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Follistatin 344

Follistatin is a natural protein that binds and blocks activin and myostatin, signals that limit muscle growth and help control the reproductive hormone FSH. "Follistatin 344" is named after the 344-amino-acid precursor; once its signal sequence is removed, the working protein is 315 amino acids long (FS315). No follistatin medicine is approved, and the human trials so far used gene therapy rather than injected protein.

Not approved · research onlyWADA Prohibited at all times (S4.3, myostatin-binding proteins)Checked October 2026
Open the Follistatin 344 calculatorSources

Approval status

Not approved as a medicine anywhere. Products labeled "follistatin 344" are sold on the black market, and in one lab analysis only 9 of 17 actually contained follistatin.23

What it’s used or studied for

  • Human studies

    Gene therapy delivering the FS344 gene into thigh muscles improved walking distance in 4 of 6 people with Becker muscular dystrophy. This is a different form from injected protein.4

  • Human studies

    The same gene therapy improved walking in 6 people with sporadic inclusion body myositis, compared with untreated patients. It is a different form from injected protein.5

  • Animal & lab only

    Mice engineered to make extra follistatin in muscle had large increases in muscle mass.6

  • Animal & lab only

    In monkeys, follistatin gene delivery into muscle increased muscle size and strength without harming major organs.7

  • No studies

    Injected follistatin protein is used by bodybuilders to build muscle; no trials have tested this in people.8

Each label shows how strong the evidence is: an approved use, human studies, animal or lab studies only, or a popular claim with no published studies. Being studied is not the same as being proven safe or effective.

Side effects

Serious or rare

  • Fluid under the retina with blurred vision (central serous chorioretinopathy), reported after injections

There are no trials of injected follistatin protein. A case series described 11 male bodybuilders who developed this eye problem after injecting it; it cleared in those who had injected once but came back in three who had injected several times. The Becker gene therapy trial reported no adverse effects.84

Cautions

  • In a doping lab analysis, 8 of 17 black-market "follistatin 344" products contained no follistatin, and some held other growth peptides instead.2
  • The products that did contain follistatin carried a lab His-tag and many clumped (oligomer) forms, unlike natural follistatin.2
  • Eye doctors suggest asking about follistatin use in people with central serous chorioretinopathy.8

Sources

  1. UniProt P19883: Follistatin (human)
  2. Reichel C et al. Detection of black market follistatin 344 (Drug Test Anal, 2019)
  3. WADA 2026 Prohibited List
  4. Mendell JR et al. A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy (Mol Ther, 2015)
  5. Mendell JR et al. Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes (Mol Ther, 2017)
  6. Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth (PNAS, 2001)
  7. Kota J et al. Follistatin gene delivery enhances muscle growth and strength in nonhuman primates (Sci Transl Med, 2009)
  8. Dağ U et al. Central serous chorioretinopathy associated with high-dose follistatin-344: a retrospective case series (Int Ophthalmol, 2020)

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