, sterile abscess, or infection are plausible with any self-injected peptide, and immunogenic/allergic skin reactions to a foreign or impure protein are possible. Not formally studied.
HPTA suppression & recovery
Suppression: Theoretically plausible but unquantified in humans; no direct data for the injected peptide
Follistatin suppresses pituitary FSH via activin neutralization, so systemic exposure could in principle perturb the hypothalamic-pituitary-gonadal axis and gonadal function, but there are no human hormonal or recovery data for injectable FST-344. Any hormonal disturbance, suspected suppression, or fertility concern should be evaluated and managed by a qualified endocrinologist with baseline and follow-up bloodwork; do not attempt self-directed hormonal 'recovery' protocols. This monograph does not endorse any SERM or other restart protocol for this compound because there is no evidence base to guide one.
Monitoring
Cadence: Establish baseline before any use, then recheck under clinician guidance (e.g., every 4-8 weeks during use and if new symptoms arise). There is no validated monitoring schedule because the peptide is unstudied; err toward more frequent clinician contact.
- Injection-site infection, spreading redness, severe pain, or abscess
- Allergic/immune reaction: rash, swelling, difficulty breathing
- New or worsening testicular/reproductive symptoms or changes in libido/fertility
- Unexplained fatigue, chest symptoms, shortness of breath, or edema
- Any new lump or rapidly changing mass (seek prompt evaluation given untested tumor-pathway effects)
- Jaundice, dark urine, or right-upper-quadrant pain
Contraindications
- Any active or history of malignancy or elevated cancer risk (myostatin/activin inhibition affects proliferative and tumor-related pathways; systemic safety is untested).
- Pregnancy, breastfeeding, or anyone trying to conceive (activin/FSH suppression is central to reproduction; fertility effects untested).
- Known or suspected hormonal/pituitary-gonadal disorders without endocrinology oversight.
- Any use of unverified, non-pharmaceutical-grade gray-market product (contamination, endotoxin, mislabeling, and immunogenicity risks).
- People unwilling or unable to obtain clinician supervision and baseline/follow-up bloodwork.
- Known hypersensitivity to the product or excipients.
Interaction profile
- ContraindicatedWith DNP: Additive cardiovascular strain
Check a specific combination in the interaction checker.
Reducing harm & when to stop
- Understand that the injectable 'Follistatin 344 peptide' is not what was tested in humans; the only human studies used a viral gene-therapy vector injected into muscle under research conditions. Do not treat 'no adverse events' from those trials as evidence the peptide is safe.
- Because the peptide is unstudied in humans, there is no evidence-based dose; this monograph does not provide one. Any clinical dose figures in the literature refer to viral vector genome copies (vg/kg) in gene therapy, which are not translatable to peptide dosing.
- Get clinician supervision and baseline bloodwork (liver, kidney, CBC, reproductive/pituitary hormones) before considering use, and repeat during use.
- Stop immediately and seek medical care for signs of infection at the injection site, allergic/immune reactions (rash, swelling, trouble breathing), jaundice, chest symptoms/shortness of breath, or any new or rapidly changing lump.
- Given the mechanism's reach into activin/FSH and reproductive biology, discuss fertility and hormonal risks with an endocrinologist; avoid entirely if pregnant, breastfeeding, or trying to conceive.
- Avoid if you have any cancer history or elevated cancer risk until systemic safety is established.
- Gray-market protein products carry real risks of contamination, endotoxin, mislabeled contents, and immunogenicity; there is no way to verify identity or purity outside a regulated supply chain.
- This is informational harm-reduction content, not an endorsement or instruction to use.
Citations (5)
Every clinical claim above is tied to a primary source. Overall evidence grade D — this compound lacks adequate human data; claims rest on preclinical or mechanistic evidence.
- 01
All human experience with FS344 comes from AAV gene-therapy delivered by direct intramuscular quadriceps injection, not from an injected recombinant peptide; the FS344 isoform was chosen to limit off-target binding.
Case seriesA phase 1/2a follistatin gene therapy trial for becker muscular dystrophy.PMID 25322757 ↗
- 02
In a phase 1/2a Becker muscular dystrophy trial (n=6), AAV1.CMV.FS344 produced histologic muscle hypertrophy, reduced endomysial fibrosis and central nucleation, with 6-minute-walk improvements in some patients and no reported adverse events; follistatin acts as a potent myostatin antagonist.
Case seriesDOI 10.1038/mt.2014.200 ↗
- 03
In a sporadic inclusion body myositis gene-therapy trial (n=6), rAAV1.CMV.huFS344 into quadriceps improved 6-minute walk distance (+56 m/year treated vs -25.8 m/year untreated, p=0.01) with decreased fibrosis and improved regeneration.
- 04
Across the Nationwide Children's follistatin/muscular-dystrophy gene-therapy program, follistatin was used as a potent myostatin antagonist delivered by AAV to increase muscle size and strength, with no adverse events encountered in the reported trials.
- 05
There is no adequate human data on the injectable recombinant follistatin-344 peptide sold to consumers; efficacy, pharmacokinetics, and safety in that form are unstudied (statement of evidence absence).
Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice