Follistatin (FST-344)
Follistatin (FST-344) — myostatin inhibition, what the evidence really shows & status

Class: protein
Synonyms: Follistatin
FST
FST-344 / FS344 (the "research-peptide" isoform)
FST-315 (the circulating form)
FST-288 (the tissue-bound form). INCI: none
Function tags: muscle / myostatin inhibition
(studied:) muscular-dystrophy gene therapy — research context
Overall evidence grade: D
WADA prohibited: yes
WADA note: Sport: WADA-prohibited — myostatin-inhibiting agents are explicitly banned (Section S4.4, Hormone and Metabolic Modulators). Do not touch it in tested sport.
Last updated: 2026-07-13

In brief: Follistatin is a natural myostatin inhibitor whose biology is spectacular — releasing the body's brake on muscle produces the "double-muscling" seen across species. But "follistatin, the injectable muscle drug" does not exist in any proven form: the dramatic gains are from transgenic animals and gene therapy, the human gene-therapy trials were modest and unapproved, the pharma myostatin-inhibitor programs mostly failed on function or safety, and the online "FST-344" is a large protein of dubious authenticity. Fascinating science; unproven and risky as a product. Banned in sport.

## Identity Follistatin is a naturally occurring **glycoprotein** produced in the gonads, pituitary and liver. At **~300+ amino acids it is a protein, well above the "peptide" range** — which matters, because a large protein is far harder to synthesise, verify and keep stable than a short peptide. **FST-344** (the 344-residue isoform sold online) is converted in the body to **FST-315**, the systemic circulating form. ## Mechanism (as proposed) follistatin binds and sequesters **myostatin (GDF-8)** plus **activin A, GDF-11 and several BMPs**, removing multiple TGF-β "brakes" on muscle → increased protein synthesis and reduced degradation → hypertrophy. Its breadth ("biological shotgun") is why it out-performs myostatin-only antibodies in animals — and also why its **off-target effects** (reproductive, vascular, bone) are a genuine concern. FST-344 converts in vivo to the systemic FST-315.

Evidence grades by outcome:
outcome: Muscle hypertrophy via myostatin inhibition (the biology)
base: Transgenic + gene-therapy animals (20–40%, more in overexpression); Lee 2007
effect: Dramatic muscle growth
caveat: This is the biology in transgenic / gene-therapy models — not injected protein in humans
outcome: Injected follistatin protein → muscle in healthy humans (the marketed use)
grade: F
base: None
effect: Not demonstrated
caveat: No human clinical evidence; the animal gains were transgenic / gene-therapy
outcome: Gene therapy for muscular dystrophy / IBM
grade: D
base: Phase 1/2 (Mendell)
effect: Modest
caveat: Well tolerated locally, but modest; never became an approved therapy
outcome: Broader myostatin-inhibitor drug class
base: Antibodies (MYO-029 etc.) + ACE-031
effect: Largely failed
caveat: Antibodies: modest/no functional benefit; ACE-031 terminated on vascular safety
outcome: Safety
base: Preclinical + gene-therapy + mechanism
effect: Real concerns
caveat: Suppresses FSH/LH (reproductive); vascular risk (class); may lower bone density (GDF-11 block); muscle may outpace tendon; theoretical oncogenic concern; effects may not "cycle off"; gray-market product often mislabelled/contaminated; WADA-banned

Legal status:
region: INT
status: See note
note: - Everywhere — not approved for any human indication. Sold only as a "research" chemical — and because it's a large protein, gray-market "Follistatin-344" is frequently underdosed, mislabelled or contaminated.
- ⚠ Sport: WADA-prohibited — myostatin-inhibiting agents are explicitly banned (Section S4.4, Hormone and Metabolic Modulators). Do not touch it in tested sport.

Cosmetic claims:

Note: Reference entry — not sold here. The muscle world's holy grail — myostatin inhibition — and the honest reality behind the hype. Two things to know up front: it's a large protein, not a peptide, and the jaw-dropping numbers you've seen come from transgenic animals and gene therapy, not from an injected protein. No dosing published here.
Honest note: the myostatin story is real and thrilling — but the gap between "myostatin knockout doubles muscle in mice" and "inject this vial and grow" is enormous and unbridged. The compounds that tried to bridge it with rigorous trials mostly failed (function or safety), and what's sold online is a large, fragile protein you can't verify. Respect the biology; don't trust the product.

Sources:
Lee SJ. (Follistatin overexpression roughly doubles muscle mass; effect on a myostatin-null background.) PNAS, 2007.
Mendell JR, et al. (AAV-follistatin gene therapy trials in muscular dystrophy / inclusion-body myositis.) Phase 1/2.
Kota J, Sumner CJ, et al. Inhibition of myostatin, with emphasis on follistatin as a therapy for muscle disease. (Review; PMC2717722.)
ACE-031 clinical-termination reports; myostatin-antibody trial reviews (MYO-029 and successors).