Follistatin 344
Myostatin Inhibitor | TGF-β Antagonist for Muscle Research
Follistatin 344 (FS-344) is a naturally occurring glycoprotein that inhibits myostatin and activin A, members of the TGF-β superfamily that suppress muscle growth. The FS-344 gene encodes a 344-amino acid precursor that is cleaved to produce the circulating FS-315 isoform. Gene therapy studies in primates and human clinical trials for muscular dystrophy have demonstrated significant muscle hypertrophy and strength gains. However, injectable peptide use has very limited human data, a short half-life (~90 minutes), and most research involves gene delivery rather than exogenous peptide administration. Follistatin has been banned by WADA since 2019.
Daily dose
100-200 mcg (anecdotal only)
Frequency
Once daily during cycle
Cycle length
10-30 days
Storage
-20°C (lyophilized)
Key benefits
Myostatin and activin A inhibition, potential muscle growth enhancement, studied for muscular dystrophy therapy
How it works
Binds and neutralizes myostatin and activin A, preventing their interaction with ActRIIB receptors. Blocks TGF-β signaling that suppresses muscle growth.
Dosage protocols
Goal
Research Protocol (Anecdotal)
Dose
100 mcg · Once daily
Route
SubQ
Goal
Higher Dose Protocol (Anecdotal)
Dose
200 mcg · Once daily (max)
Route
SubQ
Research indications
muscle Growth
recovery
Administration
Interactions
Safety notes
Most safety data from gene therapy, not injectable peptide
Potential FSH suppression affecting reproduction
Minor LDL increase possible
Case report of vision impairment at high dose
Not recommended during pregnancy
WADA banned since 2019
Research studies
Follistatin Gene Delivery Enhances Muscle Growth in Nonhuman Primates (2009)
Primate | AAV1-FS344 | 1×10¹³ vg | Cynomolgus macaques | 15 months
AAV1-FS344 injection into quadriceps produced 15% circumference increase at 8 weeks, persisting 15+ months. Muscle fiber diameter increased significantly (87.7 μm vs 65.5 μm control). No adverse effects on cardiac, reproductive, or organ systems.
View study →Phase 1/2a Follistatin Gene Therapy for Becker Muscular Dystrophy (2015)
Human | AAV1.CMV.FS344 | 6 BMD patients | Intramuscular | 1 year
First human gene therapy trial. Patients showed 6-minute walk test improvements up to +125 meters. High-dose patients showed muscle fiber diameter increase (40→59 μm) and 35-43% reduction in fibrosis. No serious adverse events. Hormone levels remained normal.
View study →Long-term Enhancement of Muscle Mass by Myostatin Inhibitors (2008)
Mouse | Multiple vectors | Transgenic & AAV | Long-term follow-up
Transgenic mice expressing high follistatin levels showed 194-327% muscle mass increase. FS-344 produced greatest effects among tested inhibitors. Effects persisted over 2 years without adverse events in animal models.
View study →Follistatin Induces Muscle Hypertrophy via Satellite Cell Proliferation (2009)
Mouse | In vivo & in vitro | Muscle regeneration model
Demonstrated follistatin promotes muscle growth through both myostatin inhibition AND activin A blockade, plus direct satellite cell proliferation. Double-action mechanism explains greater efficacy than myostatin-only inhibitors.
View study →Inhibition of Myostatin with Emphasis on Follistatin as Therapy (2008)
Review | Multiple studies | Therapeutic potential assessment
Comprehensive review establishing rationale for FS-344 over other isoforms. FS-315 circulating form preferred due to reduced binding to reproductive tissues and limited off-target effects compared to FS-288.
View study →