Follistatin 344
Follistatin isoform FST-344 (recombinant human follistatin, 344 amino acids)
Follistatin 344 is a naturally occurring glycoprotein that inhibits myostatin, activin, and other TGF-beta superfamily members. AAV-delivered gene therapy showed muscle hypertrophy in a Phase 1/2a trial for Becker muscular dystrophy (PMC5240576) and strong preclinical models (PNAS 2008; Kota et al. 2009). The injectable recombinant peptide form lacks controlled human data, and product quality on the gray market is extremely unreliable (PMID 31758732).
Last updated: 2026-03-10
Safety Summary
Safety data is extremely limited for injectable recombinant follistatin. No consistent side-effect profile for direct FS344 peptide use has been established. The most controlled data comes from AAV gene therapy trials: the Phase 1/2a BMD trial (6 patients) reported no adverse effects (PMC5240576; PMC4426808), and the sIBM trial (3 patients at low dose) reported no adverse events (PMC5240576). Minicircle's gene therapy program (500+ patients) reported the most common side effect as a slight LDL increase (~8 mg/dl) in roughly one-third of patients, which they describe as not clinically significant (minicircle.io). Community reports of injectable use include flu-like symptoms from a single online communities user, and weakened ligaments/tendons from bodybuilding community reports (SelfHacked). These effects may be attributable to product impurities rather than follistatin itself, given that only 9/17 tested products contained actual follistatin (PMID 31758732). Myostatin is expressed in cardiac tissue, and chronic inhibition raises theoretical cardiovascular concerns, but no long-term human data addresses this (peptidesunleashed.com). Black-market adulteration remains a separate and substantial safety concern (PMID 31758732).
Known Side Effects
common (roughly one-third of gene therapy patients)
uncommon (reported in community use)
uncommon (reported in community use)
unknown
unknown (theoretical)
Who Should NOT Use This
WADA lists follistatin under the prohibited S4 class (agents preventing activin receptor IIB activation / myostatin inhibitors). WADA has specifically funded detection and administration studies for black-market FS344 and FS315 products (WADA resource 19C09CR; PMID 31758732).
Follistatin promotes cell proliferation while restricting metastasis. Follistatin expression has been associated with improved survival in breast cancer but shortened survival in lung, ovarian, and gastric cancers (PeptideWiki EXA-708870cf9d11; SelfHacked tavily-safety). The dual role of follistatin in cancer biology warrants extreme caution.
Minicircle's FST gene therapy is not recommended for pregnant individuals or those planning pregnancy within 12 months (minicircle.io). Follistatin regulates reproductive hormones including activin and FSH (PMC2717722).
Only 9 of 17 tested black-market products actually contained follistatin; others contained His-tagged oligomers or different growth-promoting peptides entirely (PMID 31758732). Source authenticity is a major safety concern.
Cell studies show rapamycin reduces follistatin levels, particularly in prostate cancer cells (SelfHacked, tavily-safety). Potential interaction that may reduce follistatin effectiveness. In vitro data only.
Follistatin binds activin and can suppress FSH secretion, potentially affecting reproductive function. The FS344/315 isoform has approximately 10-fold lower activin affinity than FS288, and gene therapy trials showed no reproductive effects at tested doses (PMC2717722; PMC5240576). However, effects of injectable recombinant follistatin on fertility are not characterized.
Talk to Your Doctor
Before considering Follistatin 344, discuss it with your healthcare provider. Ask about potential interactions with your current medications, whether it is appropriate for your health conditions, and what monitoring may be needed.
Sources: [1-10]
Evidence Assessment
Tier 4: Phase 1/2a human clinical trial data exists for the AAV gene therapy form of FS344 in Becker muscular dystrophy (Mendell et al. 2015, PMC5240576; PMC4426808) and early sIBM trials, with broader dystrophy programs reviewed (PMID 34490244, Yao et al. 2021). Strong preclinical data in mice (>2 years of benefit, PNAS 2008) and nonhuman primates (Kota et al. 2009). However, these trials use AAV-mediated gene therapy, NOT injectable recombinant peptide. No Phase 3 trials exist, no replicated randomized controlled trial program, and no approved formulation. The injectable peptide form has no controlled human clinical trials for any indication. Most data on injectable follistatin comes from animal studies and community self-experimentation. Product quality is extremely unreliable (PMID 31758732, only 9/17 black market products contained actual follistatin).
1Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors
Haidet AM et al. - Proceedings of the National Academy of Sciences (2008) - Animal study (mouse) - Multiple mouse cohorts (normal C57BL/6 and dystrophic mdx)
Single AAV1 injection of FS344 produced sustained muscle size and strength enhancement lasting >2 years in both normal and dystrophic mice. FS344 was more potent than GASP-1 or FLRG in head-to-head comparison. Older mdx mice maintained increased grip strength through 560 days. Authors suggest FS344 may offer a more powerful strategy than targeting myostatin alone due to additive effects from involvement in multiple signaling pathways.
Limitations: Animal study only. Gene therapy delivery, not injectable peptide. Exact cohort sizes not fully available in bundled excerpts.
2Follistatin gene delivery enhances muscle growth and strength in nonhuman primates
Kota J et al. - Science Translational Medicine (2009) - Animal study (nonhuman primate) - Cynomolgus macaques with contralateral controls; exact N not fully specified in available excerpt
AAV1-FS344 injection into quadriceps enhanced muscle mass and strength in macaques. FS344 transgene had 98% homology with primate follistatin. Well tolerated with no treatment-related pathologic changes in major organ systems. Animals maintained on immunosuppressants (tacrolimus + mycophenylate mofetil). No adverse effects on reproductive capacity.
Limitations: Nonhuman primate study. Required immunosuppression protocol. Gene therapy delivery, not injectable.
3Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease
Rodino-Klapac LR et al. - Muscle & Nerve (Review) (2009) - Review - N/A (review)
Comprehensive review of the strategy using alternatively spliced FS344 cDNA delivered by AAV to muscle, producing FS-315 peptide that circulates systemically. Translational studies showed increased muscle size and strength from mice to monkeys. Adverse effects avoided; no organ pathology or change in reproductive capabilities. Established the rationale for using FS344 over FS317 to minimize off-target effects.
Limitations: Review article synthesizing preclinical data. No original human clinical data.
4A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy
Mendell JR et al. - Molecular Therapy (2015) - Phase 1/2a clinical trial (gene therapy) - 6 male patients with Becker muscular dystrophy
AAV1.CMV.FS344 gene therapy improved ambulation (6-minute walk test) in BMD patients. No adverse effects encountered. Histological changes showed reduced endomysial fibrosis and reduced central nucleation. FS344/FS315 isoform had no effect on pituitary hormones. Also describes early sIBM trial (3 patients at low dose, no adverse events, transient improvement). Excellent safety profile noted across both disease populations.
Limitations: Small sample size (6 BMD, 3 sIBM patients). Open-label dose-escalation design. Gene therapy delivery, not injectable peptide. Disease-specific population, not healthy users.
5Current Pharmacological Strategies for Duchenne Muscular DystrophyPMID 34490244
Yao S et al. - Frontiers in Cell and Developmental Biology (2021) - Review - N/A (review article)
Lists intramuscular rAAV1.CMV.huFollistatin344 as a Phase I/II strategy for DMD. Summarizes beneficial mdx and primate data. Frames follistatin as one of the downstream muscle-function approaches under active translational consideration for dystrophy.
Limitations: Review article, not primary efficacy data. Some cited programs were ongoing or historical at time of publication.
6Detection of black market follistatin 344PMID 31758732
Reichel C, Gmeiner G, Thevis M - Drug Testing and Analysis (2019) - Analytical / anti-doping market surveillance study - 17 black market products tested
Only 9 of 17 black market FS344 products actually contained follistatin. All 9 positive products contained His-tagged FS344 with high degree of oligomers. Some negative products contained other growth-promoting peptides (MGF, GHRP-2). Detection method developed: immunomagnetic purification + SDS-PAGE + Western blot. His-tag allows unambiguous differentiation from endogenous follistatin. Detection limit ~0.1 ng/mL in 10 mL urine, ~5 ng/mL in 100 uL serum.
Limitations: Analytical study focused on detection methodology and market surveillance, not therapeutic outcomes. Limited to European/online market sampling.
7AAV-mediated follistatin gene therapy improves functional outcomes in the TIC-DUX4 mouse model of FSHDPMID 30429376
Giesige CR et al. - JCI Insight (2018) - Animal study (mouse model of FSHD) - Multiple mouse cohorts
AAV-mediated follistatin gene therapy improved functional outcomes in the facioscapulohumeral muscular dystrophy (FSHD) mouse model.
Limitations: Animal study. Gene therapy delivery. Not in the filtered paper set but referenced in raw data sources.
8Plasmid-Mediated Gene Therapy in Mouse Models of Limb Girdle Muscular DystrophyPMID 31890729
Guha TK, Pichavant C, Calos MP - Molecular Therapy: Methods & Clinical Development (2019) - Animal study (mouse model of LGMD) - Multiple mouse cohorts
Plasmid-mediated follistatin gene therapy showed efficacy in limb girdle muscular dystrophy mouse models.
Limitations: Animal study. Plasmid-mediated delivery, not injectable peptide. Not in the filtered paper set but referenced in raw data sources.
9Circulating resistin and follistatin levels in obese and non-obese women with polycystic ovary syndrome: A systematic review and meta-analysis
Raeisi T et al. - PLOS ONE (2021) - Systematic review and meta-analysis - 9 studies for follistatin (815 cases; 328 controls)
Circulating follistatin levels significantly higher in PCOS women vs controls (WMD = 0.44 ng/ml; 95% CI = 0.30-0.58, P<=0.001). Effect independent of obesity status. Significant heterogeneity detected (I2 = 99.2%).
Limitations: Observational studies only, no interventional data. Significant heterogeneity. Small number of follistatin-specific studies (9). Measures endogenous follistatin levels, not exogenous supplementation.
10Clinical Intramuscular Gene Transfer of rAAV1.CMV.huFollistatin344 Trial to Patients With Duchenne Muscular DystrophyNCT02354781
Mendell JR (PI) - ClinicalTrials.gov (2015) - Clinical trial registration (gene therapy) - Not specified in available data
Registered clinical trial for AAV-follistatin gene therapy in DMD patients. Related to the FDA Orphan Drug Designation granted November 19, 2012.
Limitations: Gene therapy, not injectable peptide. Full results not available in source data.