Follistatin-344

Myostatin inhibitor - an isoform of endogenous follistatin that binds and neutralises GDF-8 (myostatin). Most of the impressive data comes from AAV gene-therapy studies, not the injected peptide form. Don't conflate the two.

Research chemicalWADA S4 prohibitedInjectableMyostatin inhibitor

Key facts

Common routesSC, IM

Half-lifeHours

Typical range100–200 mcg/day

Summary

Follistatin-344 (FS-344) is a specific isoform of the naturally occurring follistatin protein. It binds and neutralises myostatin (GDF-8) - the body's signal that limits muscle growth. "Take the brakes off muscle growth" is the popular phrasing, and mechanistically it's accurate. Unlike the FS-315 isoform, FS-344 avoids binding activin, giving it a somewhat more muscle-vs-reproductive-tissue specific profile (the specificity is contested).

The critical distinction: the impressive "mighty mouse" data used AAV-follistatin gene therapy, not injected FS-344 peptide. The peptide form has a very short half-life and weak bioavailability versus the gene-therapy vector. Human data for the peptide form is almost non-existent - bodybuilding community logs are the only available signal.

Mechanism notes

Myostatin (GDF-8) inhibition

Binds myostatin ligands and neutralises them before they can attach to the activin type IIB receptor on muscle cells. Removes the body's "stop signal" for muscle growth.

Activin inhibition (selective)

While FS-344 also binds activin, the 344 isoform is described in literature as more favourable for muscle vs reproductive tissue than other isoforms. The specificity is contested - no guarantee that reproductive effects are excluded.

Peptide vs gene therapy (critical distinction)

Most bodybuilding excitement traces back to AAV-follistatin gene-therapy studies (Kota 2009 in mice, early human work for muscular dystrophy indications). The injected FS-344 peptide isn't the same - shorter half-life, weaker systemic action. Expectations should not be calibrated from the gene-therapy data.

Dosing patterns

Myostatin-suppression blast

100–200 mcg daily SC for 10–20 days. Users prefer short high-dose blasts because myostatin levels are reported to rebound quickly. Longer cycles are avoided to reduce adaptation.

Local (IM) use

Some operators inject IM into the target muscle. Mechanistically the action is systemic, so the local advantage is anecdotal. Site-hypertrophy reports are community-log grade.

Tendon reality check

Rapid muscle growth without corresponding tendon strengthening can lead to tears. Myostatin is also involved in tendon maintenance; blocking it may weaken connective tissue. This is the biggest practical concern with follistatin use.

Evidence snapshot

Tier: Limited data. High-quality data comes primarily from gene-therapy studies (AAV-follistatin), not the peptide form. Human safety data for FS-344 is almost non-existent outside anecdotal bodybuilding logs.

Animal / gene-therapy data

Kota 2009: AAV-follistatin in mice produces "mighty mouse" phenotype with massive hypertrophy. Haidar 2019 and similar studies cover gene therapy for muscular dystrophy indications. Important: that's not the FS-344 peptide use case.

Human data (peptide form)

Outside gene-therapy contexts (DMD research), no published trials of injected FS-344 peptide. Community logs are the only human signal - mixed, often in the context of AAS and other growth factors.

Safety considerations

High-risk class with thin human safety basis. Main concerns: tendon weakening relative to muscle growth (real and under-discussed), anecdotal kidney-stress reports, sourcing quality (follistatin synthesis is non-standard and frequently mis-made), and the expectations mismatch between gene-therapy data and peptide reality.

Common cautions

Tendon strengthening lags muscle growth - tear risk if training aggressively during cycle
Anecdotal reports of dark urine and kidney stress on high-dose cycles
Frequently mis-made in the grey market; FS-344-specific synthesis is non-standard
Expectations mismatch: the "mighty mouse" data is gene therapy, not peptide